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Item 6C - Ord. No. 1030A- Stormwater Master Plan ORDINANCE NO. 1030A AN ORDINANCE ADOPTING THE STORMWATER MASTER PLAN AS AN ELEMENT OF THE SOUTHLAKE COMPREHENSIVE PLAN, THE CITY'S COMPREHENSIVE PLAN UPDATE. WHEREAS, a Home Rule Charter of the City of Southlake, Texas, was approved by the voters in a duly called Charter election on April 4, 1987; and, WHEREAS, the Home Rule Charter, Chapter XI requires an update to the City's comprehensive plan elements, WHEREAS, the City Council recognizes that the Stormwater Master Plan is an element of the City's Comprehensive Plan, WHEREAS, the City Council has deemed that the Stormwater Master Plan has been formulated with adequate public input, WHEREAS, the City Council has deemed that the recommendations in the Stormwater Master Plan herein reflect the community's desires for the infrastructure of the City, WHEREAS, the City Council has deemed it is in the best interest of the public's health, safety and welfare to establish a policy framework for stormwater infrastructure as it pertains to its operation, maintenance and investment within the Stormwater Master Plan. NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF SOUTHLAKE, TEXAS, THAT: Section 1. All of the findings in the preamble are found to be true and correct and the City Council hereby incorporates said findings into the body of this ordinance as if copied in its entirety. Section 2. The statements in `Exhibit 1' are hereby adopted as the Stormwater Master Plan of the Southlake Comprehensive Plan. Section 3. The different elements of the Comprehensive Plan, as adopted and amended by the City Council from time to time, shall be kept on file in the office of the City Secretary of the City of Southlake, along with a copy of the ordinance and minute order of the Council so adopting or approving the same. Any existing element of the Comprehensive Plan which has been heretofore adopted by the City Council shall remain in full force until amended by the City Council as provided herein. Section 4. This ordinance shall be cumulative of all provisions of ordinances of the City of Southlake, Texas, except where the provisions of this ordinance are in direct conflict with the provisions of such ordinances, in which event the conflicting provisions of such ordinances are hereby repealed. Section 5. It is hereby declared to be the intention of the City Council that the phrases, clauses, sentences, paragraphs and sections of this ordinance are severable, and if any phrase, clause, sentence, paragraph or section of this ordinance shall be declared unconstitutional by the valid judgment or decree of any court of competent jurisdiction, such unconstitutionality shall not affect any of the remaining phrases, clauses, sentences, paragraphs and sections of this ordinance, since the same would have been enacted by the City Council without the incorporation in this ordinance of any such unconstitutional phrase, clause, sentence, paragraph or section. Section 6. The City Secretary of the City of Southlake is hereby authorized to publish this ordinance in book or pamphlet form for general distribution among the public, and the operative provisions of this ordinance as so published shall be admissible in evidence in all courts without further proof than the production thereof. Section 7. This ordinance shall be in full force and effect from and after its passage and publication as required by law, and it is so ordained. PASSED AND APPROVED on the 1st reading the 7' day of April, 2026. MAYOR ATTEST: CITY SECRETARY PASSED AND APPROVED on the 2nd reading the 21St day of April, 2026. MAYOR ATTEST: CITY SECRETARY APPROVED AS TO FORM AND LEGALITY: CITY ATTORNEY DATE: ADOPTED: EFFECTIVE: Ot CITY OF SOUTLA f "� ��•• 101K. K• ILL y J RMWATE STO MASTER PLAN An Element of the Southlake Comprehensive Plan Alm Adopted by the Southlake City Council Ordinance No. 1030A April 21, 2026 ,> �, .� t.. 06 i r 1 c � k= x r; �, K" fH° �Ft J'" a {-u� � r,x.: ��^ �t i. f,(�. s il,:�. .',ay,.._1� A SOUTHLAKE Comprehensive Plan City of Southlake City Council Mayor Shawn McCaskill Mayor Pro Tem Randy Williamson Deputy Mayor Pro Tem Kathy Talley Councilmember Place 2 Randy Robbins Councilmember Place 3 Frances Scharli Councilmember Place 4 Austin Reynolds Councilmember Place 5 Chuck Taggart Planning & Zoning Commission Corridor Planning Committee Chair Daniel Kubiak Kathy Talley - City Council Vice Chair Mike Forman Randy Robbins - City Council Commissioner Gina Cannova Austin Reynolds - City Council Commissioner David Cunningham Frances Scharli - City Council Commissioner Michael Springer Daniel Kubiak - Planning & Zoning Commissioner Stacy Driscoll Michael Springer - Planning & Zoning Commissioner Lora Gunter David Cunningham - Planning & Zoning Gina Cannova - Planning & Zoning Mike Forman - Planning & Zoning Magdalena Battles - Parks Board Chad Patton - Ex-Officio City of Southlake Staff City Manager Alison D. Ortowski Assistant City Manager James Brandon Assistant City Manager Stacey Black Chief Financial Officer Sharen Jackson Interim Director of Public Works Lauren LaNeave City Engineer Jeff Ginn Director of Economic Development & Tourism Daniel Cortez Director of Planning and Development Services Dennis Killough Deputy Director of Planning and Development Services Jenny Crosby Assistant to the Director Ryan Firestone Senior GIS Analyst Jesus Gabriel ae p x 5 p e , Southlake's Stormwater Master Plan ("Plan") establishes a long-range framework for managing stormwater as a core public service that supports public safety, infrastructure reliability, environmental stewardship, and quality of life. This master plan is written to be accessible to the public while providing implementation clarity to guide future decisions, study updates, ordinance refinement, and investment planning. It is supported by technical studies and field observations, but it does not replace regulatory compliance or engineering design. Instead, it establishes how the City will make stormwater decisions in a consistent, transparent, and strategic way and how the City will communicate stormwater responsibilities and expectations clearly across the community. Stormwater management in Southlake is shaped by three interconnected components: Regulatory: The legal and program requirements that govern stormwater quality, construction controls, and floodplain management. Southlake operates as a regulated small Municipal Separate Storm Sewer System under the Texas Pollutant Discharge Elimination System administered by the Texas Commission on Environmental Quality. The City also participates in the National Flood Insurance Program and administers floodplain regulations consistent with FEMA standards. (tceq.texas.gov) (fema.gov) Public: The City-owned and City-maintained network of inlets, pipes, culverts, channels, and drainage facilities within public rights-of-way and improved easements that manage runoff, protect public infrastructure, and reduce hazards. Private: The private grading, private drains, HOA-maintained facilities, and long-term maintenance of post-construction stormwater controls that influence drainage performance and drive many day-to-day resident concerns. Beyond infrastructure, the Plan supports the development of targeted programs and public education initiatives that promote responsible stormwater practices, protect water quality, and increase community awareness. The plan allows Southlake to manage stormwater in a coordinated, resilient, and cost-effective manner while enhancing public safety and quality of life, reducing long-term costs, and ensuring the City remains resilient in terms of public infrastructure to face the future. STORMWATER MASTER PLAN TABLE OF CONTENTS PAGE CHAPTER 7 CHAPTER 1: INTRODUCTION 13 CHAPTER 2: SYSTEM AND WATERSHED OVERVIEW 27 CHAPTER 3: HYDROLOGY, HYDRAULICS, AND LEVEL OF SERVICE 35 CHAPTER 4: COLLECTING DATA AND UPDATING MODELING 43 CHAPTER 5: EVALUATING THE STORMWATER SYSTEM 51 CHAPTER 6: WATER QUALITY & REGULATORY COMPLIANCE 57 CHAPTER 7: GREEN INFRASTRUCTURE & RESILIENCE 61 CHAPTER 8: STORMWATER INFRASTRUCTURE AND ASSET MANAGEMENT 69 CHAPTER 9: DEVELOPMENT STANDARDS AND FRAMEWORK 75 CHAPTER 10: COMMUNITY ENGAGEMENT, PARTNERSHIPS, AND FUNDING 81 CHAPTER 11: PLAN ADOPTION & PUBLIC ENGAGEMENT 85 STORMWATER MASTER PLAN POLICY STATEMENTS SUMMARY 87 GLOSSARY OF TERMS ORDINANCE r 'dF'R Stormwater management is one of the most visible infrastructure services a city provides. Residents experience it through street ponding, driveway and yard flow paths, creek and channel erosion, debris accumulation at inlets, and the City's response during heavy rainfall. Because storm events are episodic, attention to Stormwater can rise quickly during intense storms and then fade during dry periods. A master plan creates continuity and consistency before, during, and after storms by establishing shared expectations for how stormwater is managed, how risks are described, and how the City evaluates maintenance, studies, and capital improvements. Stormwater runoff is generated when rain flows over impervious or semi-impervious surfaces such as roofs, streets, sidewalks, driveways, and compacted soils. As development increases within a community, infiltration declines and runoff volumes and peak flow rates can increase. Effective stormwater management helps communities accommodate these changes while minimizing localized flooding, reducing channel erosion, and protecting water quality. Stormwater runoff can also carry sediment, nutrients, bacteria, oils, and debris into the storm drainage system and local water bodies. These materials can reduce drainage performance (for example, clogging inlets or depositing sediment in pipes and channels), degrade water quality, impair aquatic habitat, and contribute to nuisance conditions. Stormwater management therefore includes the planning, control, collection, storage, conveyance, and treatment of runoff to reduce hazards, protect water quality, and safeguard public and environmental resources. It requires infrastructure, programs, maintenance, and enforcement, and it must be planned and funded strategically and cost-effectively. PURPOSE OF THE STORMWATER MASTER PLAN This Stormwater Master Plan provides a long-range policy and planning framework for managing stormwater infrastructure, programs, and risks across the community. It sets direction for how stormwater issues are evaluated, how priorities are determined, and how the City balances maintenance, studies, and capital improvements to sustain reliable system performance over time. This Stormwater Master Plan provides a long-range framework for: • Protecting life safety and emergency access by reducing stormwater-related hazards. STORMWATER MASTER PLAN I SOUTHILAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN • Sustaining reliable system performance as infrastructure ages and the community approaches build out. • Maintaining compliance with applicable federal and state stormwater and floodplain requirements. • Improving water quality through best management practices and program delivery. • Reducing erosion risk where it threatens public infrastructure and property. • Clarifying public versus private stormwater responsibilities. • Establishing a strategic foundation for a future dedicated Asset Management Plan. • Strengthening customer response, public education, and community understanding of stormwater processes and responsibilities. This plan is intended to be used as a decision framework. It helps staff and decision-makers answer consistent questions: • What is the issue and what is driving it? • Does the concern primarily involve public infrastructure, private drainage conditions, or both? • What is the appropriate response pathway (maintenance, inspection, study, ordinance tool, education)? • How should the City communicate findings, expectations, and next steps to residents? RELATIONSHIP TO THE STRATEGIC MANAGEMENT SYSTEM The City's Strategic Management System emphasizes measurable outcomes, accountability, and disciplined service delivery. Stormwater management fits naturally within this structure because it combines routine operational service such as inspection, maintenance, customer response, and regulatory program responsibilities with longer-term infrastructure planning that must be sustained over decades. A stormwater program cannot rely solely on responding to issues as they appear; it must also build and maintain the information, processes, and reinvestment practices that reduce risk over time. This Plan supports that disciplined approach by establishing clear program building blocks (data and inventory, study cycles, maintenance and asset management, ordinance tools, and education) that staff can carry forward year after year. This Plan also supports performance management by identifying stormwater outcomes that can be tracked consistently and communicated transparently. Examples include inspection and maintenance coverage of critical assets, response timeliness for higher-priority concerns, reduction of repeat service "hotspots" through targeted maintenance, progress on scheduled watershed study updates, and advancement of capital improvements that reduce documented risk. These measures help the City demonstrate how routine work and planned investments contribute to reliable system performance and public safety especially during intense storms when residents most directly experience the stormwater system. Over time, using these measures as part of regular reporting strengthens public confidence, improves internal coordination, and supports defensible budgeting decisions grounded in documented needs rather than anecdotal pressure. RELATIONSHIP TO THE CITIZEN SATISFACTION SURVEY The Citizen Satisfaction Survey reinforces that stormwater drainage is viewed as a core, "must-work" service in Southlake on par with other foundational infrastructure functions. In the 2025 survey, managing stormwater drainage was rated as very or somewhat important by 97% of respondents (with 81% rating it "very important"), placing it among the City's highest-priority infrastructure services. Residents also report strong satisfaction with stormwater drainage compared to most other services, with 84% indicating they are very or somewhat satisfied. Stormwater drainage appears within the survey's highest-satisfaction group of City services, alongside utilities and key public safety measures. That said, a 14-percentage-point gap between importance and satisfaction ratings indicates that resident expectations for this service run slightly ahead of perceived performance, a signal the Plan takes seriously even though the gap remains below the City's established prioritization threshold of 2S%. Verbatim comments and open-ended responses provide additional insight into how residents experience Stormwater. Comments tend to cluster around a few recurring themes: localized street ponding during heavy rainfall, drainage patterns affecting individual lots or driveways, debris accumulation at inlets, and concerns about creek or channel erosion over time. In many cases, these comments reflect specific locations or recent storm events, rather than ongoing or widespread issues. Where dissatisfaction does surface in the open-ended responses, it centers on three recurring concerns: uncertainty about whether a drainage issue is the City's responsibility or a private matter, a perception that development has worsened drainage conditions in established neighborhoods, and frustration when a reported concern did not result in a visible response or a clear explanation of next steps. Residents also frequently express a desire for clearer communication regarding what is normal during intense rainfall, what actions the City can take, and what responsibilities fall to property owners. This combination of high importance, strong satisfaction, and location-specific feedback supports the Plan's emphasis on sustaining system performance through disciplined maintenance, clear service expectations, and long-term reinvestment planning. For Stormwater specifically, where concerns are often event-driven and emotionally charged, this also underscores the importance of a consistent and predictable customer response approach. The Stormwater Master Plan therefore includes a stronger customer response and public education framework, including consistent intake and triage of concerns, clear public versus private responsibility messaging, and reliable "closure" communication following inspections, maintenance actions, or storm events. In other words, the plan is not only about engineering outcomes. It is also about meeting resident expectations through clearer explanations, predictable processes, and better follow-through when residents need answers most. By incorporating both quantitative survey results and qualitative feedback, the City can respond more effectively to concerns while maintaining a balanced, data- informed approach to stormwater management. RELATIONSHIP TO THE SOUTHLAKE COMPREHENSIVE PLAN The Stormwater Master Plan is a supporting element A of the City's comprehensive planning framework and is intended to translate community-wide goals into practical /a\ direction for stormwater policy, investment, and service delivery. Comprehensive planning provides the long- range "why" for City decisions protecting quality of life, —��— maintaining strong neighborhoods,guiding development, and coordinating public investments. Stormwater plays a direct role in each of these outcomes. Drainage constraints and flood risk shape where and how development/ SOUTHLAKE redevelopment can occur, how streets function during major rain events, and how reliably residents and visitors Comprehensive Plan STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN can move through the community. Creek corridors and drainageways are also part of Southlake's open space network and contribute to community character, environmental stewardship, and long-term resilience. When stormwater is considered early and consistently within the broader planning structure, the City is better positioned to avoid avoidable risk, reduce lifecycle costs, and ensure that capital projects and redevelopment efforts reinforce rather than undermine long-term community goals. Integrating stormwater planning into the comprehensive planning structure also improves coordination across City programs that affect runoff and drainage performance. Land use policy, street and mobility projects, park and open space planning, code enforcement, and capital improvement programming all influence how stormwater behaves in the built environment. This Plan supports a coordinated approach by establishing shared expectations for stormwater levels of service, identifying where watershed constraints should shape project decisions, and setting a clear framework for when technical studies are needed to support major investments. In short, it provides the planning "bridge" between the Comprehensive Plan's long-range vision and the day-to-day decisions that determine whether stormwater performance remains reliable as the community reinvests in existing areas. STORMWATER STUDIES AND PROGRAM LIBRARY Southlake has invested in stormwater planning and evaluation for several decades, resulting in a well-developed body of technical information that supports both day-to-day decision-making and long-term capital planning. This work includes watershed-level studies, structure-specific evaluations, citywide screening analyses, and regulatory mapping updates that collectively provide a comprehensive understanding of how the drainage system performs under a range of conditions. The studies highlighted on this page represent key efforts that inform system-wide planning; however, they are not an exhaustive list. The City also maintains a broader library of supporting analyses, including smaller-scale studies, development-driven drainage analyses, downstream assessments, and site-specific engineering evaluations completed as part of both public and private projects. These efforts provide additional localized data and continue to expand the City's understanding of drainage conditions across the system. These studies are used in different but complementary ways. Detailed watershed studies develop hydrologic and hydraulic models to understand flow behavior, evaluate flood risk, and test potential improvement alternatives in known problem areas. Citywide screening tools, such as the 2D rain- on-mesh analysis, help identify localized flooding conditions that may occur outside of FEMA- mapped floodplains, providing additional insight into areas that are not captured by regulatory floodplain mapping but may still experience drainage challenges during certain storm events. Structure-focused evaluations assess the performance of bridges, culverts, and crossings against current criteria and identify where deficiencies or constraints exist. Regulatory mapping efforts, including FEMA and Risk MAP updates, provide consistent floodplain data used for development review and flood risk communication. Together, this information allows the City to move from identifying issues to understanding causes and evaluating solutions in a structured, data-informed way. The City also maintains supporting program documentation, including MS4 permit requirements and floodplain management practices, which guide compliance and ongoing system maintenance. The City's current Stormwater Management Program (Permit Term IV, 2025-2029), prepared in March 2025 under TPDES General Permit TXR040000 (Permittee Authorization No. TXR040007), establishes the active BMP commitments and measurable goals governing the MS4 program for the current permit term and serves as the primary compliance framework alongside this master plan. Study/ Studied/ Year Report Defined Recommended/Resolved 2030 Drainage Citywide Reviewed previous studies and combined the project Master Plan baseline; recommendations into one document for Capital Improvement 2012 Report (07-11- basin Project Ranking, Planning and Implementation. Included review 2012) constraints of completed and current construction projects to remove from project planning recommendations. Drainage Crossing Full H&H analysis for the culvert sizing/configuration associated 2014 Study - Zena hydraulics with the extension of Zena Rucker. This project led to a new bridge Rucker Rd culvert crossing of Tributary BB-6 with the Zena Rucker extension Culvert (TNP) project. NCTCOG CTP Risk MAP Comprehensive flood study and mapping update of eight streams FY14 TSDN mapping totaling 10.44 stream miles within the City's Big Bear Creek 2014 - Southlake technical watershed area. Information from this study was submitted to (Completed in basis FEMA to revise the effective Flood Insurance Study and Flood 2016) Insurance Rate Maps for the study streams. As of 2020, these remain in Preliminary Review with FEMA. Highland & Crossing Full H&H analysis to make bridge culvert upsizing recommendations Dove Creek constraints/ for the W. Highland at S. Fork Kirkwood Branch and Dove Creek 2019 Trail Culvert overtopping at Dove Creek Trail, Mission and San Saba culvert crossings. This Report (JIC) risk project upsized Critical Drainage Structures 10 and 11 in addition to two other culverts. Drainage Plan Critical Reanalyzed the recommended sizing and conceptual project costs - TNP structures for the 55 Critical Drainage Structures identified in the 2012, 2030 2020 screening Drainage Master Plan Report. This included those structures not included in the priority rankings to confirm these are adequately sized. Citywide 2D 2D depth Utilized a 2D rain-on-mesh analysis to develop local floodplain Rain-on-Mesh grids; mapping and depth grids, identify hot spot locations (areas of Memo - Halff identified potential flooding) and assist with CIP planning by ranking areas 2020 hotspots outside of the FEMA-regulated floodplain for future analysis outside and potential capital im-provements. Fifty initial locations were of FEMA ultimately pared down to ten for conceptual review. floodplain Kirkwood/ Watershed The Kirkwood/Higgins Watershed Study updated the existing Higgins H&H; riverine flood models for the Kirkwood Branch Watershed, including Watershed alternatives S. Fork Kirkwood Branch and Higgins Branch. The FEMA model was Study (Halff) with rain-on- revised based on updated LiDAR and field survey data, and results mesh from the 2D rain-on-mesh analysis were incorporated and refined 2021 to evaluate three hot spot areas for conceptual improvements. The study directly informed three capital improvement projects: the N. White Chapel at Kirkwood Branch Bridge Project, N. Peytonville at Raven Bend Drainage Improvements, and the drainage improvements from 1330 N. Peytonville to Higgins Branch at Post Oak Trail. Erosion Erosion/ Preliminary Engineering field inspection, easement, right-of-way 2022 Control stability risks and permit analysis and recommendations for the stabilization of Improvements four separate locations where the City's wastewater infrastructure PER (J&C) is being impacted by stream erosion. 10 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN Study Studied/ Year Report Defined Recommended/Resolved Stormwater Citywide The purpose of this study was to create a roadmap for the City to Master Plan baseline; take a holistic approach to the City's Stormwater Program. Phase Study - Phase prioritization 1 reviewed data collected from prior studies and existing drainage 1 (Halff) ordinances. The study focused on ranking both watershed study updates and CIP prioritization. Subsequent phase recommendations 2022 include Stormwater Utility Rate Study, CIP Program Updates for CIP and Erosion Assessment, Watershed Studies, Pilot Programs inspection and maintenance, Revised Stormwater Criteria by updating City Drainage Ordinance and Stormwater Criteria Manual, and to consider/evaluate participation in FEMA's Community Rating System. Open Space Open space Identified and ranked five conceptual regional detention locations Drainage drainage across Kirkwood Branch, Dove Creek, and Big Bear Creek 2023 Enhancement concepts watersheds; Area 4 (Timarron Creek/Continental Boulevard) ranked Study highest based on flood benefit, CIP alignment, and open space criteria. Stormwater Permit-term Establishes BMP commitments and measurable goals for the 2025 Management BMPs/goals City's MS4 program under TPDES General Permit TXR040000 for Plan - Permit permit term IV (2025-2029); serves as the City's active compliance Term IV framework under TCEQ authority. This master plan does not reproduce individual technical reports. Instead, it organizes and applies the findings from these efforts to support policy direction, program alignment, and future investment decisions. It also identifies where additional or updated analysis will provide the most value as conditions change over time. The City's experience across these studies shows that stormwater challenges are often localized and influenced by site-specific conditions such as drainage patterns, infrastructure constraints, channel meandering, or maintenance needs. With a strong foundation of existing data and tools,the City is positioned to consistently evaluate these conditions, determine appropriate responses, and prioritize improvements in a way that supports system performance and community expectations. VSn !!! 3 AN* ism --kL --- — d' IV- CHAPTER2: SYSTEM AND WATERSHED OVERVIEW Southlake is a substantially developed community. Most of the land is covered by rooftops, pavement, and maintained turf, and the natural landscape that once absorbed rainfall has largely given way to surfaces that move water quickly. When it rains, that water has to go somewhere. Understanding where it goes, how the City's stormwater system manages it, and why certain outcomes occur is the foundation for every stormwater decision Southlake makes. Stormwater is not confined to a single street or neighborhood.It moves along connected drainageways that cross subdivisions, road corridors, and jurisdictional boundaries before discharging to receiving waters. This means stormwater outcomes in Southlake are influenced by conditions both inside and outside the City's direct control, and decisions made in one part of the community can affect conditions in another. A watershed-based planning approach accounts for these connections, allowing the City to evaluate stormwater behavior at the right scale, identify where cumulative impacts concentrate, and sequence studies and investments based on documented risk rather than isolated symptoms. This chapter provides the context for a consistent public message: Southlake's stormwater system can perform well overall while still experiencing localized issues that residents notice such as temporary street ponding, nuisance flooding in low areas, overtopping at select crossings during severe events, or erosion in certain channels and outfalls. Citywide analyses and targeted watershed studies help distinguish between issues driven by public infrastructure capacity, maintenance conditions, natural exceedance behavior, and private drainage responsibilities. For example, citywide modeling and hotspot screening have been used to map flood depth patterns and identify locations where follow-up investigation or improvements may be warranted, while more detailed corridor studies incorporate storm drain infrastructure, crossings, and complex overflow areas to evaluate alternatives. By grounding this chapter in the best available data and study findings, the Plan documents how Southlake understands its watersheds today and how the City will use that understanding to guide maintenance, study cycles, ordinance tools, and future investments. HOW STORMWATER WORKS IN SOUTHLAKE Stormwater is the rainwater that falls on homes, streets, parking lots, sidewalks, lawns, and open space, then runs downhill to the nearest low point. In a natural landscape, much of that water infiltrates into soil or is slowed by vegetation before reaching a creek. In a developed community, 12 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN - — — — — — — — — — — — — — — • Where It Flows, - Goes When it rains,snows,or sleets,water hits hard surfaces and takes anything on that surface with it,through drains, pipes,and ditches to local rivers, lakes,and streams. Cigarette butts in the gutter become a real problem when they reach wildlife. Trash left on sidewalks _ T T T gets swept into sewers and contaminates Pet waste v hes U stormwater. ' pollutes parks and washes down I �I 1 into pipes,and lea�ed from cars and w. recreation areas drains and into downstream. waterways. Lawn fertilizer and pesticides can _6rwashing wash across pavement and down chemicals can harm drains,taking toxics;with them. fish and animals. D 91 Where • ■ Flows, Everything Goes 1100EPA Figure 2-1:EPA-Stormwater Flow Infographic roofs, pavement, compacted soils, and curb-and-gutter streets reduce infiltration and move water quickly. Runoff can also increase following extended dry periods, when soils become hardened and less able to absorb water, causing rainfall to run off more rapidly than it would under normal moisture conditions. Where terrain is steeper, water reaches drainage channels faster and with greater force, which can intensify peak flows and increase erosion risk in those corridors. That speed matters. Faster runoff increases peak flow, which influences street ponding, culvert performance, channel stability, and downstream water quality. Southlake's stormwater system is best understood as a combination of: • Surface drainage (streets and overland flow paths), • Underground conveyance (inlets, pipes, junction structures, culverts), • Open channels and creek corridors (natural or improved drainageways), • Storage and controls (detention/retention and outlet controls where present), and • Programs and practices (maintenance, inspections, regulatory compliance, public education, and customer response). When rainfall intensity exceeds what local inlets and pipes can capture quickly, water uses surface pathways. That is why some street ponding is normal during intense rainfall even in a well-functioning system. The key is whether ponding occurs in a way that creates life safety risk, disrupts emergency access, damages public infrastructure, or increases risk to insurable structures. .10 40 H ............ .._ . 0017 Taffant County r.4&.1F,11 P, miff 2Kim r MAl 1. SmllI SEMNI4,mmI111111 Ill mom""Aififfifflfi — • IN 11 Rill]&mom H *All WIN.1&1 mommoon, E �■Il�l�lriiiii�.� ':%��r.� � +O E �� - r��i�i"■��_�''� � __ _?'� ll en■ .'.■: 1 �� n41ir L......111i:::�••.I�w: 'r �-■' �•'::=�'� ::: - '':..If _ ■ ism VMS." 'ME oil "ON .112, MIMI 9 •00 .11 1 . 1 14 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN WATERSHEDS, SUB-BASINS, AND JURISDICTIONAL CONTEXT Southlake's stormwater system does not operate in isolation. Runoff generated within the City ultimately leaves through connected creek systems and receiving waters that are part of larger regional basins. Water does not recognize jurisdictional boundaries, and it does not stop at the edge of a subdivision or roadway corridor. Stormwater outcomes in Southlake are therefore influenced by upstream conditions (including runoff volumes, timing, and sediment loads entering the City from adjacent communities) and by downstream constraints such as channel capacity, tailwater conditions, and floodplain conveyance that can limit the effectiveness of local improvements. Likewise,decisions made within Southlake,whether related to development,redevelopment,roadway projects, maintenance practices, or capital improvements, can influence conditions beyond any single neighborhood. PoficV Statement A watershed-based framework provides the structure WS1. Ground watershed-based needed to understand these cause-and-effect relationships planning and stormwater and to avoid isolated solutions that reduce a symptom in investment decisions in technical one location while unintentionally shifting risk elsewhere. studies that account for upstream Sub-basins are a practical way to break the community and downstream impacts and into manageable planning units that align with how overall system behavior. runoff actually flows. This supports consistent evaluation of stormwater concerns at the right scale: some issues are localized and can be addressed through targeted maintenance or a small improvement, while others are driven by basin-wide conditions such as downstream constrictions, cumulative impervious cover effects, or channel stability trends that require broader study and coordination. For that reason, stormwater investment decisions in Southlake are grounded in technical studies that account for upstream and downstream impacts and overall system behavior, not just the location where a symptom is visible. This is the foundation of Policy Statement WS1 and the organizing principle for how the City sequences studies, evaluates improvements, and communicates with residents about what the data shows and what it supports. Watershed mapping also creates consistency in how the City prioritizes work and communicates with residents. It organizes study updates and supports a repeatable cycle for sequencing watershed work by risk and community priority. It clarifies where cumulative impacts concentrate, especially as redevelopment changes runoff patterns in incremental but compounding ways over time. It helps residents understand that drainage is connected and basin-wide, and it provides a clear visual tool for explaining why an issue observed at one location may be influenced by conditions upstream or downstream. Southlake shares creek corridors and drainage basins with neighboring communities including Grapevine, Keller, Colleyville, and Westlake. Where those corridors cross municipal boundaries, watershed mapping supports the regional coordination needed to evaluate shared constraints and pursue solutions that are effective at the basin scale rather than just within city limits. Southlake's MS4 program, the City's required stormwater pollution prevention program for its Municipal Separate Storm Sewer System (MS4),further anchors this watershed context by identifying receiving waters and reinforcing that runoff from across the community ultimately discharges to those waters. This is why public education, construction controls, illicit discharge prevention, post- construction controls, and municipal good housekeeping practices are essential components of the City's stormwater approach, not separate tasks. Watershed-based planning is the unifying structure that connects physical drainage behavior, infrastructure performance, cumulative development impacts, regulatory stewardship, and clear public communication into one consistent program framework. NATURAL DRAINAGE SYSTEMS Southlake's natural drainage system, including its creeks, tributaries, drainageways, floodplains, and low-lying storage areas, provides the foundational framework within which the built stormwater system operates. During typical rainfall events, roads, inlets, and pipes manage most runoff in developed areas. As storms become larger or more intense, the natural drainage network becomes increasingly important because it provides the primary pathways and storage areas for flows that exceed the capacity of underground systems. The natural system is not separate from stormwater management; it is an essential part of how the community safely conveys, stores, and releases stormwater during higher-volume events. Creek corridors also serve as the primary receiving point for outfalls and local drainage systems across multiple neighborhoods, which means their stability, conveyance, and maintenance condition can directly influence whether localized drainage issues persist or improve over time. Creeks and drainageways convey storm flows downstream, and floodplains provide space for water to spread out when channel capacity is exceeded. This spreading is a normal hydraulic function that reduces velocities and provides temporary storage, which can help limit the severity of downstream peak flows. Where wetlands or low areas exist, they can offer additional benefits such as temporary storage, sediment settling, and pollutant filtering, particularly when they remain connected to natural drainage pathways. Over time, the ability of natural drainage features to function as intended can be affected by encroachments, erosion, sediment deposition, vegetation changes, and modifications to outfalls or crossings. Because of that, the City treats creek corridors and floodplain function as long- term assets that require proactive stewardship and clear standards for how adjacent development and maintenance activities occur. Floodplains are often misunderstood as evidence of system failure, when in reality they exist because water has always needed a place to go during large storms. A floodplain is not a design flaw; it is the area that naturally carries and stores floodwater when a channel overtops. Responsible stormwater management does not attempt to eliminate floodplain behavior. Instead, it manages risk by: 1. Ensuring development in flood hazard areas is designed and regulated appropriately; 2. Protecting conveyance so risk is not increased by fill, structures, obstructions, or other encroachments; 3. Maintaining creek corridors so debris, obstructions, and erosion do not create avoidable hazards or reduce conveyance; and 4. Communicating clearly how flood risk is defined using objective federal standards. This approach supports transparent decision-making and avoids creating a false expectation that any flooding within a mapped floodplain is evidence of system deficiency. It also reinforces that floodplain management is fundamentally about long-term risk reduction and protecting people, property, and infrastructure through consistent, enforceable standards. Floodplain administration also connects directly to FEMA (Federal Emergency Management Agency) and NFIP (National Flood Insurance Program) requirements that define key compliance terms and triggers. One of the most important examples is substantial damage, which FEMA defines as damage where the cost to restore a building to its pre-damage condition equals or exceeds 50 percent of the building's market value before the damage occurred. This standard is used to determine when floodplain compliance upgrades are required and provides an objective, consistent basis for floodplain regulation and risk communication separate from perceptions or sentiment about the severity of an event. FEMA flood insurance records show 14 total claims filed in Southlake over the past 45 years, a figure that reflects limited structure-level flood exposure consistent with the community's generally low incidence of residential and commercial structure flooding. 16 STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN Using these federal definitions helps the City be factual and consistent in how it discusses building risk and compliance expectations, while still recognizing that nuisance flooding and disruption can be significant quality-of-life issues that warrant appropriate response through maintenance, studies, education, and targeted improvements. (FEMA Desk Reference: Substantial Improvement/ Substantial Damage Desk Reference FEMA P-7581May2010) (https.//www.fema.gov/sites/default/ files/do cum ents/fema nfip_substantial-improvement-subs tan tial-damage-desk-reference.odt) BUILT STORMWATER SYSTEM Southlake's built stormwater system is the engineered network that works in tandem with the natural drainage system to collect, - convey, and manage runoff from developed - areas. It includes the structures that capture = water from streets and public spaces, the underground and open conveyanceelements that that move flow downstream, and the crossings and controls that govern - - • how water passes through constrained •�=' ' locations. In practical terms, this system is _ what residents most often associate with y' "storm drainage" the inlets along curbs, the - pipes beneath streets, the culverts under roadways, and the visible outfalls and channels where runoff is discharged and conveyed. Because these assets are distributed citywide and are interconnected, system performance is not determined by any single component. It is a chain: if the inlet is blocked, the pipe can't receive flow; if the pipe discharges to an eroding outfall, the outfall can fail and undermine upstream performance; if a downstream crossing is constrained, upstream water levels can rise even when upstream pipes are functioning properly. This is why the master plan treats the built system as an integrated network rather than a collection of isolated parts. The built stormwater infrastructure includes: 1. Inlets and capture structures such as curb inlets, grate inlets, and catch basins that intercept runoff from streets and convey it into the storm drain network; 2. Storm drains/pipes and junction structures that transport runoff beneath streets and through developed areas; 3. Culverts and bridge openings where drainageways pass under roads and where hydraulics are often most sensitive to overtopping risk, downstream tailwater, and debris accumulation; 4. Outfalls and energy dissipation features that control discharge and reduce erosive velocities where pipes connect to channels; 5. Improved channels in select corridors that convey runoff more efficiently or protect infrastructure where natural channels are vulnerable; and 6. Detention/retention and outlet controls where provided to reduce peak flows, manage release rates, and protect downstream conditions. Many of these features are out of sight most of the time, which is part of why stormwater systems can be misunderstood: the system may function well for years, but performance can change quickly when a storm reveals a weak link or when long-term degradation (sedimentation, erosion, structural aging) reaches a tipping point. The built system is designed to reduce flood risk under defined storm conditions, but performance depends not only on original design and capacity, it depends on how the system is maintained and how watershed conditions evolve over time. Even well-designed systems can experience reduced effectiveness due to debris and sediment accumulation that blocks inlets, fills catch basins, reduces pipe conveyance, or creates localized backwater conditions. Long-term condition of pipes and culverts matters because deterioration can reduce structural integrity, allow infiltration/exfiltration, create settlement or voids, and ultimately lead to failure or collapse often at the worst time, during major events. Channel stability and vegetative conditions influence how effectively the system can discharge and convey water; an eroding channel can undermine outfalls and crossings, while excessive obstruction can reduce capacity. Downstream tailwater effects can cause upstream systems to back up, meaning a constraint far downstream can create flooding symptoms upstream even when upstream infrastructure appears adequate. Finally, development and redevelopment causing incremental imperviousness can change runoff timing and volume; small changes across a basin can accumulate over years and shift performance outcomes, especially in short-duration intense storms that stress local capture and conveyance. These realities reinforce a central theme of this Plan: stormwater infrastructure and stormwater programs are inseparable. Maintenance and asset management are not "extras" they are what preserve the function and reliability of the built system over time. A citywide network of inlets, pipes, culverts, and channels will only perform as intended if the City sustains proactive inspection and cleaning practices, tracks condition and criticality, renews aging assets before failure, and maintains clear documentation of where the system is performing well and where it is vulnerable. In the same way, development standards, construction controls, and post-construction maintenance expectations are part of system performance because they determine whether new or redeveloped areas add avoidable burden to the system. For Southlake, this master plan's role is to connect these elements: built infrastructure, maintenance discipline, study updates, standards, and clear communication into one consistent framework that supports reliable performance today and protects that performance as the community continues to reinvest over time. ROADWAY CROSSINGS AND LOW-WATER CROSSINGS Roadway crossings and low-water crossings are among the most critical components of Southlake's stormwater system because " they sit at the intersection of infrastructure performance and immediate public safety. Unlike most stormwater assets,which are out of sight and only noticed when something goes wrong, crossings are highly visible and directly experienced during storm events. When water rises to the roadway surface or overtops a crossing, the risk to motorists increases quickly, especially at night or when flow is moving fast enough to push vehicles off the roadway. Overtopping can also restrict emergency access, disrupt normal traffic circulation, and temporarily isolate neighborhoods or corridors until water recedes and debris is cleared. These impacts can occur even when building flooding is not present, which is why the City treats roadway crossings as high-consequence assets that warrant special attention in levels of service, monitoring protocols, and reinvestment planning. 18 STORMWATER MASTER PLAN I SOUTHILAKE • STORMWATER MASTER PLAN Crossings are also hydraulically sensitive locations because they are natural pinch points in the drainage system. Creeks and drainageways often narrow at road embankments, and the crossing opening (culvert or bridge) becomes the controlling feature that governs upstream water levels during large events. Crossing performance is influenced by many factors beyond the size of the opening, including upstream drainage area, downstream tailwater conditions, debris loading, sedimentation, and channel stability upstream and downstream. Even a crossing that performs well most of the time can become vulnerable if debris partially blocks the opening, if sediment deposition reduces conveyance, or if channel erosion undermines inlets/outlets and changes flow patterns. In addition to overtopping risk, crossings can become drivers of long-term maintenance and capital needs because they can accelerate channel erosion at outlets, create scour and undermining around structures, and lead to repeated repairs if instability is not addressed at the source. Southlake's past studies provide the evidence base for understanding and managing these risks. The City's technical study library, shown in Chapter 1, includes structure-focused evaluations, crossing-specific hydraulic analyses, and citywide screening tools that have helped identify and screen crossings and major structures that may be vulnerable based on hydraulic performance, overtopping potential, and system role. Regional mapping and technical support documentation also helps establish the broader watershed and floodplain context that influences crossing behavior and informs how the City communicates flood risk objectively. Together, these resources support a consistent and defensible approach to crossing decisions: 1. Identify crossings that are higher consequence due to safety and access impacts; 2. Understand the factors contributing to vulnerability (capacity, tailwater, debris, erosion, approach grades); 3. Evaluate improvement concepts where feasible; and 4. Prioritize actions based on life safety first, then system resilience and continuity of access, and then property protection as permitted by law. Operational readiness is just as important as structural evaluation because even robust infrastructure can be exceeded during extreme Policy events. The City maintains clear procedures for storm monitoring, including identifying SP1. Convey, store, and manage stormwater locations most susceptible to overtopping, to protectsafety, • establishing trigger points for closures, and property • - and coordinating barricade placement and reduction and sound system performance public alerts. A predictable operational plan • reduces risk during an event and supports stronger post-event documentation L_ - capturing what overtopped, for how long, and under what conditions — so the City can determine whether the appropriate response is routine maintenance, targeted inspection, a study update, or a capital planning consideration. Roadway closures and barricades are safety actions taken to protect motorists and emergency responders. They are not an indication that the City is ignoring the issue. When backed by documented monitoring and consistent follow-up practices, this approach directly supports Policy Statement SP1: convey, store, and manage stormwater to protect life safety, public infrastructure, and property through proactive risk reduction and sound system performance practices. REGIONAL FACILITIES AND INTER-JURISDICTIONAL COORDINATION As noted earlier in this chapter, Southlake's watersheds are shared with neighboring communities, and conditions upstream and downstream of city limits can directly influence local stormwater outcomes. Regional coordination is how the City manages that reality in a structured, productive way. It becomes especially important when downstream constraints limit the benefit of local improvements, when shared creek corridors cross jurisdictions, or when infrastructure decisions in one community influence risk in another. Watershed-based planning provides the structure needed to evaluate those relationships and pursue solutions that are effective at the basin scale rather than only within city limits. Regional coordination also supports consistency in how flood risk is understood and communicated. Floodplain mapping is a practical example. FEMA Risk MAP updates and Cooperating Technical Partner (CTP) materials document the datasets, methods, and mapping processes used to define flood hazard areas. When floodplain mapping is updated regionally, it can affect how the City administers development in flood hazard areas, communicates risk to residents, and evaluates whether projects require map revision processes such as a CLOMR or LOMR. Consistent mapping inputs and assumptions reduce confusion when residents compare flood risk information across neighboring jurisdictions. Consistent regional criteria and standards, such as integrated stormwater management practices and aligned drainage requirements, also help ensure that redevelopment and infrastructure decisions across a shared watershed are working toward the same outcomes rather than at cross purposes. Regional partnerships create practical implementation benefits as well. Many grant programs and external funding opportunities are more competitive when they demonstrate basin-wide benefit or documented multi-jurisdictional coordination. Shared models, shared mapping updates, and shared understanding of constraints can reduce duplication and improve technical analysis. Regional coordination also provides a pathway for addressing locations where a local stormwater issue is driven by conditions upstream or downstream that are outside the City's sole control. In those cases, coordination allows the City to evaluate the problem accurately, identify feasible options, and pursue improvements or studies with the appropriate partners rather than investing in isolated fixes that do not resolve the underlying constraint. KNOWN CONSTRAINTS AND PROBLEM AREAS Stormwater concerns tend to concentrate in predictable places because stormwater behavior is shaped by topography, drainage constraints, and how the system has evolved over time. Constrained culverts and bridges, channels with stability issues or constrictions, low areas where surface flow naturally converges, and locations with high debris loading are common drivers of recurring service requests in most communities. In Southlake, these conditions can be further influenced by local development patterns and private property factors such as fence lines, grading modifications, privately maintained swales or channels, and HOA detention and outlet controls that may not be maintained consistently. These conditions do not necessarily mean the citywide system is inadequate. They reflect that stormwater is highly location-specific and that a small number of constrained locations can create disproportionate community concern during intense storms. A central purpose of this Plan is to ensure that recurring issues are handled consistently and transparently over time. That begins with a documented inventory of known concern locations grounded in objective information, not only anecdotal reports. This inventory should be built and maintained using multiple inputs: resident requests and repeat calls, field observations and maintenance history, storm event documentation including road closures, overtopping observations, and post-storm inspections,and watershed study findings and mapping products. Citywide modeling • STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN k"L° a Grapevine. ° Kirkwood C O ! Branch +.- . C 0 Unnamed 0� t V Tributary to + y� ? Kirf kwood Branch O South Fork 6 0 C ; no -..-.. k Kirkwood , C j 0� Branchlll' r ^Dove Cr ee k 0 k CC 0 Ct' 1 H ginsC' C C� �J^ ; ` ` CB ranch � :. ' O C C C'" 0 C 0 s 9 West Jones C �: - O Branch ` O O 0 y 4ones , Branch Tributary Tributary C CC » C BB-9 0 BB-8cc S ,r - O Cc" 0.�.� _O Timarron :: n 1 n Tributary Creek -, i" ` Unnamed ' ` ri `C so r ILTributary 4 C 0. BB-7 0 On 0 C t W r I C C Tributary. a f To Big Bear C k � r C a s Watershed Study Basin Prioritization: Flood Risk Priority Raw Data Points µ Southlake Stormwater Master Plan September 2021 -A VO # 42929 Legend SOUTHLAKE ■■■ "t' j = Southlake City Limits ■■■ HALFF © ... r ,,,.,.,�- .•,� -' � Major Watersheds Notes: 1) Imagery Source: Nearmap Emergency Response Locations 2) Green hatched watersheds indicate local i, d basins recommended for future detailed 2D analysis as needed. ,. Low Water Crossings 0 0.5 1 2 = Miles ,_ - Drainage Complaints Figure 2-3:Halff Associates Inc.Recommended Watershed Basin Prioritization and Complaint Origins from 2016-2021 and hotspot screening tools can further support this by identifying areas where flood depths or overflow pathways cluster, providing an additional data layer to validate what residents experience and to reveal emerging risks that may not yet be reflected in complaint history. Maintaining this inventory serves several essential functions. It creates continuity so staff can see what has happened at a location over time, what maintenance actions were taken, and whether a problem is repeat or worsening. It supports strategic prioritization by allowing the City to distinguish between issues driven by public infrastructure capacity, maintenance conditions, natural exceedance behavior, or private property responsibilities. Finally, it supports transparency and public trust. Residents should be able to see that the City tracks issues, applies consistent decision criteria, and can explain why certain locations are prioritized for action and why others may require private maintenance or longer-term evaluation rather than immediate capital investment. REDEVELOPMENT AND IMPERVIOUS COVER TRENDS As Southlake approaches buildout, stormwater drivers increasingly shift from large-scale new development to redevelopment, infill, and incremental changes that occur parcel by parcel over time. In a mature community, stormwater system performance is less influenced by the creation of entirely new drainage basins and more influenced by how existing basins evolve through tear-down/ rebuild projects, commercial reinvestment, site reconfiguration, small expansions, and ongoing property improvements. This transition matters because the stormwater impacts of redevelopment are often less obvious to the public than greenfield growth, yet the cumulative effect can be just as significant over time. Redevelopment changes how rainfall interacts with the built environment: it can add impervious area, increase the efficiency of runoff conveyance toward streets or inlets, and reduce the amount of land that historically absorbed or slowed water. It can also alter the timing of runoff, producing sharper peaks that stress local capture systems during short-duration intense storms, exactly the kinds of storms that generate highly visible street ponding and neighborhood complaints. Redevelopment can increase effective imperviousness even when total impervious area oficy Statement appears similar. For example, a site may maintain . a similar amount of paved area but change DR1. Continue to administer and enforce grading, curb alignments, drainage patterns, and stormwater regulations requiring new surface connectivity in ways that move runoff developmentredevelopment more quickly to inlets and pipes. Redevelopment demonstrateresponsible management can also alter overland flow paths, sometimes impacts under in unintended ways by raising a portion of a lot, consistent with applicable state law and adding walls or fences, reconfiguring driveways, City ordinances, including upstream or changing how water exits a property. These and _ changes can shift where water concentrates applicable. during storms, potentially creating new localized issues or increasing pressure on downstream inlets and crossings. Pofic)t Statement Redevelopment can also increase runoff volume DR3. Explore ordinancedevelopment and peak flow during short-duration storms creditapproaches ' - rvious because newer surfaces are often more connected coverageand runoff mitigationreduce and less porous than older site configurations. cumulative impacts ' ver time, while TThe City's existing drainage ordinance already maintaining fairness andadministrative addresses these concerns by requiring that stormwater runoff from redevelopment not 22 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN ,x r E RM ANN - x ZEE ��.�='` exceed pre-development conditions and by allowing the City to require retention or detention when actual development generates runoff beyond what was anticipated (City of Southlake Code of Ordinances, Chapter 9.5, Sections 9.5-64(d) and 9.5-190). These tools are not about preventing reinvestment; they are about ensuring that reinvestment does not unintentionally increase risk for neighbors or create long-term burdens for the public system. Policy Statements DR1 and DR3 reflect this direction: DRl continues the City's established expectation that development and redevelopment demonstrate responsible management of stormwater impacts under buildout conditions, and DR3 supports exploring ordinance and incentive approaches that reduce cumulative impacts over time while maintaining fairness and administrative clarity. A key challenge with redevelopment is that stormwater impacts tend to be incremental, cumulative, and distributed. One project may not appear to change basin behavior on its own, but over a decade, dozens of projects in a sub-basin can meaningfully shift runoff patterns, peak timing, and downstream channel stability. This reality supports the Plan's policy direction to treat buildout as the correct long-term planning condition for stormwater decisions, which is already established in the City's drainage ordinance through its requirement that all runoff calculations be based on ultimate conditions of development (City of Southlake Code of Ordinances, Chapter 9.5, Section 9.5-22). The City has applied this standard consistently, and DRl continues that expectation by requiring development and redevelopment to demonstrate responsibly managed stormwater impacts under buildout conditions, including upstream and downstream considerations where applicable. DR3 builds on this foundation by supporting the exploration of ordinance and incentive approaches that reduce cumulative impacts over time while maintaining fairness and administrative clarity. These tools are not about preventing reinvestment; they are about ensuring reinvestment does not unintentionally increase risk to neighbors or create long-term burdens for the public system. When standards and incentives are structured clearly and applied consistently, they help the City protect long-term system performance while providing predictable expectations for property owners and developers. PUBLIC VS. PRIVATE RESPONSIBILITIES One of the most common sources of confusion and frustration residents experience with stormwater is not knowing who is responsible for a drainage risk or concern. That frustration is understandable. Drainage issues often occur near property lines, along shared corridors, or in areas that are difficult to identify as clearly public or private. The boundaries between public and private stormwater responsibility in Southlake are grounded in the Texas Constitution, state law, City ordinance, and development agreements that have been in place for decades. A consistent stormwater program requires clear, repeatable definitions of responsibility both to manage public risk effectively and to communicate with residents fairly. What the City is responsible for. Southlake's role is to maintain and improve public stormwater infrastructure within public rights-of-way and publicly dedicated drainage easements, to manage stormwater planning and capital improvements, and to administer required regulatory programs including MS4 stormwater quality requirements and floodplain administration. This authority is defined by what the City owns, what has been dedicated to the public through plat or easement, and what the City is legally empowered to maintain using public funds (City of Southlake Code of Ordinances, Chapter 9.5). Where the City has legal authority and documented need, it acts. That commitment is consistent and enforceable. What property owners and HOAs are responsible for. Private property owners and HOAs are responsible for maintaining drainage features on their property or within private easements, including yard drains, private swales, privately maintained channels, and detention facilities and outlet controls established as part of a development. They are also responsible for ensuring drainage-related activities comply with City ordinances and permitting requirements, including prohibitions on unpermitted fill, grading, encroachments into drainage easements, and modifications to stormwater facilities without City approval. This responsibility is grounded in Texas Water Code Section 11.086, which establishes liability for anyone who impounds, alters, or diverts the natural flow of surface waters in a manner that damages other properties, and in City of Southlake Code of Ordinances, Chapter 9.5. For many Southlake developments, it is further defined through Residential Developer's Agreements, which assign maintenance of ponds and drainage facilities to the developer and subsequently to the HOA. r -1` 24 „ • E STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE STORMWATER MASTER PLAN For developments approved after 2023, stormwater facility maintenance agreements are required under Chapter 9.5, Section 9.5-191, providing an additional documented record of that obligation. These private systems are not City assets. In many Southlake neighborhoods, HOA- maintained detention ponds, outlet structures, and drainage channels play a significant role in how stormwater is managed, and their condition directly affects how well the overall drainage system performs for surrounding properties. Private maintenance is not a secondary concern; it is a core part of how the system is intended to function. The confusion residents experience is understandable. Many stormwater concerns reported to the City are driven at least in part by private maintenance conditions: clogged yard drains, blocked swales, modified grading, fences or landscaping that disrupt historic flow paths, unpermitted encroachments into drainage easements, HOA channels that have accumulated debris, or detention outlet controls that have become partially blocked over time. These conditions can produce the same visible symptoms as a public infrastructure problem — water in a yard, slow drainage after storms, or recurring ponding in a low area — without the City having authority or obligation to resolve them. Texas law prohibits the expenditure of municipal funds for improvements to private property, as doing so would constitute a gift of public funds in violation of the Texas Constitution, Article III, Sections 52 and 44. The Texas Supreme Court has established that the purpose of any public expenditure must be to accomplish a public purpose rather than benefit a private party, that the entity must retain public control over the fund, and that the entity must receive a return benefit. This is not an abdication of responsibility; it is a legally required and fiscally appropriate boundary that protects all residents equitably. Texas stormwater law, rooted in the Texas Supreme Court's decision in Kraft v. Lankford and codified in Texas Water Code Section 11.086, also establishes that lower properties should anticipate receiving flows from upstream properties, and that liability for stormwater damage arises when someone impounds, alters, or diverts the natural flow of surface waters in a manner that damages other properties. When these conditions persist without clear communication, residents may understandably assume the City should fix the drainage, even when the City does not have ownership authority or legal responsibility for the affected feature. Getting that communication right is a core program obligation, not an afterthought. The goal of this Plan is to get ahead of that frustration through proactive education, plain-language guidance materials for residents and HOAs, and consistent staff responses that clearly identify whether a concern involves public infrastructure, private maintenance, or a combination of both. The best time to communicate responsibilities is before a problem becomes a crisis, through HOA outreach, guidance materials, and consistent messaging during development and redevelopment review when post-construction controls are first established. When a complaint is received, staff will document the issue, identify the responsible party, explain what the City can and cannot do, and provide a clear path forward including what the private party needs to do and what follow-up will occur. Education remains the first step, but enforcement must remain available as a targeted backstop when private failures create hazards or violate ordinances. PolicK Statement This approach supports both public safety and AM3. Clearly define public versus fairness: the City maintains what it owns and is private stormwater responsibilities and responsible for, while private owners and HOAs strengthen how the City communicates, maintain what they own, especially when private supports, and verifies private systems influence neighbors and public outcomes. maintenance obligations where those Policy Statement AM3 reflects this commitment: facilities influence public risk. the City will clearly define public versus private stormwater responsibilities and strengthen how those obligations are communicated, supported, and verified over time. ♦�► f WON ANCHAPTER 3: - - HYDROLOGY, HYDRAULICS, AND LEVEL OF SERVICE Stormwater decisions are often confusing to the public because the system's performance is event-driven and because no two storms are alike. The same intersection that drains quickly after a moderate rainfall may pond for hours after a short, intense burst. A culvert that performs reliably for years can become a chokepoint when debris accumulates before a storm. A creek that stays within its banks during a ten-year event may overtop its corridor during a storm twice that size. Chapter 3 bridges that gap between technical evaluation and public understanding, explaining the foundational concepts the City uses to assess Stormwater behavior and communicate performance expectations in a consistent, factual way, without turning the plan into an engineering report. It explains how storm events are measured and compared, how rainfall becomes runoff and why that process changes as land cover and drainage patterns evolve, and how the physical characteristics of pipes, culverts, channels, and crossings determine whether water stays within the system or spills into streets and floodplains. These concepts matter because they are what the City uses to evaluate whether a flooding concern reflects a system that is undersized, a maintenance condition that has reduced effective capacity such as a blocked inlet or a partially silted culvert, a storm that exceeded what the infrastructure was designed to manage, or a private drainage issue that falls outside the City's maintenance authority. Without this shared baseline, it is difficult to give residents accurate, consistent answers about what happened during a storm and what the appropriate response is. This chapter also establishes the framework for levels of service and risk management, which are among the most important concepts in the plan. Stormwater systems are not designed to eliminate all flooding under all storms. They are planned and maintained to reduce risk, protect life safety, preserve emergency access,and manage infrastructure performance under defined storm conditions, with the understanding that exceedance can and will occur during extreme events. By establishing these expectations clearly, the City can communicate more consistently about what the stormwater system is intended to do, why certain outcomes occur even when the system is functioning as designed, and how technical evaluation guides maintenance, study updates, standards, and long- term investment decisions. DESIGN STORMS AND INTENSITY TRENDS Stormwater systems are evaluated using rainfall statistics that describe the intensity, duration, and frequency of storm events. For decades, these events have been described using shorthand terms 26 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN like the "2-year storm," 10-year storm," or 100-year storm." These labels refer to probability, not prediction. A 100-year storm does not mean a storm that happens once every hundred years; it means a storm that has a 1 percent chance of occurring in any given year. That distinction matters because it is widely misunderstood, and the misunderstanding can create unrealistic expectations about flooding risk. In fact, a community can experience multiple 100-year storms within a short span of years, and that possibility increases as rainfall patterns shift over time. The terminology itself is also evolving. Federal agencies including FEMA have moved toward annual chance and annual exceedance probability language to better communicate what these statistics actually mean. Under this framework, what was called a 100-year storm" is now more accurately described as a 1 percent annual chance event, meaning there is a 1 percent chance it will be equaled or exceeded in any given year. This probability-based measure is referred to as Annual Chance Exceedance, or ACE, and that term will appear in technical references, study documents, and the rainfall table on this page. A 10-year storm" becomes a 10 percent annual chance event. This plan uses both terms because residents and technical references still use the older language, but the annual chance framing is more precise and is the direction the profession is moving. The goal in either case is the same: to compare storm events on a consistent, probability-based scale so infrastructure can be designed, evaluated, and communicated about accurately. The table to the right illustrates what these storm Precipitation Frequencies - 24-Hour Duration frequencies mean in practical rainfall terms for Recurrence Interval Depth (inches) Southlake, based on NOAA Atlas 14 data. In Texas, NOAA Atlas 14 precipitation frequency data is 50% ACE (2-Year) 3.9 the current standard reference for rainfall depths 10% ACE (10-Year) 5.8 across different storm durations and frequencies. 1% (100-Year) 9.1 Atlas 14 matters because rainfall assumptions drive Figure 3-1:PDS-based precipitation frequency estimates with nearly every conclusion in a stormwater evaluation: 90%confidence intervals estimated runoff volumes and peak flows, predicted water surface elevations at crossings, expected ponding depths at low points, and the mapped extents of flooding in model outputs. When the City and its consultants use the same rainfall reference across studies, performance comparisons between watersheds, corridors, and projects are grounded in fact rather than subjective impressions of whether a storm was large. The table included in this section shows representative 24-hour rainfall depths for Southlake under selected storm frequencies, illustrating how significantly rainfall intensity increases as events become rarer. This chapter reinforces a consistent message that is important for public understanding: no practical stormwater system can prevent all flooding under all possible storms. Even well-designed systems can be exceeded during rare, high-intensity events, particularly short-duration storms that overwhelm local capture capacity and create rapid street ponding before runoff can fully enter inlets and pipes. Responsible stormwater management focuses on risk reduction, life safety, infrastructure protection, and defined levels of service, while maintaining clear pathways for water to move safely when storms exceed design conditions. HYDROLOGIC METHODS: RUNOFF DRIVERS AND ASSUMPTIONS Hydrologic methods estimate how rainfall becomes runoff — how much water reaches the stormwater system and how quickly it arrives. These methods rely on assumptions about soils, land cover, slopes, drainage connectivity, and storage features such as detention basins or natural low areas. Those assumptions matter because they drive the calculated peak flows and volumes that determine whether pipes, culverts, and channels are predicted to perform adequately under defined storm events. In practice, hydrology is where development and redevelopment patterns become stormwater drivers: when land cover changes, runoff changes. A storm event that produced limited runoff decades ago may produce higher peaks today if impervious cover has increased and flow paths have become more connected and efficient. A central driver is imperviousness, but its is not just the total amount of pavement or roof area. It is also effective imperviousness '` — meaning how directly runoff is connected to the drainage system. Redevelopment can increase effective imperviousness by creating more direct direct pathways to streets, inlets, and pipes, reducing infiltration, and shortening the time it takes water to concentrate and reach a low point. Soil type also plays a significant role. More clayey soils absorb less water and generate more runoff than sandy soils. This is directly relevant in Southlake, where expansive clay soils are common. Those soils can become hard and less permeable during dry periods, causing rainfall to run off more rapidly than it would under normal moisture conditions. When soil type, slope, impervious cover, and drainage connectivity are combined, the result is a picture of how quickly and how much water the stormwater system must handle during a given storm event. For Southlake, this supports two practical expectations for studies and development Policy Statement review. First, the City requires all development - Continue to _ nforce and redevelopment to submit drainage studies _ administer requiring new prepared by a licensed professional engineer,with development redevelopment clear documentation of hydrologic assumptions, demonstrateresponsible so that outputs can be understood, compared, impacts _r full buildout conditions, management and validated over time (City of Southlake consistent with nd Code of Ordinances, Chapter 9.5, Sections 9.5- licable state law " 41 through 9.5-43). Second, those studies must City ordinances, including upstream evaluate both existing conditions and reasonable considerations future conditions including buildout, becauseapplicable. stormwater performance is ultimately shaped by cumulative change across a watershed; the ordinance expressly requires that all runoff calculations be based on ultimate conditions of development (Chapter 9.5, Section 9.5-22). A project that appears manageable in isolation may contribute to downstream problems when combined with dozens of similar changes over time. Where an individual development submittal triggers the need for broader watershed-level evaluation, that analysis is the responsibility of the developer or applicant (Chapter 9.5, Sections 9.5-41 through 9.5-43 and 9.5-122). This directly supports Policy Statement DR1, which continues the requirement that development and redevelopment demonstrate responsibly managed stormwater impacts under buildout conditions, including upstream and downstream considerations where applicable. Consistent assumptions also support fairness: when the same methods are applied across projects and neighborhoods, residents and applicants alike can trust that evaluations reflect comparable standards. 28 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN HYDRAULIC CRITERIA: CAPACITY, TAILWATER, AND FREEBOARD Hydraulic evaluation answers the next question after hydrology: once runoff enters the stormwater system, can the system convey it safely and reliably? Hydraulic performance depends on the capacity and condition of pipes, culverts, channels, and crossings, and on downstream conditions that can limit how efficiently the system discharges. The system behaves as a connected network, so an upstream improvement may not perform as expected if a downstream constraint governs water surface elevations during larger events. Hydraulic evaluation is therefore essential for understanding where constraints concentrate risk and for ensuring that investments reduce that risk rather than shifting it elsewhere. One of the most important hydraulic influences in Southlake, as in many communities, is tailwater. Tailwater occurs when downstream water levels limit discharge and push water back upstream, a condition known as backwater. It can be driven by downstream channel constraints, floodplain elevations, or coincident creek flooding that raises water levels near outfalls and crossings. When tailwater is high, upstream pipes and culverts may be functioning as designed but still experience elevated water levels because the system cannot release water efficiently. This is why localized flooding or roadway overtopping can appear to originate upstream of the actual controlling condition, and why watershed-based evaluation is necessary to understand cause and effect. It is also why some improvement projects must be evaluated not only at the immediate site but in the context of the full downstream corridor. Another critical concept, particularly for roadway crossings, ponds, floodplains, and other key Policy Statement drainage corridors, is freeboard. Freeboard is the vertical clearance between an anticipated WS4. Exploreand water surface elevation and a roadway surface, design flood" structure top, or other critical elevation. It functions areas based ' as a practical safety buffer that accounts for recognizing that risk is not uniform real-world uncertainty factors such as debris the city and - ' - to vary accumulation, sedimentation, model limitations, by context. and storm variability, all of which can cause water to rise higher than a model predicts. In crossing evaluations, freeboard is used to assess whether overtopping is likely under a given storm event and to help determine whether a crossing should be prioritized for improvement, monitored operationally, or managed through closures during rare extreme events. Southlake's technical study library treats freeboard and overtopping frequency as key indicators of public safety risk at crossings and critical drainage structures. These concepts directly support Policy Statement WS4, which recognizes that risk is not uniform across the city and that standards and evaluation depth may need to vary by context, particularly in higher-risk flood areas or where critical assets and emergency access corridors are involved. Clear hydraulic criteria and consistent evaluation practices allow the City to make those determinations transparently and to prioritize improvements in a way that is grounded in documented technical findings. LEVEL OF SERVICE: CLEAR EXPECTATIONS FOR SAFETY, RELIABILITY, AND RESPONSE Levels of service are how the City translates stormwater expectations into practical, repeatable commitments. Without defined levels of service, stormwater performance becomes subjective: one resident expects a street to drain immediately after a storm, while another accepts some temporary ponding as normal. One neighborhood's standard becomes another's complaint. A clear level-of- service framework removes that ambiguity. It does not promise no flooding. Instead, it establishes what outcomes the City is working to deliver, how performance will be evaluated across different locations and contexts, and how staff will communicate consistently when impacts occur. Levels of service are also a transparency tool: they help residents understand the difference between outcomes the system is designed to manage routinely and exceedance outcomes that are expected during larger events even when the system is functioning correctly. A practical level-of-service framework clarifies three things. It identifies what performance is expected for different asset types and locations, recognizing that a neighborhood inlet and local street network serve a different function than a major trunk corridor, a creek crossing, or infrastructure adjacent to a hospital or fire station. It defines how the City distinguishes between nuisance impacts such as temporary shallow street ponding that clears as the system catches up and higher-consequence hazards such as roadway overtopping that threatens motorist safety or blocks emergency access. And it provides the basis for how maintenance and reinvestment decisions are made, allowing the City to evaluate where the system is meeting expectations, where gaps exist, and whether the appropriate response is routine maintenance, targeted study, or capital improvement planning. Because risk is not uniform across the city, the level-of-service framework differentiates between four common stormwater contexts. Local street drainage typically manages frequent storms through inlets, pipes, and surface pathways; shallow ponding during intense short-duration rainfall can occur even when the system is functioning as intended. Trunk system corridors serve larger drainage areas, and SP2. Define constraints in these corridors can influence multiple stormwaterservice for safety, neighborhoods simultaneously, so performance reliability, and response, using these expectations emphasize system-wide reliability over expectations cons _ isolated symptoms. Roadway crossings and low-water planning, maintenance, _ nd investment crossings are treated as high-consequence assets decisions. because overtopping creates immediate safety hazards and can restrict emergency access, so levels of service at these locations prioritize life safety and operational readiness alongside structural performance. Areas near critical facilities such as public safety infrastructure require heightened consideration because even temporary loss of access can materially affect emergency response and community function. Different contexts justify different performance targets, monitoring practices, and investment priorities, and the level-of-service framework is what makes those distinctions consistent and defensible over time. Establishing clear levels of service directly supports Policy Statement SP2, which uses community expectations for safety, reliability, and response to guide planning, maintenance, and investment decisions. It also strengthens the City's customer service by improving consistency in how concerns are evaluated and explained. When residents ask why something happens at a particular location, staff can reference the applicable service context, explain what the system is designed to manage, describe what exceedance looks like under larger storms, and outline what actions the City will take. That consistency reduces confusion, builds trust, and supports transparent long-term decision- making. PRIORITIZATION HIERARCHY: LIFE SAFETY, RESILIENCE, PROPERTY PROTECTION Stormwater decisions must be strategic, repeatable, and aligned with what matters most when storms occur. A clear prioritization hierarchy provides that consistency. It ensures that staff and decision-makers apply the same logic whether they are responding during a storm event, scheduling maintenance, selecting study updates, or recommending capital improvements. Without a clear hierarchy, stormwater decisions can become reactive driven by which location is most visible or which concern is most recent rather than guided by consistent risk-based principles. A prioritization 30 STORMWATER MASTER PLAN I SOUTHILAKECOMPREHENSIVE STORMWATER MASTER PLAN r hierarchy also helps communicate to residents why some issues are addressed sooner than others, especially when resources are constrained. The City's prioritization hierarchy begins with life safety. This includes reducing hazardous roadway overtopping risk, maintaining emergency access routes where feasible, and responding rapidly to conditions that can cause injury or swift water hazards. Second is system resilience and continuity of access and critical infrastructure. This includes protecting major corridors that serve multiple neighborhoods, preserving the reliability of crossings and conveyance elements that prevent cascading impacts, and safeguarding infrastructure whose failure would disrupt broader community function. Third is property protection as permitted by state law. While property protection is important, the City's approach recognizes that stormwater systems cannot eliminate all flooding under all storms, and that investments should be evaluated carefully to ensure they measurably reduce risk without shifting impacts to other locations. This hierarchy does not require a rigid scoring formula. Instead, it establishes the consistent policy basis for how staff evaluates and communicates why certain actions occur first. Project-specific factors such as feasibility, cost-effectiveness, coordination opportunities, and implementation readiness still inform decisions, but those factors are applied within a framework that consistently prioritizes safety and resilience above other considerations.This is the foundation of Policy Statement SP3: decisions remain grounded in objective criteria, defensible over time, and consistent across storm events so that residents and decision-makers alike can understand how and why stormwater resources are directed. DEFINING "DAMAGE" AND COMMUNICATING PROPERTY IMPACTS OBJECTIVELY When stormwater concerns are reported, the language used to describe them matters. Terms like "flooding," "damage," and "drainage failure" carry different meanings depending on whether they refer to a temporary inconvenience, a recurring quality-of-life issue, or a formal regulatory determination. When these distinctions are not made clearly, residents can receive inconsistent information, compliance expectations can be misapplied, and the City's credibility is undermined. Consistent, factual language about stormwater impacts is therefore a core program responsibility, not a communications afterthought. The City's communications must distinguish between three different concepts that are often conflated: 1. Disruption refers to temporary impacts during or immediately after a storm event F — street ponding, yard flooding, or brief - access issues that resolve as flows recede. These are normal outcomes during intense rainfall and do not automatically indicate a ,1 system deficiency. - 2. Nuisance impacts are repetitive, recurring conditions that affect convenience and quality of life without causing structural ` - damage, such as chronic low areas that pond after moderate storms or yard areas that drain slowly. These conditions matter. i� They drive maintenance targeting, inform ar 4, study priorities,and are tracked in the City's hotspot inventory. They are evaluated ,� differently, however, than conditions that create safety risks or trigger regulatory w requirements. 3. Regulatory damage determinations are objective, federally defined standards t used in floodplain administration. The most significant is FEMA's substantial damage standard: damage where the cost to restore a structure to its pre-damage condition equals or exceeds 50 percent of the building's market value before the damage occurred. This threshold determines when a structure in a flood hazard area must be brought into compliance with current floodplain regulations, with significant regulatory and financial implications for property owners. Applying this standard consistently ensures the City's determinations are factual and legally defensible, not subject to variation based on perception of a storm's severity. (FEMA Substantial Improvement/ Substantial Damage Desk Reference, FEMA P-758, May 2070) The purpose of maintaining these distinctions is not to minimize what residents experience. Policy Disruption and nuisance flooding are real impacts that warrant appropriate responses through Prioritize actionsrojects and by life safety, followed by consistent, maintenance, education, targeted improvements, objective criteria that support defensible and and consistent communication. The purpose is repeatable decision-making. to ensure that when the City describes building damage risk, triggers compliance requirements, or communicates about flood hazard, it does so using accurate and consistent standards. That consistency protects residents, supports fair and lawful floodplain administration, and strengthens public trust by keeping the City's messaging grounded in established, objective criteria. This is the practical foundation for Policy Statement SP3: consistent, objective criteria cannot guide defensible decisions if the terms being used to describe impacts mean different things to different people or in different contexts. 32 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN Ir �J it , jA pq 4 ram' ComS_ I 1 V r I VID- 74 Y t s, sue.- `a Ir 1r 40 CHAPTER4: COLLECTINGDATA AND UPDATING MODELING Good stormwater decisions depend on good information. Southlake's ability to respond effectively during and after storm events, to prioritize maintenance and investment consistently, and to communicate clearly with residents about what is happening and why, all depend on having reliable data about what assets exist, where they are located, how they connect, and what their performance history looks like overtime. This chapter establishes the approach to data management, field observation, and technical analysis that supports those decisions. It treats these activities as core program functions, not optional support activities. Southlake enters this planning cycle with a meaningful foundation. The City has a well-developed technical study library, established field inspection practices, and GIS-based asset information that continues to improve. At the same time, the City does not yet have a formal real-time monitoring infrastructure. Current practice relies on manual inspection, most notably the practice of inspecting low-water crossings before and after storm events, combined with staff field observations and storm event documentation. This is a practical and appropriate starting point, and it is the honest baseline from which this chapter builds. Policy Statement WS3 directs the City to combine data-driven analysis with staff field knowledge to act early in emerging problem areas while ensuring major investments are supported by appropriate technical evaluation. In practice, "acting early" means using the tools currently available, manual inspection, field reconnaissance, documented complaint patterns, and technical study findings, to identify emerging risks before they become crises, while building toward more systematic data collection over time as resources allow. This chapter describes how the City strengthens that foundation across three areas. First, inventory and data management: establishing and maintaining a GIS-based system of record that captures what the City owns, where it is, what condition it is in, and what has been done to maintain it. Second, monitoring and field reconnaissance: using structured observation practices, storm event documentation, and targeted field checks to build the evidence base needed for consistent, defensible decisions. Third, modeling and scenario planning: using hydrologic and hydraulic tools to quantify system behavior, test improvement concepts, and understand cumulative impacts as redevelopment continues over time. INVENTORY AND DATA MANAGEMENT A reliable stormwater program depends on accurate records of what assets exist, where they are located,and how they connect.Stormwater infrastructure is highly distributed across the city,and many 34 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN critical elements are out of sight — underground pipes, culverts, junction structures, and private Poficy Statement connections that influence performance but are AM1. Manage stormwater infrastructure as invisible until something goes wrong. Southlake's . portfolio of public by maintaining GIS inventory is the foundation for managing an inventory, understanding condition these assets as a connected network rather than and criticality, and applying risk-based a collection of isolated parts. It functions as more maintenance and reinvestment. than a map; it is the structured dataset the City relies on to make decisions, linking asset location, characteristics, ownership, access authority, inspection history, work order documentation, and supporting records. Building and maintaining that dataset to a consistent standard is the central expectation of this section and is the foundation for Policy Statement AM1, which directs the City to manage stormwater infrastructure as a portfolio of public assets by maintaining an inventory, understanding condition and criticality, and applying risk-based maintenance and reinvestment. The City's current GIS inventory provides a working foundation, and the expectation of this plan is that it continues to improve in completeness, consistency, and integration with work order history. An inventory that is fragmented across spreadsheets, paper files, or staff memory creates real operational risk: it slows storm response, makes it harder to defend investment recommendations, and creates inconsistency in how similar situations are handled. Perhaps most practically,stormwater knowledge that exists only in staff memory does not stay with the organization when staff change. A maintained, structured system of record ensures that institutional knowledge is documented and available regardless of personnel transitions. That continuity matters to residents because it means the City's understanding of a recurring problem, what has been tried, what was observed, and what the next step is, does not reset every time there is a staff change. A consistent system of record supports several essential outcomes. It improves storm response by reducing the time required to locate structures, determine ownership, and understand how flow is expected to move through a corridor. It improves maintenance planning by allowing staff to identify hotspots, repeat inlet blockages, sedimentation-prone segments, debris-concentration locations, and schedule proactive work based on risk rather than reaction. It strengthens capital planning by tying project concepts to verified asset locations, sizes, constraints, easements, and connection points, which reduces uncertainty and improves cost estimation. It supports MS4 regulatory documentation, which requires mapping, inspection records, asset and outfall tracking, and measurable demonstration of program implementation. And it supports the City's ability to communicate clearly with residents about what is known, what is being evaluated, and what actions are planned. At a minimum, the inventory should include unique asset identifiers with consistent naming conventions, standardized attributes such as asset type, size and material where known, ownership and maintenance responsibility, and right-of-way or easement status. Where as-built drawings and historical documentation exist, the inventory should link to those records. Over time, maintenance history and inspection findings should be tied back to asset identifiers so the City can see which assets or corridors are repeatedly stressed, where condition is trending downward, and where renewal or rehabilitation may be more cost-effective than continued reactive repair. The inventory does not need to be perfect on day one, but it must be structured in a way that improves continuously and becomes more valuable with every inspection cycle, maintenance action, and project update that is documented within it. MONITORING, HIGH-WATER MARKS, AND FIELD RECONNAISSANCE Stormwater decisions improve when field observations and real-world evidence are used Storm SewerUSOUTHLAKE CITY OF Infrastructure T� Box Culvert o Channel Pipe ocz-= Drainage Basin Divide O 100-Year Flood Plain/ 1%Annual Chance Flood ji tEao�oE ao t $I ao Esao I Denton County ................................................................ .o ......■.. .......................................:............................................................... p, '\ Tarrant County I , N Imir- b SJ T T1t l*oo— i______ ,■. �ERo,'i r � —� __ ___s, Scale 1:54000 p ! 1 inch = 4500 feet It l l� � >Y` •� r• � •� � a�5 - L tuJ �I � r�r:; s • � _ �� o, Lip DISCLAIMER A This data has been compiled for The City of Southlake and is for informational purposes. Various official and unofficial sources were used to gather this data, and it does not represent an on-the-ground survey. Any property boundaries shown are approximate only. Every effort was made to ensure the accuracy of this data, but it was not prepared for and may not be suitable for legal,engineering,or surveying purposes. As such,no guarantee is given Last Storm Sewer Revision- 1/1/2025 Office of Performance Excellence or implied as to the accuracy of this data. Last Basema Revision- 1 1 2026 p / / Geographic Information Systems 36 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN alongside technical studies and models. Southlake's current practice relies primarily on manual inspection and staff field observation rather than instrumented monitoring infrastructure. That is an honest starting point, not a deficiency. The City's staff have direct knowledge of how the system behaves, where recurring problems concentrate, and what conditions look like before and after significant storms. That field knowledge is valuable and, when documented consistently, becomes one of the most durable inputs to stormwater decision-making available to the City. The most established field practice is the manual inspection of low-water crossings before and after Policy Statement storm events. This practice directly supports life safety and operational readiness by giving staff WS3. Combine data-driven timely information about where overtopping is staff field knowledge to act early in occurring, how long conditions persist, and whether emerging problem ' closures are warranted. It also builds an evidence major investments are supported base over time. When the same crossing overtops appropriate repeatedly across multiple events, documented observations support defensible prioritization and help staff evaluate whether the issue is driven by structural constraint, debris loading, tailwater, or approach grade conditions. That evidence is exactly what Policy Statement WS3 calls for: combining data-driven analysis with staff field knowledge to act early in emerging problem areas while ensuring major investments are supported by appropriate technical evaluation. The key phrase in WS3 is "appropriate", acting early does not mean acting without evidence. It means using the evidence the City has consistently and building toward better evidence over time. Beyond crossing inspections, field reconnaissance during and after storm events provides important context. Staff observations of debris blockages at inlets, sediment deposition in channels, outfall undermining, erosion progression, and private drainage conditions that redirect flow paths help distinguish between capacity-driven problems and maintenance or condition-driven problems. That distinction is essential for choosing the right response. A blocked inlet that causes street ponding calls for a maintenance action. A crossing that repeatedly overtops under moderate storms despite being clear of debris calls for a study update or capital planning consideration. Field observation is what allows staff to make that determination accurately. High-water marks after significant events provide additional evidence for understanding flood extents and checking whether modeled results align with observed conditions. Even without permanent gauges or automated sensors, staff can document water levels at known reference points, photograph conditions at key locations, and record timing and duration of overtopping events. These observations cost little but build significant long-term value. Looking ahead, the City should evaluate opportunities to expand its observation capabilities at strategic locations, particularly at higher-consequence crossings and known problem corridors, as resources allow. This does not require a large infrastructure investment to begin; simple, consistent documentation practices are the foundation, and instrumented monitoring can be added incrementally where it provides the greatest decision-support value. Maintaining a consistent storm event documentation process is one of the highest-value improvements the City can make. A standard process improves the City's ability to explain what occurred in clear terms, compare observed outcomes to expected performance, identify whether maintenance, a study update, or capital planning is the appropriate response, and maintain continuity of evidence over time. That documentation record becomes the foundation for everything else in this chapter: it feeds the inventory, informs watershed study priorities, and supports the defensible, data-backed communication that residents and decision-makers expect. MODEL PLATFORMS, NETWORK BUILD, AND DECISION-SUPPORT TOOLS Hydrologic and hydraulic models support scenario testing and allow the City to quantify system behavior under defined storm events. Modeling does not replace field knowledge; it organizes it, tests it, and helps the City communicate why certain outcomes occur in a way that is consistent and defensible. Southlake's technical study library demonstrates that stormwater evaluation occurs at multiple scales. Corridor-level and watershed-level studies evaluate specific basins and improvement alternatives. Citywide screening tools, such as two-dimensional rain-on-mesh modeling and hotspot mapping, provide a broader view that identifies where localized flooding risk concentrates and where follow-up study or concept development may be warranted. Integrated modeling approaches are particularly valuable in built-out communities because stormwater behavior is often shaped by the interaction between underground infrastructure and surface overflow pathways. Integrated models that represent storm drain networks alongside two- dimensional surface flow behavior can help explain why water ponds in certain locations, where overflow routes travel,and how changes in one corridor may influence downstream conditions.These tools can also help the City quantify the impacts of current and future redevelopment by allowing alternative scenarios to be tested consistently under the same storm assumptions. That consistency is what allows performance comparisons between corridors and projects to be grounded in fact rather than inference. Southlake's current modeling tools represent a meaningful foundation, and maintaining that Policy foundation over time requires ongoing investment. WS5. Regularly explore opportunities to Models need to be updated as development improve the City's stormwater decision- patterns change, as new data becomes available, support . . by evaluating, procuring, and as study findings reveal where assumptions and/or participating in advanced need refinement. The City's decision-support tools watershed and infrastructure modeling are only as useful as their currency and calibration. platforms and regional collaboration Policy Statement WSS reflects this reality: it studies as warranted, so staff can directs the City to regularly explore opportunities consistently quantify current and future to improve its stormwater decision-support tools by evaluating, procuring, and participating in development impacts, test system advanced watershed and infrastructure modeling performance under defined storm events, platforms and regional collaboration studies as and support knowledgeable, data-driven warranted, so staff can consistently quantify current capital . . . standards . . . and future development impacts, test system performance under defined storm events, and support knowledgeable, data-driven capital planning and standards updates. WS5 is intentionally aspirational rather than prescriptive. It does not require a specific platform or investment level. It establishes the expectation that the City actively evaluates its modeling capabilities on a regular basis and pursues improvements where they deliver the greatest decision-support value relative to cost. A key value of modeling is that it supports transparent communication. When residents experience localized flooding or roadway overtopping, model outputs and mapped results, when used carefully, can help explain what is happening and why. That communication must remain grounded and responsible. Models are decision-support tools that rely on assumptions, and those assumptions should be clearly documented and explained. When model results are paired with field observations and documented storm event evidence, they reduce uncertainty and support consistent, basin- scale decision-making that residents and decision-makers can understand and trust. 38 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN CALIBRATION, VALIDATION, AND CREDIBILITY Stormwater decisions must be credible both technically and in public communication. Credible studies document inputs, assumptions, model methods, and validation steps. Where observed flood extents, roadway overtopping records, or high-water marks exist, validation strengthens confidence that model results reflect real-world behavior. Where direct evidence is limited, studies should clearly explain why assumptions are reasonable and how sensitivity testing was used to understand uncertainty. This matters for public trust: the City should be able to explain why a study indicates that system performance is generally sound while still acknowledging the localized concerns that residents observe and experience. Major capital recommendations should reference the technical basis for the recommendation in a way that is understandable. This includes the storm events evaluated, why those events were chosen, what alternatives were considered, and what evidence supports the recommended approach. Documenting the basis for recommendations improves defensibility, reduces the risk of inconsistent decision-making over time, and helps align Council, staff, and community understanding. Policy Statement WS3 directs the City to combine data-driven analysis with staff field knowledge to act Policy Statement early in emerging problem areas while ensuring major investments are supported by appropriate technical WS3. Combine data-driven evaluation. Acting early is the goal, but it depends staff field knowledge to act early in on available evidence. Without real-time monitoring, emerging problem some problems will only become apparent during major investments are supported • storm events. The City's commitment under WS3 appropriate technical evalu- • is to act on available evidence consistently and systematically, and to build a stronger evidence base over time so that earlier identification becomes increasingly achievable. SCENARIO DEVELOPMENT: EXISTING, MITIGATED, BUILDOUT, EMERGENCY Scenario planning is essential because stormwater performance depends on both current physical conditions and how those conditions change over time. Southlake's technical studies use four standard scenarios that together provide a complete picture of how the system performs today, what improvements can achieve, and what the future may bring: 1. Existing conditions establish the baseline — how the system performs now under defined storm events, where constraints concentrate risk, and what the starting point is for evaluating any proposed change. 2. Mitigated conditions reflect proposed improvements and allow decision-makers to evaluate what a project actually achieves before committing resources. Comparing mitigated results to existing conditions is what demonstrates whether an investment measurably reduces risk and whether it shifts that risk elsewhere. 3. Buildout and redevelopment conditions reflect cumulative long-term change as parcels redevelop over time. This scenario is important because individual projects may appear minor in isolation but can meaningfully shift runoff patterns and peak flows across a basin when aggregated over years of incremental change. 4. Emergency or exceedance conditions illustrate how the system behaves when storms exceed design expectations. This scenario supports honest communication with residents about what can still occur during extreme events even when the system is functioning as designed, and it identifies where residual risk concentrates so the City can evaluate operational readiness and exceedance pathways. Scenario planning supports investment decisions by showing what improvement concepts achieve and by helping decision-makers compare alternatives using consistent storm events and assumptions. It also supports public communication by helping the City explain why localized impacts may occur during extreme events and why targeted improvements reduce risk in the most consequential locations. These scenarios are not a one-time exercise. Policy Statement WS2 establishes a regular cycle of watershed studies sequenced by risk, conducting at minimum one basin-level evaluation every other year, so that scenario assumptions are refreshed as redevelopment patterns evolve and new data becomes available. Policy Statement WS4 recognizes that risk is not uniform across the city and that evaluation depth may need to vary by context, particularly in higher-risk flood areas or where critical assets and emergency access corridors are involved. Together, WS2 and WS4 reinforce that scenario planning must remain current and appropriately calibrated to the conditions it is intended to reflect. EstablishPolicy Statement Po/icy Statement . . floodstormwater and watershed studies, conducting design evaluation in higher-risk basedat minimum one basin-level evaluation every areas on technical study findings, supportother year sequenced by risk, to refresh recognizing that risk is not uniform across priorities, validate assumptions, and long-termcapital .7' 40 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN s i 's e br �11 l � I z at}' a� — t Ir �J it , jA pq 4 ram' ComS_ I 1 V r I VID- 74 Y t s, .�h. � vet� "�- � �� .:� a rJ` • r . - ^�_ j,��,a _•' Je;!oi A When a resident contacts the City about water in their street, erosion along a creek corridor, or flooding in their yard, the City's response should be accurate, consistent, and explainable. That requires more than good intentions. It requires a shared framework for understanding what causes stormwater problems, how different causes call for different responses, and how the City distinguishes between routine outcomes that are expected during intense rainfall and higher- consequence hazards that warrant escalated attention. Chapter 5 provides that framework. This chapter is not a design report. It is a structured evaluation narrative that organizes what the City knows based on field experience, storm event observations, and existing technical studies. It describes how the system behaves, where stormwater risks tend to concentrate, and how the City moves from identifying a symptom to understanding its cause and determining the appropriate response. Stormwater concerns are often reported as a single visible symptom — water in a street, erosion along a creek, or a flooded yard — but the underlying cause can vary widely: public infrastructure capacity limits, debris or sedimentation reducing effective capacity, downstream tailwater effects, channel instability, or private drainage maintenance conditions. Getting the cause right determines whether the appropriate response is a maintenance action, a field investigation, a study update, or a capital improvement concept. Consistent, accurate diagnosis is what makes consistent, accurate communication with residents possible. This evaluation approach directly supports the policy direction that runs throughout this Plan, and it strengthens the City's ability to communicate with residents consistently about what is happening, why it is happening, and what the appropriate next step is. CAPACITY CONSTRAINTS Stormwater system performance varies by location because each watershed corridor has different drainage area characteristics, slopes, upstream constraints, downstream tailwater influences, and infrastructure age. The system is also affected by how development and redevelopment have evolved over time, including incremental changes in impervious cover and how runoff is connected to the drainage network. For that reason, "system performance" is not measured by a single factor like pipe size or channel width—it is a function of how multiple components interact under storm conditions and how effectively the City preserves capacity through maintenance and renewal. 42 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN Southlake's technical studies and field experience show that many stormwater concerns arise from a limited set of recurring mechanisms: • Inlet capture limitations during peak rainfall. During intense short-duration storms, water may pond in streets because local inlets cannot capture runoff as quickly as it is generated. This is especially true where overland flow converges toward a low point, where curb geometry limits inlet efficiency, or where inlet spacing is wider than ideal for local street conditions. Some degree of temporary street ponding can occur even when systems are functioning as intended, which is why the Stormwater Master Plan emphasizes clear levels of service and consistent exceedance messaging. • Localized storm drain capacity constraints. Storm drain pipes and junction structures are typically sized for defined storm events and local design criteria. Constraints can occur where pipe sizes step down, where alignment creates energy losses, where junctions are surcharged, or where downstream tailwater limits outflow. These constraints can manifest as repeated ponding in the same low area or as nuisance flooding during heavier events. • Culvert and bridge constraints at crossings. Crossings are natural hydraulic constraints. The opening, roadway profile, and debris susceptibility can control upstream water surface elevations during larger events. Even where upstream pipe networks are functioning, a crossing constraint can cause upstream water levels to rise, resulting in overtopping or localized flooding that appears "upstream" of the actual controlling condition. • Channel conveyance limits and obstructions. Open channels and creek corridors convey basin-wide flow. Conveyance can be reduced by sediment deposition, vegetation changes, debris accumulations, or natural constrictions. Channel stability also matters: erosion can undermine banks and outfalls, changing geometry and increasing sediment loads that affect downstream capacity. • Debris and sedimentation reducing effective capacity. Many "capacity" complaints are actually "effective capacity" issues. A pipe or culvert may have sufficient theoretical capacity, but debris, sediment, or blockage reduces how much flow it can actually convey. This is why proactive inspection and maintenance are central to performance and why asset management is part of system evaluation rather than a separate topic. Southlake's overall stormwater outcomes are strong relative to many communities, particularly in avoiding widespread building flooding. This reflects a long history of drainage planning, floodplain administration, and targeted investment. vailable FEMA flood insurance claim data is consistent with this assessment, showing 14 total claims filed in Southlake in about 45 years, with three properties in the NFIP repetitive loss database, each carrying two claims on record. However, "strong citywide outcomes" does not mean "no localized impacts." Localized nuisance flooding and street ponding can occur when rainfall intensities exceed practical capture capacity, when downstream constraints limit discharge, when tailwater conditions back water up into local systems, or when temporary maintenance conditions reduce effective system function. The Stormwater Master Plan's role is to describe these mechanisms plainly and provide a consistent framework for response and investment decisions. A consistent evaluation process supports defensible decisions and consistent communications: 1. Confirm the primary driver: public infrastructure capacity, maintenance/condition, private drainage, or mixed. 2. Document observed conditions and recurrence: location, storm event context, photos, and whether this is a repeat hotspot — noting particularly whether the location poses risk to public safety, public infrastructure, or private property. 3. Determine the appropriate response: no further action where conditions reflect expected system behavior, education and outreach where private maintenance or resident behavior is the primary driver, maintenance action, targeted field investigation, study update, or capital improvement concept development. 4. Communicate clearly: what was observed, what the City will do next, what is private responsibility if applicable, and realistic timelines. This structured approach is the practical ofic)�Statement expression of Policy Statement SP1, which directs . the City to convey, store,and manage stormwater SP1. Convey, store, and manage stormwater to protect life safety, public infrastructure, and to protect property through proactive risk reduction and and propertyproactive sound system performance practices. Consistent reductionperformance evaluation and clear communication are what make that policy direction real for residents. FLOODPLAIN ENCROACHMENTS AND OUT-OF-BANK CONDITIONS Out-of-bank flow is a natural function of creek systems during larger storm events. Floodplains convey and store floodwater when channel capacity is exceeded, and they reduce downstream peak intensity by providing space for flows to spread and slow. A key evaluation point is that out-of-bank flow does not automatically indicate a deficiency in the storm drain network. In many cases, floodplain inundation during rare events is expected and reflects the system functioning as designed. When residents report flooding in or near a creek corridor, the first evaluation question is whether the water behavior is consistent with mapped flood hazard conditions or whether it reflects a condition that warrants further investigation — such as encroachments that have reduced conveyance, debris that has altered flow paths, or channel instability that has changed how flows are distributed. Floodplain performance can be degraded when conveyance is reduced by encroachments: fill, structures,fences,landscaping,or other obstructions placed within flood hazard areas.Encroachments 500 Year Floodplain 0.2%Annual Chance FEMA 1 D4-Year Floodplain I%Annual Chance FEMA Flood Fringe FEMA Floodway f 1 Foot Encroachment Area ------I'r ��Encroachment Area Base flood elevation after encroachment on floodplain. ' Base flood elevation before encroachment on floodplain. One Foot Increase in Base Flood Elevation - Section View STORMWATER MASTER PLAN can increase water surface elevations, shift risk to adjacent properties, and reduce the reliability of upstream conveyance by increasing tailwater effects. Identifying and addressing encroachments is a core element of floodplain administration and system evaluation, and it directly supports the City's obligations under the National Flood Insurance Program (NFIP). NFIP participation requires the City to administer consistent floodplain development standards, apply FEMA's objective definitions for substantial damage and substantial improvement, and ensure that changes in flood hazard areas do not increase risk for neighboring properties. These standards are not discretionary; they are the legal and regulatory framework within which the City must operate. From an evaluation standpoint,the City manages floodplain-related concerns through a combination of mapped flood hazard understanding, consistent regulatory administration, maintenance of creek corridor function, and targeted mitigation where it measurably reduces risk. When a concern is reported, the evaluation should distinguish between conditions that reflect expected floodplain behavior during larger events and conditions that indicate an avoidable problem — encroachment, debris accumulation, channel instability, or a change that has shifted risk in a way the City can and should address. That distinction is what allows the City to respond accurately and communicate honestly with residents about what is occurring and what the appropriate response is. EROSION, CHANNEL STABILITY, AND BANK PROTECTION NEEDS Erosion evolves gradually and often becomes costly when it reaches infrastructure thresholds when a bank failure threatens a trail, utility line, roadway embankment, outfall structure, or private improvement. Channel stability is therefore a central element of system evaluation because it influences both system performance and long-term maintenance burden. Erosion can undercut banks, damage or expose utilities, destabilize outfalls, and change channel geometry in ways that alter conveyance capacity and sediment transport downstream. Over time, erosion and sediment deposition can create a cycle: erosion produces sediment, sediment reduces downstream capacity, reduced capacity increases upstream water levels, and higher levels can increase erosion stress at sensitive locations. Southlake's erosion evaluations and preliminary engineering work identify stabilization needs and recommended improvements where risk is greatest. This supports three key master plan practices: • Proactive inspection and maintenance of channels and outfalls. Erosion is easier and less costly to address early before failures threaten assets. Regular inspections after major storm events and targeted monitoring at known erosion sites help the City act before the risk escalates. • Risk-based prioritization of stabilization. Not all erosion requires immediate structural intervention. A risk-based approach focuses first on locations where erosion threatens public infrastructure, critical conveyance, or documented risk to structures, and then on locations where stabilization provides multi-benefit outcomes (improved conveyance reliability, reduced maintenance burden, enhanced open space value). • Clear responsibility definitions near private property. Erosion can occur along the boundary between public corridors and private property. The City's approach should clearly define where the City maintains channels within public rights-of-way or improved easements and where private owners/ PolicK Statement HOAs have responsibility. Where erosion reliableAM2 system function threatens public assets, the City must be Sustain ctive inspection, maintenance, able to access and address the issue;wherethr renewalnd • ' " inlets, culverts, erosion is solely private, the City's role isdrainage often guidance and ensuring that private and actions do not increase downstream risk. Because erosion is both a performance and asset management issue, it belongs in system evaluation, it directly affects capacity, reliability, and lifecycle costs. Policy Statement AM2 reflects this by directing the City to sustain reliable system function through proactive inspection, maintenance, and renewal of public inlets, culverts, channels, and drainage facilities. DETENTION/RETENTION ADEQUACY AND RELEASE CONTROLS Detention and retention can reduce peak flows, manage release rates, and protect both upstream and downstream conditions, but performance depends on design, outlet controls, tailwater conditions, and critical maintenance. In a built-out community, detention facilities often become long-term operational assets. Over time, sedimentation can reduce storage volume, vegetation can alter flow paths, and outlet controls can become partially blocked or modified. When these conditions occur, a facility that once provided meaningful peak reduction may gradually lose effectiveness. The impacts can run in both directions, downstream neighborhoods may experience increased nuisance flooding, and areas that historically drained to the facility may experience increased backwater or ponding if outlet controls are undersized, blocked, or not functioning as designed often without a clear single failure event to point to. This is why detention evaluation must include both technical performance and ownership and maintenance clarity. Where detention is privately owned or HOA maintained, the City's ability to protect public risk depends on clear documentation of responsibility and tools to verify maintenance where private facilities materially influence public outcomes. Policy Statement AM3 directs the City to clearly define public versus private stormwater responsibilities and strengthen how those obligations are communicated, supported, and verified. Policy Statement DR1 reinforces that development and redevelopment must continue to demonstrate responsible management of stormwater impacts under buildout conditions, including upstream and downstream considerations which means post-construction controls must be designed, maintained, and documented to remain effective over the long term, not just at the time of construction. A practical evaluation approach includes: Policy Statement PoficV Statement DR1. Continue to administer and enforce AM3. Clearly define public versus stormwater regulations requiring new private stormwater responsibilities and development and redevelopment to strengthen how the City communicates, demonstrate responsible management of supports, and verifies private impacts under full buildout conditions, maintenance obligations where those consistent with applicable state law and facilities influence public risk. City ordinances, including • - • downstream considerations where applicable. • Documenting known detention facilities and outlet controls in the inventory where they influence public performance. • Identifying high-consequence facilities that warrant periodic verification. • Ensuring post-construction maintenance responsibilities are enforceable and communicated clearly. • Using resident concerns and field observations as triggers for follow-up inspection where warranted. 46 STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN ROADWAY/BRIDGE PERFORMANCE AND OVERTOPPING FREQUENCY Roadway crossings are among the highest-consequence assets in Southlake's stormwater system because overtopping creates immediate hazards for motorists and can restrict emergency access routes. Crossings are also hydraulically sensitive: the opening size, roadway approach profile, debris susceptibility, downstream tailwater, and channel stability around the structure all influence when and how overtopping occurs. Even a crossing that performs reliably under most conditions can become a hazard when debris partially blocks the opening before a storm, when tailwater from downstream flooding backs water up, or when channel erosion at the outlet changes how flow enters and exits the structure. Evaluating crossing performance in this chapter's context means understanding not just whether a crossing has sufficient theoretical capacity, but whether it is performing as expected under actual storm conditions and whether maintenance, operational protocols, or capital investment is the appropriate response. Southlake's current practice of manually inspecting low-water crossings before and after storm events is the primary tool for building that evidence base. Those inspections document when overtopping occurs, how long conditions persist, and what field conditions are present, information that distinguishes a one-time exceedance event from a recurring problem that warrants escalated attention. The City manages crossing risk through storm event documentation capturing where,when, duration, and observed conditions; monitoring and closure protocols that protect public safety during events; maintenance actions that reduce avoidable blockage and preserve effective capacity; and capital planning where improvements provide measurable risk reduction without creating downstream harm. Roadway closures and barricades at overtop-prone crossings are safety actions, not evidence of system failure. Communicating that distinction consistently is part of how the City maintains public trust during and after storm events. This approach directly supports Policy Statement SPI, which directs the City to protect life safety, public infrastructure, and property through proactive risk reduction and sound system performance practices, and Policy Statement SP3, which directs the City to prioritize actions first by life safety. Poficy Statement Policy Statement SP1. Convey, store, and manage stormwater SP3. Prioritize projects and actions first to protect - - safety, consistent, systemand property through proactive risk objective criteria that support defensible and reduction and sound - repeatable CRITICAL FACILITIES AND EMERGENCY ACCESS Stormwater risk management intersects with emergency management because roadway overtopping and localized flooding can disrupt access to critical facilities and limit emergency response routes. A key evaluation step is identifying where stormwater hazards can create consequences beyond property impacts isolating neighborhoods, restricting access to public safety facilities, or disrupting routes used during emergencies. These are the locations where stormwater and life safety converge most directly, and they warrant the highest level of attention in system evaluation, operational planning, and investment prioritization. Policy Statement SP1 directs the City to protect life safety, public infrastructure, and property through proactive risk reduction and sound system performance practices. Policy Statement SP3 establishes that life safety is the first priority when making stormwater decisions. Together, these policies require the City to actively identify, monitor, and address locations where stormwater conditions can affect emergency access and response. This master plan supports maintaining: • A current inventory of critical routes and facilities that could be affected by stormwater hazards; • A map of known closure locations and monitoring points so staff know where to focus attention before and during storm events; • Clear procedures for barricade placement and public alerts so residents and emergency responders receive timely, accurate information; and • Post-event documentation that records what closed, when, and under what conditions — because that record is what allows the City to evaluate whether operational protocols are sufficient or whether a location warrants study, maintenance attention, or capital planning consideration. ENVIRONMENTAL AND HABITAT CONSIDERATIONS Creek corridors and open space are not separate from Southlake's stormwater system, they are part of it. Channels and drainageways convey storm flows, floodplains provide storage and spreading room, and vegetated banks stabilize soils that would otherwise contribute to erosion and sedimentation. When these natural system components function well, they improve conveyance reliability, reduce maintenance burden, and support water quality outcomes. When they degrade, through encroachment, erosion, vegetation changes, or debris accumulation, system performance suffers and maintenance costs increase. For that reason, environmental and habitat conditions in creek corridors are a legitimate element of system evaluation, not a separate consideration reserved for park planning or environmental permitting. System evaluation should recognize where channel stability and vegetative conditions are affecting stormwater performance and where improvements can be structured to deliver both drainage function and community benefit. Stabilization projects and channel improvements should consider habitat continuity, long-term maintenance access, and the potential for native vegetation to reduce erosive stress and improve long-term stability compared to hardened solutions that may require repeated repair. Where feasible, projects that align stormwater performance with open space goals — improving creek corridor safety and function while enhancing trail access, vegetation, or community amenity — are more durable investments because they generate broader public support and reduce the likelihood that future encroachments will undermine the improvement. That said, environmental and community co-benefits are a secondary consideration in stormwater evaluation. The primary question is always whether a project reduces risk, improves system reliability, and addresses a documented stormwater need. Where co-benefits can be achieved without compromising the stormwater purpose, they are worth pursuing. Chapter 7 addresses green infrastructure and nature-based strategies in greater depth, including how they can be applied in Southlake's built-out context to complement traditional infrastructure rather than replace it. 48 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN Southlake,Texas Legend Southlake Drainage Master Plan EXHIBIT 1 !Southlake City Limits CRITICAL STRUCTURES 1 September 2019 Southlake Drainage Basin ---- St-nu tnp 0 Drainage Complaints Critical Structures - � • Deftient I 'i., ' • Improved fl • Inaccessible • • Need Data . ��. • Non-Structure � • Sufficient,YsR'`r' 1l • under conenuamn I Q © YC=1T C'�TT;CS, -..,. l D a €f - • coa ,s Gal ON ..yI rc1s. Ci w m a m 00 m m � SO :m >• >• f33�i Fri � • o ui 1,� • .r Figure 5-2:TNP Critical Structures Map Exhibit J 00 CHAPTER6 : WATER QUALITY 1 . R 1 • R COMPLIANCE Chapter 6 explains the regulatory responsibilities that shape Southlake's stormwater program and establishes a clear baseline for what the City is required to do to protect water quality and reduce flood risk. Stormwater management is not only an infrastructure function; it is also a compliance obligation that must be implemented and documented consistently over time. This chapter summarizes the core requirements that apply to the City, including the Municipal Separate Storm Sewer System (MS4) program administered by the Texas Commission on Environmental Quality (TCEQ) and the floodplain management responsibilities associated with participation in the National Flood Insurance Program. These requirements influence how the City conducts inspections, manages construction and post-construction controls, delivers public education, responds to illicit discharges, and administers floodplain development standards. This chapter is also intended to support clear communication and accountability. Residents often experience stormwater primarily during major storms, but many stormwater responsibilities occur year-round through education, inspections, operational practices, and recordkeeping that reduce pollutants and prevent avoidable risk. By defining these obligations in plain language,the Stormwater Master Plan strengthens the City's ability to explain why certain program activities are required, how compliance supports local outcomes, and how stormwater policy choices such as ordinance updates, maintenance practices, and public education enhancements help Southlake meet both regulatory requirements and community expectations for safety, reliability, and environmental stewardship. REGULATORY CONTEXT (MS4, NFIP, FEMA, STATE/FEDERAL MANDATES) Stormwater management is not only an infrastructure function, it is also a regulatory responsibility. The City's stormwater program must protect water quality and reduce flood risk through a combination of planning, ordinance administration, operations, inspections, maintenance, and public education. In Southlake, these responsibilities fall into two primary regulatory areas: water quality requirements under the MS4 program and flood risk management requirements through floodplain administration and National Flood Insurance Program participation. Together, these requirements shape what the City must do day-to-day and provide the baseline "musts" that the Stormwater Master Plan builds upon. • STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN MS4 / water quality compliance. Southlake operates a regulated small Municipal Separate Storm Sewer System under the Texas Pollutant Discharge Elimination System (TPDES) administered by the Texas Commission on Environmental Quality (TCEQ). The City's authorization is under the statewide Phase II small MS4 general permit TXR040000, issued August 15, 2024. Under this permit, the City must implement and document a Stormwater Management Program that includes required best management practices and measurable goals tied to the City's MS4 classification level. The City is expected to reduce pollutants in Stormwater runoff to the maximum extent practicable, a core EPA Phase II concept that guides program design and ongoing improvement. Floodplain / flood risk compliance. Southlake also administers floodplain regulations through participation in the National Flood Insurance Program (NFIP), which is administered by FEMA. NFIP participation requires the City to maintain and enforce floodplain development rules consistent with FEMA standards, including permitting, documentation, and consistent determinations for "substantial improvement" and "substantial damage." FEMA's substantial damage standard is the well-known 50 percent rule (damage equals or exceeds 50% of market value of the structure before damage). These rules matter because they protect the public over time by preventing incremental changes in flood hazard areas that increase risk or reduce floodplain conveyance. Construction-related requirements.Construction is also a key regulatory focus because construction site runoff and sediment are major pollutant sources. Construction activities disturbing one acre or more are generally subject to stormwater construction permit requirements (and must implement erosion/sediment controls and inspection practices). In addition, the City's MS4 permit requires the City to have a construction runoff control program meaning the City must implement local oversight (typically through ordinance authority, plan review, inspections, and enforcement mechanisms) to reduce pollutant discharges from construction sites. So what does this mean for the City at a high-level? The City must do the following: • Maintain a Stormwater Management Program under TXR040000 with required BMPs and measurable goals and document implementation. • Implement programs that address the six minimum control measures (described later in the Chapter) and track/report progress. • Maintain floodplain development regulations and processes consistent with NFIP requirements, including substantial damage/substantial improvement determinations using FEMA's standard. • Maintain construction runoff controls through both (a) compliance expectations for City projects and (b) local oversight tools for private construction activity (as part of MS4 requirements). MS4 MINIMUM CONTROL MEASURES AND PROGRAM EXPECTATIONS The Phase II small MS4 program is organized around minimum control measures (MCMs) that define what regulated cities must implement. These measures are designed to reduce the discharge of pollutants from the municipal storm sewer system into local waterways, addressing the full range of sources that contribute to stormwater pollution including public behavior, land disturbance during construction, post-construction runoff from developed sites, and the City's own operations. The Environmental Protection Agency's (EPA) Phase II framework describes these measures and emphasizes measurable goals and ongoing implementation.TCEQ's TXR040000 permit is described as comprehensive and requires specific best management practices and measurable goals by MS4 level. For Southlake, these requirements translate into a program that must be both implemented and documented. In other words, it is not enough to say "the City educates the public" or "the City inspects construction sites." The City must be able to demonstrate what it did, how often it did it, and how it is tracking progress toward its measurable goals (typically through program records and routine reporting). Policy Statement WQ1 directs the City to protect surface water quality through consistent implementation of regional stormwater best management practices across municipal operations, construction activity, and post-construction responsibilities. Policy Statement W02 directs the City to sustain compliance with MS4 and floodplain-related requirements while using its policy flexibility to exceed minimum standards when doing so measurably improves outcomes for residents and infrastructure. Together these policies establish that compliance is the floor, not the ceiling, of Southlake's stormwater quality program. Policy Statement Policy Statement WQ1. Protect - water quality WQ2. - with MS4 and managementthrough consistent implementation of floodplain-related requirements while regional stormwater best the policy flexibility to exceed practices . .l operations, minimum standards whenmeasurably construction activity, and . .st- improves outcomesresidents construction responsibilities. The six minimum control measures (MCMs) address each of these source categories and typically include: 1. Public education and outreach 2. Public involvement/participation 3. Illicit discharge detection and elimination (IDDE) 4. Construction site runoff control 5. Post-construction stormwater management (new development and redevelopment) 6. Pollution prevention/good housekeeping for municipal operations i w 52 STORMWATER MASTER PLAN SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN What this means the City must do: • Maintain an MS4 Stormwater Management Program document that lists BMPs and measurable goals. • Carry out program activities each year (education, inspections, investigations, municipal good housekeeping, etc.). • Keep records sufficient to demonstrate implementation (e.g., outreach products, training records, inspection logs, IDDE investigations, enforcement actions, municipal facility practices). • Use performance tracking to show progress and support continuous improvement (this is where the Stormwater Master Plan's recommended dashboards and annual "program snapshot" reporting fit). IMPAIRED WATERS, POLLUTANTS, AND LOCAL WATER QUALITY PRIORITIES Stormwater commonly transports sediment, nutrients, bacteria, metals, oils/grease, and litter from streets, parking areas, construction sites, and disturbed soils to receiving waters. Even when flood outcomes are favorable, water quality outcomes can degrade if pollutants are not managed through consistent BMP implementation and maintenance. This is why water quality is a core purpose of MS4 programs: Stormwater systems discharge to surface waters, and pollutant loads can accumulate over time. For Southlake, a practical way to frame water quality priorities is to connect pollutant sources to the City's controllable levers: • Sediment and turbidity: closely tied to construction controls, disturbed soils, and channel erosion. • Bacteria/nutrients: tied to pet waste, illicit discharges, and runoff from developed areas. • Oils/metals: tied to vehicle areas, spills, and municipal operations practices. • Litter/debris: tied to public education, street sweeping, and inlet maintenance. What this means the City must do: • Use MS4 program tools (education, inspections, municipal housekeeping) to reduce pollutant sources and prevent illicit discharges. • Maintain BMPs and drainage features so they continue to function (maintenance is inseparable from water quality performance). • Document program outputs and improvements as part of compliance and program management. STRUCTURAL AND NON-STRUCTURAL BEST MANAGEMENT PRACTICES Best management practices (BMPs) include both structural and non-structural measures. Structural BMPs might include vegetated swales, water quality features integrated into detention basins, filtration or settling features, and stable outfalls that reduce erosion and sediment transport. Non-structural BMPs include street sweeping, spill prevention and response, staff training, public education, construction inspections, and consistent operational housekeeping. A core best practice is that BMP performance depends on maintenance. A Swale that fills with sediment, a detention outlet that becomes blocked, or a channel reach that erodes can lose effectiveness and, in some cases, become a contributor to pollution and capacity issues. This is why the Stormwater Master Plan links water quality outcomes to the asset management foundation: water quality controls should be treated as assets with ownership, inspection expectations, and maintenance responsibility especially for post-construction controls that are privately owned or HOA maintained. What this means the City must do: • Implement BMPs as part of MS4 program activities and development/post-construction requirements. • Ensure BMPs remain functional over time through inspection/maintenance expectations (public and private where applicable). • Maintain documentation that demonstrates BMP implementation and ongoing program activity. CONSTRUCTION AND POST-CONSTRUCTION CONTROLS Construction sediment is one of the most common pollutants affecting stormwater systems, and it also directly affects system performance by clogging inlets, filling pipes, and reducing channel capacity. Construction controls are therefore both a water quality requirement and a system reliability issue. Under the MS4 permit framework, the City must implement a construction runoff control program typically through ordinance authority, plan review, inspections, enforcement capability, and documentation. The EPA's Phase II framework includes construction runoff control and post- construction stormwater management as minimum control measures. TCEQ's permit requires measurable goals and BMP implementation at the City's MS4 level. Post-construction controls matter just as much because development impacts do not end when construction ends. Post-construction requirements address long-term runoff and pollutant transport Fs s«x 9G -i to �'b fr, .E,•k t'.��M � Y �� Y,Y � � r �p R �JNCf 54 .✓a "'' _.r- 1. le,..-�' 1, c '' '� ` �\�j _..\��`�'*..C.�"�'a'�F�� F��:. e�F'' 'a,' �`�'• , t��'9��s� STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN and establish maintenance responsibilities for permanent controls (detention, water quality features, drainageways, outlet controls). Clear ordinances, predictable submittal requirements, and enforceable maintenance expectations are essential for long-term compliance and performance. This is also where the Plan's ordinance modernization direction is most critical: development standards must remain clear, fair, and enforceable, and maintenance obligations must be documented in a way that can be verified over time especially when private facilities influence public risk. What this means the City must do: • Maintain construction runoff control authority and inspection/enforcement capability as part of the MS4 program. • Ensure City projects disturbing >_1 acre also meet applicable construction stormwater requirements (as an operator/owner where applicable). • Maintain post-construction standards that require permanent controls and establish long- term maintenance responsibility. • Document compliance activities (inspections, violations, corrective actions) as part of measurable goal tracking. ILLICIT DISCHARGE DETECTION AND ELIMINATION (IDDE) Illicit discharges are discharges to the storm drain system that are not composed entirely of stormwater (with limited allowable exceptions). These discharges can degrade water quality and, depending on the substance, may create public health concerns or visible nuisance conditions. An effective program depends on clear reporting pathways, staff training to recognize indicators, consistent investigation procedures, and documentation of outcomes. IDDE is also a credibility and trust issue. Residents are more likely to report concerns when they know how to report them and believe the City will investigate and follow up. Integrating IDDE into the broader customer response framework strengthens outcomes and supports the Stormwater Master Plan's education-first approach. What this means the City must do: • Maintain an IDDE program component as part of MS4 requirements, including procedures for reporting, investigation, and response. • Train staff and maintain consistent documentation of investigations and outcomes. • Use public education to reduce common sources of illicit discharges and improve reporting. � I 77 r Chapter 7 describes how Southlake can strengthen stormwater performance and long-term resilience by using green infrastructure and nature-based strategies alongside traditional drainage improvements. As the City approaches buildout, many stormwater opportunities will come through redevelopment, corridor reinvestment, and park projects rather than large-scale system expansion. Green infrastructure is well-suited to this context because it focuses on reducing runoff closer to where rain falls, slowing and spreading flows, improving water quality, and reducing erosive forces that can destabilize channels and outfalls over time. It is complementary to conveyance and storage, not a replacement for it. The goal is not to implement green infrastructure everywhere; the goal is to apply the right strategy to the right location where solutions can deliver measurable stormwater benefit, long-term maintenance responsibilities are clear, and lifecycle costs are understood. Successful green infrastructure depends on feasibility, maintainability, and clear responsibility for long-term operation. Features that are difficult to maintain or lack clear ownership can lose effectiveness and become liabilities over time. For that reason,this chapter focuses on strategies that match Southlake's built environment and maintenance realities, including how green infrastructure is best incorporated early in project planning, when and where nature-based approaches reduce long-term maintenance burden compared to structural alternatives, how multi-benefit opportunities can be evaluated practically, and how resilience thinking applies to Southlake's planning horizon. LOW IMPACT DEVELOPMENT AND GREEN INFRASTRUCTURE Low impact development and green infrastructure strategies complement traditional conveyance and storage by reducing runoff volume, slowing peak flows, improving water quality, and supporting streambank protection.These strategies work by increasing infiltration where feasible,adding storage and filtering functions within the landscape, and breaking up runoff connectivity so water reaches the drainage system more slowly. When used strategically, these measures can reduce nuisance ponding in localized areas, lessen downstream erosive forces, and reduce pollutant loads delivered to receiving waters supporting both system performance goals and water quality responsibilities. In Southlake's built-out context, the most effective applications of green infrastructure are typically those that can be integrated into projects the City and the private market are already doing. Green infrastructure is most successful when it is incorporated early during redevelopment, streetscape 56 STORMWATER MASTER PLAN I SOUTHILAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN improvements, parking lot reconstruction, park projects, and new development site design because that is when grading, drainage patterns, and right-of-way design can be shaped intentionally. Retrofitting is still possible, but it is usually more costly and may introduce maintenance complications if responsibility and access are not clear. This is why the Plan treats green infrastructure as a planning and standards tool as much as an engineering tool: the earlier it is considered, the more feasible and cost-effective it becomes. The City's approach emphasizes feasibility and maintainability. Green infrastructure features that cannot be maintained reliably tend to lose performance and can become liabilities. For example, a bioretention area that fills with sediment,a permeable pavement area that clogs,or a vegetated Swale that becomes overgrown and obstructed can reduce drainage function rather than improving it. The Stormwater Master Plan therefore supports a practical "maintenance-first" screening approach: • Can it be maintained with clear ownership and access? • Does it have a measurable stormwater purpose (volume reduction, peak reduction, water quality benefit, or stability benefit)? • Is it compatible with local soils, slopes, and site constraints? • Will it reduce long-term burden on the public system or downstream corridors? When those answers are favorable, green infrastructure becomes a strong tool for incremental, distributed improvement, particularly in redevelopment corridors where cumulative impervious impacts can otherwise compound over time. Regional integrated stormwater management guidance provides a helpful framework for this Policy Statement -7 approach. The North Central Texas Council of Governments (NCTCOG) iSWM Criteria Manual . - - . practices describes criteria to address and manage water by prioritizing approaches quality, streambank protection, stormwater natural hydrology conveyance, and flood control issues associated with slowing it down, - . . development and redevelopment, and it is intended through feasiblegreen to be used by cities and counties as a component infrastructure strategies. of stormwater-related development regulations. The iSWM Technical Manual provides additional technical guidance referenced by the criteria framework. Southlake can use this regional framework selectively aligning where it improves clarity and consistency while applying local provisions that reflect Southlake's context, maintenance capabilities, and community expectations. This directly supports Policy Statement DR2, which directs the City to advance iSWM-aligned practices by prioritizing approaches that mimic natural hydrology through feasible low-impact and green infrastructure strategies. NATURE-BASED SOLUTIONS AND CO-BENEFITS Nature-based solutions are a subset of green infrastructure that intentionally leverages natural systems vegetation, soils, floodplains, and stream processes to reduce risk and improve long-term stability. In stormwater management, nature-based solutions can include restoring more stable channel forms, regrading banks to reduce erosive stress, adding riparian buffers, integrating native vegetation that stabilizes soils, and using floodplain space to store and convey water more safely. These approaches can reduce erosion risk and improve long-term maintainability, especially when compared to repeated small repairs in the same corridor. For Southlake, nature-based strategies are particularly relevant in creek corridors and open space areas where space constraints may be less severe and where projects can deliver multiple community benefits. Creek corridor improvements can enhance drainage function while also supporting recreation, aesthetics, habitat protection, and community character. These benefits matter because stormwater investments are more durable and publicly supported when they solve a stormwater problem and improve community value at the same time. For example, a stabilization project that also improves a trail corridor, enhances vegetation, or restores a degraded drainageway can deliver both risk reduction and quality-of-life outcomes. 1 Nature-based solutions also provide a useful lens for prioritizing erosion control projects. Not all erosion needs immediate structural armoring. In many cases, improving stability through vegetation- based approaches, bank shaping, and selective structural toe protection can reduce the long-term maintenance burden and better match the natural behavior of the system. This aligns with the Plan's emphasis on risk-based channel management: stabilizing where erosion threatens public infrastructure, where it drives repeated maintenance, or where it threatens system reliability. DROUGHT-FLOOD NEXUS AND MULTI-BENEFIT OPPORTUNITIES Stormwater storage is primarily a flood mitigation tool. Certain sites may also offer multi-benefit opportunities where operational feasibility and maintenance responsibilities are clearly established. In a Texas context, communities often experience periods of drought punctuated by intense storms. This creates a planning opportunity: where feasible, certain stormwater facilities can be designed to provide benefits beyond peak-flow reduction, such as improved landscape resilience, better water quality treatment, or enhanced open space function. That said, multi-benefit concepts must be evaluated carefully. Storage features can introduce water quality concerns if they hold water for extended periods without appropriate design. They can also increase maintenance needs if sedimentation, vegetation growth, or debris management is not planned. For Southlake, the most realistic multi-benefit opportunities are those that: • Are located in public facilities or park/open space contexts where maintenance access is clear; • Provide measurable stormwater performance (peak reduction, reduced erosive discharge, improved water quality); and • Do not create long-term operational burdens that exceed the City's ability to maintain them. This master plan supports evaluating multi-benefit opportunities as part of corridor and park projects where they align with broader City goals and where lifecycle costs are understood. 58 STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN ADAPTIVE RESILIENCE AND CLIMATE CONSIDERATIONS Resilience recognizes uncertainty related to rainfall intensity, infrastructure aging, Policy Statement and redevelopment patterns. Over time, stormwater performance can be influenced WS2. Establishregular ' ' ' m of technical by updated rainfall standards, cumulative stormwater and watershed studies, conducting impervious changes, and aging infrastructure at minimum one ' ' evaluation that loses capacity or reliability. In this other year sequencedby context, resilience is not a single project— priorities, validatesupport" it is an approach to managing uncertaintycapital planning. through disciplined planning, monitoring, and staged investment decisions. Maintaining that regular study cycle is itself an act of resilience, and it is what Policy Statement WS2 establishes as a programmatic commitment. An adaptive resilience approach allows Southlake to take near-term actions now while defining triggers for future upgrades as new data becomes available. For example, the City can: • Improve maintenance programs and hotspot targeting immediately. • Update watershed studies and decision-support tools on a regular cycle. • Incorporate flexible standards and ordinance updates that manage redevelopment impacts. • Prioritize improvements at high-consequence assets such as crossings and critical corridors. This approach helps avoid both extremes: under-investing until failures become emergencies or over-investing based on assumptions that may change. It also supports transparent communication with residents: the City can explain what is known, what is being monitored, and what thresholds would trigger additional evaluation or future projects. The iSWM framework also supports this adaptive mindset by providing a structured way to address multiple stormwater focus areas (water quality, streambank protection, conveyance, flood control) and by supporting consistent criteria that can be refined locally over time. �}, !r,, to { CpT . -41 Ip y .� ._,�_--a cry-- �,------• �•� � —— vw fly. AMS F '�r"� i ik�»�'��s�`�w r��e p�:"-- y . ,�� f. �:'♦ • • Chapter 8 establishes the asset management foundation for Southlake's stormwater system and sets clear expectations for how the City will preserve stormwater performance as the community approaches buildout. Stormwater infrastructure is not a single facility; it is a distributed network of public assets that must work together during routine storms and during high-impact events. Many of these assets are out of sight and are only noticed when a storm exposes a weak link: an inlet that can't capture flow quickly enough, a culvert that is partially blocked, a deteriorated pipe segment, an undermined outfall, or a channel reach that has lost stability. Because of this, the City cannot rely on reactive repairs as its primary strategy. Sustained performance requires a disciplined asset management approach that ties inventory, inspection, maintenance, condition assessment, renewal forecasting, and documentation into one consistent program. This chapter is intentionally written as the"building blocks"chapter fora future dedicated Stormwater Asset Management Plan. The future plan will provide more operational detail, but this master plan must establish what staff must do now to create that future plan successfully. The expectations in this chapter are therefore assertive: staff must maintain a stormwater system of record, must standardize how condition and risk are evaluated, must establish and execute proactive inspection and maintenance cycles, and must document work and outcomes in a way that supports defensible decision-making and transparent communication. Asset management is how Southlake protects public safety, preserves emergency access, reduces avoidable failures, and sustains reliable service levels with finite resources. WHY ASSET MANAGEMENT IS ESSENTIAL Stormwater infrastructure must function reliably during storms that test the system quickly and visibly. When it does not, impacts are immediate: roadway ponding, overtopping at crossings, erosion at outfalls and channels, and rapid escalation in public concern. As Southlake approaches buildout, the City's stormwater challenge increasingly becomes a lifecycle challenge maintaining and reinvesting in aging infrastructure, improving system resilience, and reducing avoidable risk rather than expanding the network into new growth areas. Asset management is the framework that makes that possible. Asset management is not simply "having a list of pipes." It is an organized practice that ensures the City consistently knows what it owns, what condition it is in, how critical each asset is to safety 60 STORMWATER MASTER PLAN I SOUTHILAKECOMPREHENSIVE STORMWATER MASTER PLAN and system function, and what work is needed to preserve performance over time. Without this structure, the program becomes reactive and difficult to defend. With it, staff can make repeatable decisions about maintenance, rehabilitation, replacement, and targeted capital improvements and can communicate clearly why certain actions are prioritized. A mature asset management program also supports credibility: it allows the City to show that stormwater is being managed continuously not only when storms occur through documented inspections, proactive maintenance, and reinvestment planning. This Stormwater Master Plan establishes the foundation for a future Stormwater Asset Management Plan. The future plan will go deeper into operational procedures, inspection forms, cost models, and renewal schedules. However, those elements cannot be built without the expectations set in this chapter. For that reason, the master plan establishes the "musts" that staff will carry forward: maintaining a GIS-based system of record, standardizing condition and risk evaluation, implementing proactive inspection and maintenance cycles, integrating work order history, and building a renewal forecast that informs long-range funding decisions. DEFINING THE ASSET PORTFOLIO AND SERVICE OUTCOMES Asset management begins by defining what outcomes the system is expected to deliver. Southlake's stormwater program is not evaluated solely by whether water ever ponds in streets; it is evaluated by whether the City is reducing risk and sustaining reliable outcomes that matter most to the community. Those outcomes include protecting life safety and emergency access, maintaining reliable function of public streets and infrastructure during common storm events, minimizing the frequency and severity of unacceptable hazards (especially at crossings and critical routes), reducing avoidable service disruptions caused by maintenance-related capacity loss, and supporting regulatory and water quality obligations under the City's stormwater quality program. These outcomes must align with the City's levels of service framework. Levels of service express "what the community expects"; asset management expresses "how the City delivers it." If the City sets service expectations but does not connect them to the asset inventory, inspection schedules, and reinvestment plans needed to maintain performance, then expectations and outcomes will drift apart over time. This is especially important in a built-out community where performance is sustained primarily through reinvestment and maintenance rather than new system expansion. This chapter therefore establishes the expectation that asset management is the mechanism that keeps levels of service realistic, measurable, and achievable. INVENTORY AS THE "SYSTEM OF RECORD" Chapter 4 established the City's approach to building and maintaining the stormwater inventory as a decision-support tool. Chapter 8 builds on that foundation by connecting the inventory to the asset management functions it must support: condition assessment, criticality evaluation, maintenance planning, and renewal forecasting. A complete and current GIS-based inventory is the foundation for managing stormwater as a public asset portfolio. It is not simply a map layer; it is the structured dataset the City relies on to make decisions connecting asset location, characteristics, ownership, access authority, inspection history, Policy Statement work history, and supporting documentation. If AM1. Manage stormwater infrastructure as stormwater information is fragmented across ublic assets by maintaining spreadsheets, paper files, or staff memory, ortfolio of understandingan inventory, the City will inevitably lose continuity, create and sed inconsistent responses, and struggle to defend and applying . investment recommendations. Staff must " implement a staged approach to inventory maturity. The inventory does not need to be perfect on day one, but it must be structured consistently and improve over time. This is the central building block for Policy Statement AM1, which directs the City to manage stormwater infrastructure as a portfolio of public assets by maintaining an inventory, understanding condition and criticality, and applying risk-based maintenance and reinvestment. ASSET HIERARCHY AND FUNCTIONAL ROLES A clear asset hierarchy helps staff manage the stormwater system as a connected network by organizing assets by function and by the role each plays in overall performance. Southlake's stormwater asset portfolio includes the following functional categories: Collection and Capture (gets runoff into the system) • Curb inlets, grate inlets, catch basins, inlet leads • Yard drains located in public right-of-way or within improved City easements (where applicable) • Small conveyance swales and roadside drainage features that route flow to inlets Underground Conveyance (moves water beneath streets and developed areas) • Storm drain pipes (all diameters/materials) • Junction structures (manholes, junction boxes, curb inlet boxes) • Headwalls and connections to channels/outfalls • Storm drain outfalls (where the underground system discharges to open conveyance) Open Conveyance (moves water through channels and drainageways) • Natural channels and drainageways within City maintenance limits • Improved channel segments (lined or reinforced reaches where applicable) • Roadside ditches and swales maintained by the City (where applicable) • Energy dissipation features and grade control elements (where applicable) Crossings and Constrictions (often control upstream water levels and safety outcomes) • Culverts (pipe culverts, box culverts, multi-cell structures) • Bridges and bridge openings • Low-water crossings and dip crossings (where present) • Roadway embankments and other constrictions that narrow flow paths Storage and Control (temporarily stores and regulates discharge) • Detention basins or regional storage features (public where applicable) • Outlet control structures (orifices, weirs, risers, valves where present) • Spillways and emergency overflow routes • Retention features where permanent pool or extended storage exists (where applicable) Water Quality and Post-Construction Controls (reduces pollutants and protects receiving waters) • Structural BMPs (water quality inlets, hydrodynamic separators where present, vegetated treatment areas) • Water quality features integrated into detention facilities(forebays,sediment traps,vegetated benches) • Permanent post-construction controls required through development (including privately owned controls with documented maintenance responsibility) 62 STORMWATER MASTER PLAN I SOUTHILAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN This hierarchy supports practical program decisions by helping staff determine: • Which assets require more frequent inspection and proactive maintenance; • Which assets are highest consequence if they fail or underperform (especially crossings, outfalls, and major conveyance); • Which asset types most commonly drive repeat service requests (inlets, debris-prone constrictions, erosion-prone outfalls); and • How maintenance and renewal actions preserve capacity, reliability, and public safety outcomes over time. CONDITION ASSESSMENT: BUILDING USEFUL CONDITION DATA Asset condition data becomes useful when it is consistent, repeatable, and tied to action. The City does not need expensive inspections everywhere to start managing condition effectively. Staff must begin with consistent screening and escalate to targeted and detailed assessments where risk warrants. The key is not to inspect everything at the same intensity, but to apply the right level of assessment to the right assets. At the screening level, staff must document field-observable conditions that influence performance: inlet blockage, sediment accumulation, visible damage at structures, erosion at outfalls, bank instability, and undermining signs near culverts and headwalls. Screening observations are often enough to trigger maintenance actions and to identify repeat issues that require follow-up evaluation. Targeted inspection then focuses on higher-consequence assets or recurring hotspots: culvert condition checks, pipe spot checks where failure risk is suspected, channel cross-section observations at erosion sites, and outlet control inspections where detention performance may be affected. Detailed assessment, such as CCTV inspection or structural evaluation, is reserved for locations where condition indicators and consequence justify the cost such as suspected pipe collapse, recurring roadway flooding with evidence of structural constraint, or severe instability threatening public assets. Condition data becomes significantly more valuable when paired with work history. A location that requires repeated inlet clearing every storm season may indicate upstream debris loading, design limitations in capture efficiency, or a chronic condition issue that warrants a different response than repeated cleaning alone. Similarly, recurring erosion repairs at the same outfall may indicate that the underlying discharge energy or channel geometry needs a more durable stabilization approach. Staff must therefore connect condition observations to the system of record and work order history, so the City can distinguish between isolated maintenance needs and emerging reinvestment priorities. CRITICALITY AND RISK-BASED DECISION-MAKING Risk-based decision-making is the backbone of repeatable and defensible stormwater investment. Staff must evaluate assets not only by condition, but also by criticality: what happens if the asset underperforms or fails.In stormwater,consequence can beirnmediateand high.Roadway overtopping can create safety hazards. Culvert failure can disrupt emergency access. Outfall undermining can damage upstream pipes. Channel instability can threaten roads, utilities, and private property. Water quality controls that fail can degrade receiving waters and create compliance risk. Risk combines consequence and likelihood. Consequence reflects impacts to life safety, emergency access, critical infrastructure, risk to insurable structures, regulatory consequences, and community disruption. Likelihood reflects observed defects, history of overtopping or blockage, debris loading propensity, erosion susceptibility, and sensitivity to downstream tailwater. Staff must formalize this risk approach in a way that is consistent and documented. The master plan does not require a complex scoring model immediately, but it does require the City to adopt a structured approach that can be applied consistently and explained clearly. PROACTIVE MAINTENANCE PROGRAM Proactive maintenance is one of the most cost-effective ways to sustain stormwater performance, reduce repeat complaints,and prevent avoidable failures. Many localized flooding concerns are driven not by "system undersizing," but by reduced effective capacity caused by debris, sedimentation, or minor defects that accumulate over time. A proactive program improves system reliability because it preserves capacity before storms occur, and it improves public confidence because residents can see that stormwater is managed continuously, not only after major events. Staff must implement a tiered inspection and maintenance structure that reflects risk. Critical assets such as key roadway crossings, known closure locations, major outfalls, and high-consequence channels require higher inspection frequency and event-triggered checks. Known hotspots require seasonal attention because repeat issues are predictable and preventable. Standard neighborhood assets require routine cycles that keep the system functional. Low-risk assets can be inspected as part of broader sweeps or as needed. The important point is that maintenance is not ad hoc; it is a scheduled program tied to the asset inventory and risk context. Staff must also maintain a storm season readiness approach. Before periods when heavy rain is more likely, critical locations should be inspected, debris-prone inlets should be cleared, and known constrictions should be checked. After major events, staff must document conditions and perform targeted follow-up inspections, Policy particularly where overtopping occurred, where erosion is likely, and where debris accumulationreliable - function is common. This practice directly supports Policyproactive inspection, Statement AM2, which directs the City to sustainrenewalpublic reliable system function through proactivechannels, facilities. inspection, maintenance, and renewal of public inlets, culverts, channels, and drainage facilities. WORK MANAGEMENT, DOCUMENTATION, AND REPEATABILITY Documentation is not administrative overhead; it is what makes stormwater management defensible and repeatable. Staff must maintain a structured work management approach that links service request intake, field observations, work performed, follow-up communication, and "next step" classification (resolved, monitor, study input, capital concept, private responsibility). Without this linkage, the City cannot reliably identify repeat hotspots, cannot demonstrate program activity, and cannot communicate consistently with residents about what actions were taken and why. Staff must standardize service request categorization so data can be analyzed over time. Categories should Policy be consistent and meaningful such as inlet blockage, localized ponding, erosion, culvert/crossing concern, CS1. Incorporate documented resident private drainage maintenance issue, suspected drainage concerns as an input to illicit discharge, and construction runoff concern. prioritization clear, objective Standardization allows the City to identify patterns, ensuringcriteria customer service justify proactive maintenance targeting, and use informs decisions while data-driven risk customer concerns as an input to planning while feasibilityand maintaining risk and feasibility as primary drivers. 64 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN - �Jj An I l4-y+ p `0, .......... This is the operational backbone for Policy Statement CS1, which directs the City to incorporate documented resident drainage concerns as an input to prioritization through clear, objective criteria, ensuring customer service informs decisions while data-driven risk and feasibility remain primary. RENEWAL AND REINVESTMENT PLANNING Stormwater assets have life cycles. Even well-maintained assets require renewal over time, and that renewal must be planned rather than discovered through emergencies. A renewal program includes planned rehabilitation (lining, structural repairs), planned replacement of end-of-life assets, channel stabilization where erosion threatens infrastructure, and modernization where outdated configurations create recurring issues. Renewal planning is the bridge between day-to-day maintenance and long-range financial sustainability. Poficj�Statement Staff must begin building a renewal forecast structure as FN1. Explore sustainable funding part of asset management maturity. A renewal forecast strategies including rate/fee connects what the City owns, what condition it is in, what it structure options every five costs to preserve service outcomes, and how funding needs years supported by appropriate evolve over time. This aligns directly with Policy Statement financial analysis, so resources FN1, which directs the City to explore sustainable funding align with documented needs, strategies every five years supported by appropriate financial k, and long-term obligations. analysis, so resources align with documented needs, risk, and long-term obligations. PUBLIC VS. PRIVATE RESPONSIBILITIES AND PRIVATE MAINTENANCE VERIFICATION A significant portion of recurring drainage concerns originate from private property maintenance issues. Private grading changes, obstructed swales, clogged yard drains, HOA-maintained channels, and privately owned detention facilities can materially influence neighborhood drainage outcomes especially when failures redirect flows, create downstream impacts, or contribute to repeat flooding and erosion. For that reason, the City must clearly define responsibilities and maintain tools that support private accountability where private facilities influence public risk. This is not only a matter of -' licV Statement program efficiency and fairness; it is also a matter Clearly define public versus of legal and fiscal accountability. Policy Statement private stormwater responsibilities AM3 directs the City to clearly define public versus strengthen communicates, private stormwater responsibilities and strengthen supports, and verifies private how the City communicates, supports, and verifies maintenance obligations where those private maintenance obligations where those facilities facilities influence public risk. Texas law places important limits on how public funds can be used on private property. As the City explains in its published guidance, homeowners are ultimately responsible for drainage on their property because state law prohibits cities from making improvements on private land. In practical terms, this means the City generally cannot use public funds to maintain or improve private drainage features such as HOA-owned channels, private swales, or privately owned detention facilities simply because those facilities affect drainage outcomes. Public expenditures must be tied to a legitimate public purpose and supported by legal authority; absent that authority (for example, public right-of- way, dedicated public drainage easements, or other legally established public control/interest), the City's role is to require private parties to meet their responsibilities, not to take on private maintenance with public resources. This constraint is consistent with the Texas Constitution's prohibition on granting public money for private benefit without a valid public purpose and appropriate controls. Because of these constraints, staff must treat private responsibility clarity as a core program function not an informal message delivered only during complaints. The City must provide consistent guidance documents and maintenance checklists for HOAs and residents, and it must ensure that post-construction maintenance obligations for private stormwater controls are documented and enforceable. When complaints indicate a likely private facility failure that materially influences public risk, staff must use a consistent process: document observed conditions, identify the responsible party, request maintenance documentation where appropriate, and provide clear direction on corrective actions expected. Education remains the first step, consistent with policy in this Plan, but enforcement must remain available as a targeted backstop when private failures create hazards or violate ordinances. This approach supports both public safety and fairness: the City maintains what it owns and is responsible for, while private owners and HOAs maintain what they own especially when private systems influence neighbors and public outcomes. 66 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN Ir �J it , jA pq 4 ram' ComS_ I 1 V r I VID- 74 Y t s, O Fes, IIII � Al CHAPTER 9: DEVELOPMENT STANDARDS ANDFRAMEWORK As Southlake approaches buildout, the most consequential stormwater decisions are no longer about where to build new infrastructure. They are about whether each parcel change, redevelopment project, and corridor reinvestment is completed in a way that responsibly manages runoff and does not shift risk to neighbors or downstream properties. Development standards are the mechanism for ensuring that happens. They establish expectations before construction begins, when stormwater solutions are least costly and most practical to incorporate, and they create consistency and fairness by ensuring similar projects are evaluated using the same transparent requirements regardless of who is asking or where the project is located. Southlake's existing development and post-construction stormwater standards provide a working foundation for this framework. This chapter builds on that foundation by identifying where alignment with regional best practices, modernization of technical criteria, and stronger post-construction accountability will improve long-term outcomes. It addresses how buildout-based evaluation protects the public system from cumulative impacts, how iSWM-aligned practices support runoff reduction and water quality in a redevelopment context, how standards documentation supports long-term defensibility, and how private system accountability keeps post-construction controls functioning as intended over time. WHY DEVELOPMENT POLICY IS ONE OF THE MOST EFFECTIVE STORMWATER TOOLS Development and redevelopment standards are among the most cost-effective stormwater tools available to a city because they prevent avoidable risk from being created. Once a site is built, correcting stormwater problems often requires retrofits that are expensive, disruptive, and difficult to coordinate across multiple property owners. By contrast, clear standards applied during plan review and permitting allow stormwater performance, downstream protection, and long-term maintenance responsibilities to be addressed at the point when changes are least expensive and most feasible to incorporate. This is especially important in a built-out community like Southlake, where the majority of change occurs through incremental redevelopment and reinvestment rather than new greenfield growth. Southlake's stormwater decision environment is increasingly shaped by redevelopment and small incremental changes that accumulate over time. A single project may appear minor, but over a decade, dozens of projects within a sub-basin can alter runoff timing and volume, increase downstream erosive stress, and compound constraints at crossings and channels. Development 68 STORMWATER MASTER PLAN I SOUTHILAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN standards therefore serve two essential purposes: they protect the public system and downstream properties from cumulative impacts, and they provide predictability and fairness. Predictability matters for applicants, because it clarifies what is required and reduces uncertainty. It matters for staff, because it enables consistent review and defensible decisions. It matters for residents, because it reduces the chance that incremental changes will shift risk onto neighborhoods over time. Because stormwater is a watershed issue, standards also benefit from regional consistency. Creek systems and floodplains cross jurisdictional boundaries, and stormwater impacts accumulate across a basin. Coordinated practices help reduce cumulative impacts and support shared expectations across jurisdictions, especially for developers working throughout the region. This Plan supports modernization of standards in a way that preserves Southlake's local priorities while aligning with regional best practices where appropriate. BUILDOUT, DOWNSTREAM PROTECTION, AND CUMULATIVE IMPACT MANAGEMENT Policy Statement DR1 continues and reinforces the City's existing requirement that new development and redevelopment demonstrate that stormwater impacts are responsibly managed for full buildout conditions, including upstream and downstream considerations where applicable. This expectation is already established in the City's drainage ordinance, which requires all runoff calculations to be based on ultimate conditions of development, mandates drainage studies for all development prepared by a licensed professional engineer, and gives the City authority to require off-site analysis based on project scale and downstream impact (City of Southlake Code of Ordinances, Chapter 9.5, Sections 9.5-22, 9.5-47 through 9.5-43, and 9.5-722). The practical reason for this policy direction is cumulative impact management. In a built-out community, system performance and flood risk are shaped less by any single project and more by Policy the combined effect of many projects over time. -1. Continue to administer enforce If each project is evaluated only under immediate stormwater regulations _ conditions, the City risks approving incremental development changes that, in aggregate, increase peak flows, demonstrate responsible management worsen downstream constraints, or accelerate impacts under full buildout conditions, erosion in channel corridors. Buildout-based consistent with applicable state law and evaluation provides a long-term, equitable lens City ordinances, including upstream that ensures projects contribute to responsible downstreamand where watershed outcomes rather than creating future L" applicable. problems for neighborhoods and the public system. A consistent approach to buildout evaluation does not require every project to complete a complex model. It requires clarity about expectations and consistency in assumptions. For smaller projects in low-risk settings,a simplified analysis may be appropriate. For larger projects,sensitive locations, and areas with known downstream constraints, a more robust evaluation is warranted. The master plan supports the following core principles for Policy Statement DR1 implementation: documentation of assumptions, evaluation of downstream sensitivity where known, and long-term reliability of detention/outlet controls where they are part of the mitigation approach. The City's goal is not to overburden redevelopment; it is to ensure that redevelopment is not increasing risk for others and that mitigation strategies remain functional over time. ALIGNING WITH ISWM: WHY IT BENEFITS SOUTHLAKE Integrated stormwater management provides a regional framework for addressing the full set of stormwater outcomes that matter in redevelopment: water quality, streambank protection, conveyance, and flood control. NCTCOG's iSWM Criteria Manual is designed for cities and counties to use as a component of stormwater-related development regulations, and it provides a structured way to evaluate stormwater practices beyond simply "moving water offsite." (iswm.nctcog.org) For Southlake, aligning with iSWM where appropriate is beneficial for three main reasons. First, iSWM supports a balanced approach that matches Southlake's long-term needs as the city approaches buildout. Traditional drainage criteria often focus heavily on conveyance and peak rate control, but in built-out communities, the challenges are often tied to short-duration intensities, localized ponding, downstream constraints, and channel stability. iSWM's structure intentionally includes streambank protection and water quality, which are directly relevant to Southlake's creek corridors, erosion concerns, and MS4 program responsibilities. It therefore provides a framework that is compatible with how the City must manage stormwater as a long-term asset rather than a one-time development review item. Second, iSWM alignment improves consistency and predictability. Developers, engineers, and contractors who work across North Texas benefit from a common "baseline" framework. When a city's standards are aligned with regional guidance, while still reflecting local context, it reduces confusion, improves submittal quality, and supports more consistent review outcomes. That predictability helps Southlake protect stormwater performance without creating avoidable administrative friction. It also supports defensibility: when standards reference established regional practices, the City's requirements are easier to explain and justify. Third, iSWM supports practical runoff reduction strategies that directly address cumulative impacts. DR2 emphasizes feasible practices that mimic natural hydrology soaking it in, slowing it down, and spreading it out. These strategies help reduce runoff volume and peak flow, improve water quality, and reduce erosive energy delivered to channels. iSWM provides recognized guidance on how to evaluate and apply these measures in development and redevelopment contexts. The master plan supports incorporating iSWM-aligned concepts where feasible, recognizing that maintenance responsibility must be clear and reliable and performance should be measurable. Examples that are typically most practical in Southlake include vegetated swales and bioretention elements in streetscapes and parking lot reconstruction, distributed capture and filtration features where maintenance can be assigned clearly, and preservation or enhancement of natural drainage corridors where feasible. STANDARDS MODERNIZATION AND DOCUMENTATION Policy Statement DR4 directs the City to ensure stormwater ordinances and technical standards remain current, enforceable, and regionally aligned Policy Statement where appropriate, recognizing that stormwater is a watershed issue that benefits from coordinated ' - " ordinances practices. Standards modernization is not primarily and technical standards about adding complexity; it is about improving enforceable, and regionally clarity, fairness, and enforceability while ensuring where appropriate, in rec' ' technical evaluation is scaled to risk. Southlake's stormwater current ordinances and technical standards already benefits practices. address many of these elements. Modernization in this context means reviewing those standards for consistency with current rainfall data, regional alignment, and post-construction accountability requirements, not replacing the foundation that already exists. A modern standards framework typically includes: a clear hierarchy of reference manuals; use of current rainfall data; tiered drainage study triggers tied to project scale and risk context; clear • STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN �� a. i v s r downstream assessment expectations;consistent detention and outlet documentation requirements; mapping and digital submittal standards that support recordkeeping and future use; and post- construction control maintenance documentation that ensures mitigation remains functional long- term. Policy Statements WS4 and WSS reinforce the need for enhanced evaluation in higher-risk areas and for maintaining decision-support tools over time. Together, these policies support a standards framework that is context-sensitive and data-supported rather than one-size-fits-all. In Southlake's context, modernization also supports asset management and customer response. Clear documentation requirements improve the City's ability to understand how redevelopment affects drainage patterns, to verify post-construction controls, and to respond to future complaints with records that explain how a site was designed to manage runoff. This master plan therefore treats "documentation quality" as a stormwater performance tool. FLOODPLAIN ADMINISTRATION, MAP REVISIONS, AND NO-ADVERSE-IMPACT PRACTICE Floodplain administration is a core component of stormwater risk management and N FI P participation. Development in flood hazard areas must be evaluated and permitted to prevent increased risk. This includes ensuring that fill, structures, and other modifications do not reduce floodplain conveyance or increase flood elevations in a way that shifts risk to others. It also includes consistent application of FEMA concepts such as substantial damage/substantial improvement, which provide objective triggers for compliance upgrades. In certain cases, development or public projects require FEMA map revision processes (CLOMR/ LOMR) when changes affect flood elevations or floodway conditions. These processes require careful documentation and coordination and support factual communication about flood risk. In a master plan context, the key message is that floodplain management is an ongoing risk-reduction responsibility that protects residents and maintains eligibility and consistency within the NFIP framework. CHANNEL PROTECTION, EROSION CONTROL, AND MAINTENANCE ACCESS r' 1 Erosion control standards and channel protection practices reduce long-term maintenance burdens and protect channel stability. As runoff patterns change over time, channels can experience increased erosive stress. Outfalls can undermine, banks can retreat, and sediment loads can increase downstream capacity issues. Development standards therefore play a direct role in long-term channel stability by requiring appropriate outfall stabilization, protecting drainageway corridors, and avoiding changes that accelerate erosive conditions. Where appropriate,stream buffers and drainage corridor protections can preserve flood conveyance, maintain access for future maintenance, and reduce conflicts when channels migrate or erode over time. This master plan supports evaluating buffer approaches and channel protection provisions in a practical way that reflects Southlake's context and balances private property considerations with long-term public risk reduction. PRIVATE SYSTEMS AND POST-CONSTRUCTION CONTROLS: LONG-TERM ACCOUNTABILITY Private drainage systems and post-construction stormwater controls often determine long-term neighborhood outcomes. When private controls are not maintained, local issues can become chronic and can influence public risk, especially where private facilities affect downstream conveyance or contribute to repeated nuisance flooding and erosion. This master plan supports strengthening maintenance agreement requirements, documentation and verification practices, HOA guidance and compliance tools, and enforcement as a targeted backstop when necessary to protect public risk. Maintenance agreements are the mechanism that connects the benefit a control was designed to provide with the long-term obligation to preserve it. Currently, maintenance agreements are required as part of the development approval process for projects that include post-construction stormwater controls. As Southlake approaches buildout and the inventory of privately maintained stormwater controls grows, the City may wish to consider whether a broader, citywide maintenance agreement framework would strengthen long-term accountability for existing controls that were approved under prior standards or that were installed without a formal recorded agreement. This is a policy question that warrants further evaluation, including a review of legal authority, administrative feasibility, and equitable application across property types. This plan does not resolve that question but does identify it as a relevant consideration for future ordinance review. Development review does not end at final inspection. If post-construction controls are part of the mitigation strategy, the City must be able to confirm that responsibilities are assigned clearly and that tools exist to verify maintenance where needed. Education should remain the first step, but the City must preserve the ability to pursue compliance actions when private failures create hazards or violate ordinances. This ensures fairness across the community and protects both public infrastructure and neighboring properties from preventable impacts. 72 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN i q I �' yU V40 OW qqANJv r+1 " .Y• �. �. r M1' �` w �- Wt�'. �' '•`'" -4 �* sc -� - ?Ii'�¢ �� 1a >• �q�.��f � s,'�4 f'.� � "e�,'Y` ti. 6�r - � :Y�.y �, � �- '�-✓t s+' ��r �8t ��,�ti�7!'4`r+S-_+i,�'`.z •`:� r r e � c . r s- 0 -. R a AL r cs o'er IN14 '' - N _ o�4- -i�: - '°Y. C • ,-'.r 34 y ' - ,N` ,. f Hr� `r y '� :� �'+F"�il�� L� �it' r' 7 �'��. r �✓a � �r >_w �`� ��' �"''E¢ +,4�� 'r+°, ,y�¢��.r�Ci�i� �.rl.. R 4 a.. .: `y' ; r�>•. _ �, '�a 'l �h $sr' vz�"t"':3s�a i ad .dx��i.-r' N L• h�- r R e Ll !r 1 � I • � t Wtl. r y4 r�A R FA'4 t 771 Ja CHAPTER 10: PARTNERSHIPS, AND FUNDING Stormwater is one of the City services residents experience most directly, and usually during stressful moments — heavy rainfall, street ponding, roadway overtopping, or erosion near homes and trails. In those moments,the City's response is judged not only by what work is performed but by how clearly the City communicates what is happening,what actions are being taken,and what residents or HOAs may need to do.A plan that establishes strong policy but cannot explain itself clearly to the people it serves is incomplete.Chapterl 0 addresses that gap by establish i ng thecommu nication,education,and customer service framework that allows this plan's policy direction to become consistent day-to-day practice. Stormwater performance is also sustained through continuous obligations, not just occasional projects. Maintenance, inspections, emergency response, regulatory compliance, study updates, and long-term renewal all require stable resources and coordinated effort. This chapter connects implementation to partnerships and funding, defines how concerns move from intake to response and from recurring hotspot to capital planning consideration, and frames funding not as a separate financial discussion but as the necessary resource foundation for everything the plan commits to delivering. WHY IMPLEMENTATION CLARITY MATTERS A Stormwater Master Plan succeeds only if it produces consistent action and consistent communication. Stormwater concerns are often event-driven, highly visible, and emotionally charged, which can make programs drift toward reactive decision-making unless staff have clear pathways for what happens next. Implementation clarity means the City can respond to a concern in a way that is repeatable, transparent, and aligned with the plan's policy direction regardless of who receives the call, what neighborhood it comes from, or how recently a similar issue occurred elsewhere. This consistency is not only important for internal efficiency; it is one of the City's most effective tools for building public trust over time. Implementation clarity also strengthens defensibility. When a resident reports flooding or erosion, the City must be able to explain whether the issue is driven by public infrastructure capacity, maintenance condition, natural exceedance behavior, downstream constraints, or private drainage responsibilities and what the City will do as a result. This chapter establishes those pathways without prescribing a rigid scoring model. Instead, it defines the decision logic that connects day-to-day customer service to long-range planning: how recurring hotspots are identified and addressed, 74 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN when an issue moves from routine maintenance to targeted investigation, when a watershed study update is warranted, and when a capital improvement concept should be developed for future funding consideration. This chapter also ties those pathways to funding, because stormwater programs cannot be sustained without stable resources to support both routine service delivery and long-term reinvestment. CUSTOMER SERVICE AND COMPLAINT RESPONSE FRAMEWORK Stormwater concerns are visible, urgent, and often personal for residents because they affect access, PolicV Statement safety, and property. A consistent customer response resident framework improves trust and reduces repeat issuesdocumented t to by ensuring similar concerns receive similar handling, objective documentation, and communication. This framework customer service also protects the integrity of prioritization. Policyensuring Statement CS1 does not require that customer decisions while data-driven concerns become the primary driver of investmentfeasibility decisions; it requires that concerns be documented and incorporated as an input to prioritization through clear, objective criteria, while data-driven risk and feasibility remain primary. In practice, that means complaint data must be treated as actionable information captured consistently, categorized correctly, and used to identify patterns rather than handled as isolated incidents. A reliable framework starts with standard intake practices so staff obtain the information needed to respond appropriately. Intake should capture: • Precise location (address + nearest intersection if applicable) • Issue type and description • Timing (during storm vs. after storm) • Photos/video if available • Known safety hazards (swift water, roadway overtopping, blocked access) • Issue on public or private property (if known at intake) Once intake occurs, triage should classify the concern by consequence and response urgency. A practical triage framework includes: Category A - Life Safety/ Emergency Access: roadway overtopping with hazardous conditions; swift water potential; blocked critical routes; immediate hazards (collapsed inlet, severe undermining). Category 6 - Public Infrastructure Condition: pipe/culvert structural concern; inlet collapse; severe erosion threatening public assets; outfall undermining tied to public system. Category C - Localized Nuisance / Recurring Ponding: street ponding that clears; repeat low areas; overflow without immediate hazard; locations that may require investigation if recurring. Category D - Private Drainage/ Private Maintenance: yard drains, swales, private channels, HOA detention, grading obstructions, private maintenance failures. The most important part of this framework is the response pathway and communication closure. A resident does not only want the City to look at an issue; they want to understand what was observed, what the City will do, what they (or their HOA) may need to do, and what timeline applies. Response pathways should therefore include both operational steps and communication steps: Category A: immediate response, documented closure actions as needed (barricades, notifications), coordination with public safety, and a short follow-up message explaining what occurred and what will be reviewed after the event. Category B: timely field inspection, work order creation, and repair/rehabilitation planning if needed, with a follow-up message that explains condition findings and next steps. Category C: maintenance evaluation and monitoring, with the location added to a hotspot list if recurring and flagged for study input when warranted. Category D: clear private responsibility guidance, HOA/resident toolkit reference, and documented follow-up, with enforcement used only if hazards or ordinance violations exist. Finally, the City must establish a feedback loop so customer concerns improve planning rather than creating reactive churn. Complaint data should be used to identify recurring hotspots for targeted maintenance, inform watershed study sequencing, evaluate whether ordinances or standards need refinement,and improve education targeting particularly around private maintenance responsibilities. COMMUNICATIONS, PUBLIC EDUCATION, AND COMMUNITY PARTNERSHIP Public education is required under MS4 program expectations and is also one of the most effective tools for reducing avoidable stormwater issues such as clogged inlets, illicit dumping, and private maintenance failures. TCEQ's MS4 framework emphasizes program implementation and measurable goals by MS4 level, which means education is not simply a "nice to have" it is an ongoing requirement that must be delivered and documented. In Southlake's context, education is also essential to customer service integrity because many stormwater complaints are driven by misunderstandings about how drainage systems work, what exceedance looks like during extreme storms, and where public versus private responsibilities begin and end. Policy Statement EC1 prioritizes education as the first step in stormwater compliance, and Policy Statement EC2 directs the City to go beyond baseline MS4 requirements to address documented awareness gaps. Together they establish education not as a checkbox activity but as a core program function. programPolicy Statement Policy Statement EC1. Prioritize education as the first EC2. Enhance the City's stormwater public step in stormwater compliance through education baseline expanded resident and HOA awareness of requirements to address documented responsibilities, common violations, and awareness gaps and improve community practical maintenance steps, supported understanding of risks, responsibilities, and by fair and consistent enforcementprevention. voluntary compliance • - • . _A 11 L_ For Southlake, communications and education should be structured around consistent messages delivered through predictable channels. The objective is not to overwhelm residents with technical information; it is to provide clear, repeatable guidance that improves outcomes and reduces repeat complaint cycles. A robust education program should include three complementary tiers: Tier 1: Foundational stormwater literacy (year-round). These messages explain what stormwater is, where storm drains discharge, why dumping and litter matter, and how to report issues. They also introduce the City's stormwater responsibilities and the role of the MS4 program in protecting receiving waters. 76 STORMWATER MASTER PLAN I SOUTHILAKE • STORMWATER MASTER PLAN Tier 2: Private maintenance and HOA responsibilities (priority emphasis). This tier is critical in Southlake because private maintenance issues are among the most common drivers of recurring concerns. Messaging should explain what private detention and outlet controls are, what failure looks like, why blocked swales and altered grading can cause downstream impacts, and how HOA's and property owners can inspect and maintain private drainage features. Clear, nonjudgmental messaging is essential: the goal is to help residents understand how to prevent issues, not to assign blame during a crisis. Tier 3: Post-event communications and safety messaging (event-based). After major storm events, residents want to know what occurred and what the City did. Post- event messaging should describe impacts in plain language, include safety messaging (especially around roadway overtopping), explain what actions were taken (closures, inspections, debris clearing), and provide clear "what you should do next" guidance for residents and HOA's. Community partnership strengthens all three tiers. HOA's, schools, neighborhood groups, and business associations can help distribute consistent messages. Partnerships also create credibility and reach: residents are more likely to absorb guidance when it is repeated through multiple trusted channels. The master plan supports building a communications calendar and a toolkit approach so messaging is not reinvented each storm season. IMPLEMENTATION PROGRAM STRUCTURE AND PHASING Implementation is not only capital projects. A complete stormwater program includes routine service delivery, regulatory compliance activities, data and decision-support improvement, and targeted capital investments. The Stormwater Master Plan organizes implementation into two categories, programmatic actions and capital improvements because both are necessary to sustain performance, and both must be funded and documented. Programmatic actions include inspection cycles and hotspot maintenance, inlet cleaning and debris management, culvert and outfall inspections, storm event documentation and after-action review, education deliverables and compliance outreach, ordinance evaluation tasks and standards updates, and watershed study cycle management and decision-support tool upgrades. These actions are the "always-on" work that prevents avoidable failures and builds the information foundation for defensible long-range planning. Capital projects address structural constraints and high-consequence vulnerabilities such as crossing improvements, conveyance upgrades, channel stabilization and erosion control, outfall stabilization, and targeted storage/control improvements where feasible. Capital projects should be phased and coordinated with other City initiatives whenever possible, because coordination can reduce cost, reduce disruption, and improve outcomes (for example, integrating drainage improvements into roadway reconstruction or open space enhancements). FUNDING, COST RESPONSIBILITY, AND PARTNERSHIPS Stormwater services require stable resources because the program includes continuous obligations, not only occasional projects. Funding must support routine maintenance and inspection, emergency response and storm operations, regulatory compliance implementation and reporting, long-term renewal and reinvestment, and targeted capital improvements. Without stable funding, programs become reactive: maintenance is deferred, assets degrade, and the City is forced into more costly emergency repairs and higher-impact disruptions. The Stormwater Master Plan ties funding discussions to documented needs so conversations remain transparent Policy Statement and defensible. Policy Statement FN1 supports evaluating sustainable funding strategies, including rate/fee structure FN1. Explore sustainablefunding options every five years supported by appropriate financial strategies including analysis, so resources align with documented risk and long- structure options term obligations. Policy Statement FN2 supports equitable I years supportedappropriate" funding approaches that reflect demand drivers and system financial analysis, so resources impacts while maintaining transparency and administrative align with documented needs, feasibility. Policy Statement FN3 supports pursuing grants, obligations. interlocal partnerships, and cost-sharing opportunities especially where privately owned facilities materially influence public risk while maintaining clear standards and accountability for outcomes. Policy Statement FN12. Evaluate tiered . equitable Funding discussions should be grounded in the obligations fundingapproaches" - flect documented throughout this master plan: the asset stormwaterdemand - rs and inventory and renewal forecast foundation (Chapter 8), systemimpacts, documented hotspot maintenance needs, watershed transparency, study cycle commitments, known high-consequence administrative feasibility. vulnerabilities (crossings, erosion threats), and the staffing and implementation needs required for stormwater quality compliance. This framing is important because it clarifies that stormwater funding is not simply about "building projects." Policy It is about sustaining service levels and protecting public safety and infrastructure reliability over time. FN3. Pursue grants, interlocall partnerships, Partnerships are a key implementation strategy, not an opportunities especiallyfor afterthought. Regional watershed studies and shared privately owned modeling can reduce duplication and improve basin-wide materially influence public outcomes. Interlocal coordination for creek corridors can while maintaining clear standards address shared constraints that no single city can solve alone. and accountabilityoutcomes. Grant partnerships can support mitigation and stabilization projects, particularly where benefits extend beyond the city's boundaries or align with broader resilience goals. Cost-sharing approaches may also be appropriate where privately owned facilities materially influence public risk, but such approaches must be structured carefully with clear standards and accountability. In addition, the City must remain mindful of legal constraints related to the use of public funds on private property, and partnership approaches must be designed to maintain appropriate public purpose and compliance. 78 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE r"4 C. _ rT • `r 46MP� 41 t h Y l i w L��'•�y. <Y '.!`` s x,n\, _ M� i ` r CHAPTER 11 : PLAN ADOPTION & PUBLIc ENGAGEMENT This chapter documents how the Stormwater Master Plan is advanced from a drafted policy framework into an adopted element of Southlake's Comprehensive Plan. Southlake's comprehensive planning process is designed to be transparent, iterative, and community-informed. Rather than treating adoption as a single vote at the end, the City uses a sequence of public meetings to build understanding, gather feedback, and refine plan direction before formal action is taken. This approach helps ensure the plan reflects community priorities, aligns with related Comprehensive Plan elements, and provides clear implementation direction for staff and decision-makers. The adoption process also reinforces the City's commitment to meaningful public involvement. Stormwater is experienced most visibly during storm events, and residents often form impressions based on localized conditions and personal impacts. A structured adoption process provides opportunities to ask questions, review plan concepts, and provide input in a setting that is designed for dialogue not just formal hearings. Through Corridor Planning Committee work sessions, the SPIN Town Hall Forum/Open House, Planning and Zoning Commission review, and City Council readings, the City is able to incorporate feedback while keeping the plan grounded in objective data, regulatory requirements, and long-term infrastructure stewardship. ROLE OF THE CORRIDOR PLANNING COMMITTEE IN PLAN DEVELOPMENT As part of the Comprehensive Plan update process, the Corridor Planning Committee provided early guidance and direction that helped shape the development of the Stormwater Master Plan (along with the Water and Wastewater plans being updated concurrently). The Committee meetings were structured to build shared understanding, review baseline conditions, and provide policy-level feedback that staff could translate into clear plan direction. This step is important because the Committee's role is not to design project solutions, but to help establish priorities, expectations, and policy choices that guide how the City manages its infrastructure over time. The City's Comprehensive Planning webpage documents the Committee schedule and topics, including an initial overview meeting and subsequent plan-focused meetings. The Corridor Planning Committee meetings on January 13, 2026 (overview of the Stormwater, Water, and Wastewater plan updates), January 28, 2026 (Stormwater Plan focus), and February 10, 2026 (Water/Wastewater focus) provided staff with direction that informed refinement of plan chapters, policy statements, and implementation pathways. Presentation materials from these meetings are posted online to support ongoing transparency and resident awareness. : • STORMWATER MASTER PLAN I SOUTHLAKE • STORMWATER MASTER PLAN COMMUNITY ENGAGEMENT APPROACH AND COMMUNICATION STRATEGY The City's engagement approach for these plan updates is built around multiple touchpoints, clear communication, and accessible ways for residents to participate. The Comprehensive Planning webpage emphasizes that presentation materials are posted online after meetings so residents can stay informed even if they cannot attend in person.This plan also supports expanding communications beyond meetings by using a consistent set of stormwater messages and tools with plain-language explanations, clear public vs. private responsibility guidance, and predictable "what happens next" pathways for common concerns so resident feedback is informed by shared understanding. A central public engagement milestone for these plan updates was the SPIN Open House/Town Hall Forum held on March 2, 2026. The City's Comprehensive Planning page noted that this open house was intended to provide residents an opportunity to share feedback with staff on the Stormwater, Water, and Wastewater Master Plans. This event is especially valuable because it is designed for dialogue and questions in a less formal setting than a public hearing, allowing staff to identify recurring themes, clarify misunderstandings, and document input that can be reflected in final plan edits before adoption. FORMAL REVIEW AND ADOPTION SCHEDULE After incorporating Committee direction and public input, the Stormwater Master Plan proceeds through the City's formal adoption pathway. The anticipated schedule for public consideration and adoption includes: January - April 2026 — Public Feedback Opportunity with City staff March 2, 2026 — SPIN Open House / Town Hall Forum (public feedback opportunity) March 5,2026—Planning and Zoning Commission meeting(formal reviewand recommendation step) April 7, 2026 — City Council 1st Reading (initial consideration) April 21, 2026 — City Council 2nd Reading (final adoption consideration) This stepwise schedule serves two purposes. First, it provides multiple opportunities for public awareness and feedback before final adoption. Second, it supports good governance by ensuring the plan is reviewed through the City's established boards and commission structure prior to Council action. The Planning and Zoning Commission review provides an additional public forum to evaluate the plan's consistency with the Comprehensive Plan framework and its policy direction, while City Council readings provide the final legislative pathway for adoption. HOW FEEDBACK IS USED TO REFINE THE PLAN Public engagement and board/commission review are not symbolic steps; they are intended to influence the final document. Feedback from the Corridor Planning Committee and SPIN Open House is used by staff to refine plan narrative clarity, strengthen communication tools (especially public education and customer response frameworks), ensure policy statements are supported by clear rationale in earlier chapters, and confirm that implementation pathways are understandable. Feedback may also identify where additional graphics, maps, or plain-language explanations are needed to make stormwater concepts accessible to residents. The City's approach also emphasizes follow-through. Meeting materials are posted publicly, and the plan is refined transparently so residents can see that input was received and addressed. Where feedback reflects localized concerns, staff use documented information such as service request trends, known hotspot inventories, study findings, and field observations to determine whether the appropriate plan response is improved maintenance targeting, a study update priority, a standards/ ordinance topic, or a capital planning consideration. Input gathered through the Corridor Planning Committee meetings and the SPIN Open House produced a consistent set of community themes that shaped the final direction of this plan. • Technical Standards and Criteria. Participants asked whether the criteria used to evaluate drainage performance are current and consistently applied, and whether development review standards reflect today's conditions rather than those in place when ordinances were originally adopted. • Stormwater Fees and Funding Adequacy. Participants asked whether current funding levels are adequate to support the program the community expects, and whether the fee structure is evaluated on a regular, transparent cycle. • Public vs. Private Maintenance Responsibilities. Confusion about who is responsible for maintaining drainage features was among the most frequently raised themes. Participants described situations involving easements, channels, and creek corridors where they received inconsistent guidance and where long-standing problems persisted without clear accountability. • Up-to-Date Watershed Evaluations. Participants asked whether the studies the City relies on are current, and whether infrastructure is performing as designed relative to today's development patterns and watershed conditions. • Development Impacts on Existing Drainage. Participants raised consistent concerns that new development and redevelopment were shifting stormwater burden onto established neighborhoods, and asked how the City ensures that downstream impacts are accounted for during development review. • Debris at Inlets and Culverts. Participants identified debris accumulation as a visible and recurring concern and asked whether the City inspects and maintains these structures proactively or primarily responds after problems are reported. Taken together, these themes emphasize that stormwater management in Southlake is not simply an engineering challenge, but also a transparency and communication challenge. Each of these themes is directly tied to a specific policy statement within this plan, ensuring that what the community asked for is reflected in what the City has committed to do. DOCUMENTATION AND ADOPTION OUTCOME Upon adoption, the Stormwater Master Plan becomes an adopted element supporting the City's Comprehensive Plan and provides policy direction that guides: 1. Stormwater program priorities and communication practices 2. Watershed study sequencing and decision-support tools 3. Ordinance and standards modernization 4. Asset management foundations and future asset management plan development 5. Long-range funding and partnership strategies Adoption also establishes a clear basis for accountability, what the City committed to do, how progress will be tracked, and how residents can stay informed as implementation proceeds. 82 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN AFTER ADOPTION Once adoption is completed, the Stormwater Master Plan transitions from a planning document into the City's formal policy framework for Stormwater decision-making and program delivery. Adoption establishes an official, Council-recognized basis for how the City will define stormwater service expectations, evaluate risk, prioritize actions, and communicate consistently with the community. It also clarifies how technical studies and field observations will be used to support decisions and how the City will align stormwater work with the Comprehensive Plan's broader goals for infrastructure stewardship, neighborhood stability, resilience, and quality of life. In practical terms, adoption provides staff with clear direction to apply the plan's policy statements when evaluating maintenance needs, capital improvement concepts, development standards, and regulatory responsibilities. Following adoption, the City applies the plan in three primary ways. First, it becomes the guiding reference for program implementation, supporting consistent customer response pathways, improved public education and communications, and clearer public versus private responsibility messaging, including HOA and private maintenance guidance. Second, it becomes the organizing framework for decision support and long-range planning, formalizing the cycle and sequencing of watershed studies, strengthening how the City maintains and uses stormwater inventory and performance information, and establishing the building blocks for a future Stormwater Asset Management Plan.Third, it becomes the foundation for sustainable funding and partnerships allowing the City to connect documented needs (maintenance obligations, regulatory requirements, renewal and reinvestment, and prioritized improvements) to transparent funding discussions and to pursue grants, regional collaboration opportunities, and cost-sharing approaches where appropriate and legally supported. STORMWATER MASTER PLAN POLICY STATEMENTS PERFORMANCESYSTEM Convey,store,and manage stormwaterto protect life safety, public infrastructure, SP1 and property through proactive risk reduction and sound system performance practices. Define and maintain clear stormwater levels of service for safety, reliability, and SP2 response, using these expectations to consistently guide planning, maintenance, and investment decisions. SP3 Prioritize projects and actions first by life safety, followed by consistent, objective criteria that support defensible and repeatable decision-making. ASSET MANAGEMENT & MAINTENANCE (AM) Manage stormwater infrastructure as a portfolio of public assets by maintaining AM1 an inventory, understanding condition and criticality, and applying risk-based maintenance and reinvestment. AM2 Sustain reliable system function through proactive inspection, maintenance, and renewal of public inlets, culverts, channels, and drainage facilities. Clearly define public versus private stormwater responsibilities and strengthen AM3 how the City communicates, supports, and verifies private maintenance obligations where those facilities influence public risk. WATERSHED SCIENCE, STUDIES & DECISION SUPPORT Ground watershed-based planning and stormwater investment decisions in WS1 technical studies that account for upstream and downstream impacts and overall system behavior. Establish a regular program of technical stormwater and watershed studies, WS2 conducting at minimum one basin-level evaluation every other year sequenced by risk, to refresh priorities, validate assumptions, and support long-term capital planning. Combine data-driven analysis with staff field knowledge to act early in emerging WS3 problem areas while ensuring major investments are supported by appropriate technical evaluation. Explore enhanced planning and design evaluation in higher-risk flood areas WS4 based on technical study findings, recognizing that risk is not uniform across the city and standards may need to vary by context. Regularly explore opportunities to improve the City's stormwater decision- support tools by evaluating, procuring, and/or participating in advanced WSS watershed and infrastructure modeling platforms and regional collaboration studies as warranted, so staff can consistently quantify current and future development impacts, test system performance under defined storm events, and support knowledgeable, data-driven capital planning, and standards updates. DEVELOPMENT . • • D ' Continue to administer and enforce stormwater regulations requiring new development DR1 and redevelopment to demonstrate responsible management of impacts under full buildout conditions, consistent with applicable state law and City ordinances, including upstream and downstream considerations where applicable. 84 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN Advance iSWM-aligned practices by prioritizing approaches that mimic natural DR2 hydrology by soaking it in, slowing it down, and spreading it out through feasible low- impact and green infrastructure strategies. Explore ordinance and development credit approaches related to impervious coverage DR3 and runoff mitigation to reduce cumulative impacts over time, while maintaining fairness and administrative clarity. Ensure stormwater ordinances and technical standards are current, enforceable, and DR4 regionally consistent where appropriate, in recognition that stormwater is a watershed issue that benefits from coordinated practices. WATER QUALITY & REGULATORY STEWARDSHIP (WQ) Protect surface water quality through consistent implementation of regional stormwater W01 best management practices across municipal operations, construction activity, and post-construction responsibilities. Sustain compliance with MS4 and floodplain-related requirements while using the WQ2 City's policy flexibility to exceed minimum standards when it measurably improves outcomes for residents and infrastructure. EDUCATION, COMPLIANCE & COMMUNITY PARTNERSHIP (EC) Prioritize education as the first step in stormwater compliance through EC1 expanded resident and HOA awareness of responsibilities, common violations, and practical maintenance steps, supported by fair and consistent enforcement where voluntary compliance is not achieved. Enhance the City's stormwater public education program beyond baseline EC2 requirements to address documented awareness gaps and improve community understanding of risks, responsibilities, and prevention. COST RESPONSIBILITYPARTNERSHIPS (FN) Explore sustainable funding strategies including rate/fee structure options FN1 every five years supported by appropriate financial analysis, so resources align with documented needs, risk, and long-term obligations. Evaluate tiered and equitable funding approaches that reflect stormwater FN2 demand drivers and system impacts, while maintaining transparency, predictability, and administrative feasibility. Pursue grants, interlocal partnerships, and cost-sharing opportunities especially FN3 for privately owned facilities that materially influence public riskwhile maintaining clear standards and accountability for outcomes. CUSTOMER SERVICE & PRIORITIZATION INTEGRITY (CS) Incorporate documented resident drainage concerns as an input to prioritization CS1 through clear, objective criteria ensuring customer service informs decisions while data-driven risk and feasibility remain primary. ORDINANCE NO. 1030A I ADOPTED APRIL 21, 2026 GLOSSARY OF TERMS 1D / 2D — One-Dimensional / Two-Dimensional EIA — Effective Impervious Area (impervious (hydraulic/hydrologic modeling approaches) area directly connected to drainage system) AC — Acre (unit of area) EPA — U.S. Environmental Protection Agency ACE — Annual Chance Exceedance (e.g., 1% ESA — Endangered Species Act ACE = "100-year" event) FEMA — Federal Emergency Management AEP — Annual Exceedance Probability (same Agency concept as ACE/AEP; probability a flow is FF — Finished Floor (elevation) exceeded in a year) AWWA — American Water Works Association FFE — Finished Floor Elevation BFE — Base Flood Elevation (water surface FIS — Flood Insurance Study (FEMA report) elevation for the base flood—typically the 1% FIRM — Flood Insurance Rate Map annual chance) BMP — Best Management Practice (structural or FPS — Feet Per Second (velocity) non-structural stormwater practice) GIS — Geographic Information System CFS — Cubic Feet per Second (flow rate) GNSS / GPS — Global Navigation Satellite CIP — Capital Improvement Program (or System / Global Positioning System Project) HEC — Hydrologic Engineering Center (USACE) CN — Curve Number (NRCS runoff parameter) HEC-HMS — Hydrologic Modeling System CLOMR — Conditional Letter of Map (runoff/rainfall-runoff) Revision (FEMA conditional approval prior to HEC-RAS — River Analysis System (hydraulic / construction water surface profiles, 1D/2D) LOMR — Letter of Map Revision (FEMA map HGL — Hydraulic Grade Line (energy/water revision after construction / as-built) level within a pipe system) CSO — Combined Sewer Overflow (typically HSG — Hydrologic Soil Group (A, B, C, D) not applicable where storm and sanitary are separate) HUC — Hydrologic Unit Code (USGS watershed unit) CTP — Cooperating Technical Partner (FEMA program) ICM — Integrated Catchment Modeling (e.g., InfoWorks ICM—combined 1D/2D approach) DAG / Depth Grid — Depth & Analysis Grid (mapping output showing flood depths) IDF — Intensity-Duration-Frequency (rainfall statistics curves) DBY/ Detention Basin — (Common shorthand) Detention basin iSWM — Integrated Stormwater Management (North Central Texas standard/manual) DEM — Digital Elevation Model LiDAR — Light Detection and Ranging DFIRM — Digital Flood Insurance Rate Map (elevation data acquisition) DP — Detention Pond (common shorthand) LOS — Level of Service (defined performance EAP — Emergency Action Plan (often used for expectation, e.g., roadway overtopping dams; sometimes broader emergency planning) frequency 86 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN LULC — Land Use / Land Cover ROM — Rain-On-Mesh (2D rainfall-driven MAS — Mapping Activity Statement (FEMA Risk modeling approach) MAP deliverable framework) ROW — Right-of-Way MEP — Mechanical/Electrical/Plumbing (not RTK — Real Time Kinematic (survey method) stormwater-specific, but sometimes referenced in permitting contexts) SCS — Soil Conservation Service (former name of NRCS; method references like "SCS CN" MIP — Mapping Information Platform (FEMA persist system) SSURGO — Soil Survey Geographic Database MUSKINGUM-CUNGE — Channel routing method used in hydrologic models SWMP — stormwater Master Plan (or Storm Water Master Plan) NAVD88 — North American Vertical Datum of 1988 SWPPP — Storm Water Pollution Prevention Plan NAD83 — North American Datum of 1983 (horizontal datum) TCEQ — Texas Commission on Environmental Quality NSSDA — National Standard for Spatial Data Accuracy TCAD — Tarrant County Appraisal District NCTCOG — North Central Texas Council of TNRIS — Texas Natural Resources Information Governments System NFIP — National Flood Insurance Program TPDES — Texas Pollutant Discharge Elimination System (Texas implementation of NPDES) NOAA — National Oceanic and Atmospheric Administration TR-55 — Technical Release 55 (NRCS Urban Hydrology for Small Watersheds) NRCS — Natural Resources Conservation Service (formerly SCS) TSDN — Technical Support Data Notebook (FEMA submittal) NPDES — National Pollutant Discharge Elimination System (stormwater permitting USACE — United States Army Corps of framework Engineers MS4 — Municipal Separate Storm Sewer System USGS — United States Geological Survey O&M — Operations and Maintenance WOTUS — Waters of the United States PE — Professional Engineer WQ — Water Quality PFC — Peak Flow Condition (sometimes used WSEL — Water Surface Elevation internally; not a formal standard term) QA/QC — Quality Assurance / Quality Control RCP — Reinforced Concrete Pipe RCB — Reinforced Concrete Box (culvert) RMSE — Root Mean Square Error (survey/ elevation accuracy measure) 100-Year Storm / 1% Annual Chance Event — Downstream Impact Analysis — Evaluation of A storm (or flow) with a 1% chance of being how a project changes flow, velocities, and equaled or exceeded in any given year; it can water surface elevations downstream (including occur more than once in a year. "no adverse impact" considerations). 2D Rain-On-Mesh (ROM) — A modeling Effective Impervious Area (EIA) — Impervious approach where rainfall is applied directly to a area directly connected to the drainage system 2D computational mesh to simulate how runoff (produces more rapid runoff than disconnected moves across terrain (often used to identify impervious). ponding and flow paths). Energy Grade Line (EGL) / Hydraulic Grade Backwater — Rise in water surface upstream Line (HGL) — Lines representing total energy of a constriction (e.g., culvert, bridge, channel and water level in a conveyance system; used narrowing). to evaluate surcharge and capacity. Best Management Practice (BMP) — A Floodplain — The land area that can be practice used to reduce flooding, erosion, and/ inundated by a flood of a given frequency or pollutant loading (examples: detention, (commonly the 1% annual chance floodplain). bioswales, inlet protection, street sweeping, riparian buffers). Floodway — The channel and adjacent areas that must remain available to convey the base Channel Stability / Erosion — The tendency of flood without increasing flood levels beyond a channel to maintain its shape and alignment allowable criteria. under flow conditions; instability can cause bank failure, bed incision, and infrastructure Freeboard — Vertical clearance between a exposure. design water surface elevation and a critical elevation (roadway, top of bank, finished floor); Conveyance — The capacity of a channel, pipe, used as a safety margin. or structure to carry flow without unacceptable impacts (e.g., overtopping, flooding). Green Infrastructure — Practices that use vegetation/soil systems to manage stormwater Curve Number (CN) — NRCS parameter (infiltration, filtration, evapotranspiration), often representing runoff potential based on soil improving water quality and reducing runoff. group, land use, and cover; higher CN generally means more runoff. Hydraulic Modeling — Simulation of water movement and water surface elevations in Design Storm — A rainfall event (depth/ channels and structures (often HEC-RAS distribution) selected for analysis and design 1D/2D). based on frequency and duration criteria. Hydrologic Modeling — Simulation of rainfall- Detention — Temporary storage that reduces runoff processes to estimate flows and volumes peak flow by releasing runoff at a controlled over time (often HEC-HMS or similar). rate after the storm. Infiltration — Water entering the soil surface; Detention Basin/Pond — A facility that provides key process for reducing runoff volumes and detention storage; may be dry-bottom or wet- improving water quality. bottom. Lag Time / Time of Concentration (Tc) — Discharge (Flow) — The rate of water moving Measures of how quickly runoff from the past a point, typically in cubic feet per second watershed reaches an outlet; shorter times (cfs). generally produce higher peak flows. 88 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN Level of Service (LOS) — A defined standard Surcharge — Condition where a storm drain of performance (e.g., roadway overtopping system is pressurized and water level rises allowed at certain frequencies, acceptable flood above the crown of a pipe (can lead to inlet depth thresholds, or capacity targets). overtopping and surface flooding). Low Water Crossing — A roadway crossing Stormwater Utility Fee — User-based revenue designed to be overtopped during certain mechanism to fund stormwater program storm events; requires public safety and activities, often tied to impervious area. emergency response coordination. Subbasin — A smaller drainage unit within a Manning's n (Roughness Coefficient) — watershed used in hydrologic modeling and Parameter that represents resistance to planning. flow due to channel/overbank materials and vegetation; used in hydraulic modeling. Watershed — The land area draining to a common outlet; the organizing unit for Mitigation — Actions that reduce flood risk, stormwater planning and capital prioritization. erosion, or pollutant loading (structural projects, policy standards, maintenance, and Water Quality Volume (WQv) — Volume education). of runoff targeted for treatment (capture/ filtration) to reduce pollutant loads (term and Model Calibration / Validation — Adjusting method vary by local criteria). or checking model parameters so results are consistent with observed conditions (high water Workmaps / Floodplain Mapping Products — marks, gages, known flooding). Mapping exhibits showing cross sections, flood limits, and other modeling outputs used for Non-Structural Measures — Policies and actions documentation and review that reduce risk without building conveyance/ storage (e.g., development standards, floodplain management, maintenance, warning systems). Peak Flow — The maximum flow rate during a storm event; often used for sizing conveyance and detention. Pollutant Load / Loading — The mass of a pollutant delivered to a waterbody over a period of time (often evaluated in water quality planning). Positive Overflow / Overland Relief — Planned surface route for excess stormwater when underground capacity is exceeded. Return Period — Average interval between events of equal or greater magnitude (e.g., 10-year storm"); commonly paired with annual chance language. Runoff — Portion of rainfall that becomes surface flow and enters the drainage system.