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Item 6 - Stormwater Master Plan - DRAFT - PZ - 02.27.2026 - REDUCED SIZE (3) CITY OF SOUTHLAK .R �{ o � t f STORMWATE R MASTER PLAN An Element of the Southlake Comprehensive Plan e. :'9w _ ' - ... �" � - ��''� 'fir .y.Z;► � � �� ' . Adopted by the Southlake City Council Ordinance No. 1030-A mac , Month Day, 2026 ' i'''' /- '•�r �' 'r;.l; f-.:_�' ,,•=r. ems= 7: �e ,t� ��,. �MK.\Z/ �$' �i� t� r `at��.1r r� �;•.. ; �t 4 K��jy��r1y.. •',j f''` Zlc�. <'L�� y ~:s �/' �tiu..� .r3 ,` :"... Z 1 r Al y STORMWATER MASTER PLAN s It.,4 t. ir —�. yf ! it"i� .7�I71 3�f rid 59'lry�► .� i � � � tb d _ i'l. �.�t� �'fl�Ll �1 .'S .h_� _�i7�'%t4ir1' fi7t.'��liY.. x_ !�.df.}•e I _,A.ewe i�l. - �i�•. �R . .._ 1'.S�L' ..i� Q'r."....,.. � •fir .._ 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 2: n 0 wl� S * ; Southlake's Stormwater Master 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 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. enhances public safety, reduces long-term costs, and ensures the City remains resilient in terms of public infrastructure to face the future. STORMWATER MASTER PLAN TABLE OF CONTENTS PAGE CHAPTER 6 CHAPTER 1: INTRODUCTION 10 CHAPTER 2: SYSTEM AND WATERSHED OVERVIEW 24 CHAPTER 3: HYDROLOGY, HYDRAULICS, AND LEVEL OF SERVICE 30 CHAPTER 4: COLLECTING DATA AND UPDATING MODELING 36 CHAPTER 5: EVALUATING THE STORMWATER SYSTEM 42 CHAPTER 6: WATER QUALITY & REGULATORY COMPLIANCE 48 CHAPTER 7: GREEN INFRASTRUCTURE & RESILIENCE 52 CHAPTER 8: STORMWATER INFRASTRUCTURE AND ASSET MANAGEMENT 60 CHAPTER 9: DEVELOPMENT STANDARDS AND FRAMEWORK 66 CHAPTER 10: COMMUNITY ENGAGEMENT, PARTNERSHIPS, AND FUNDING 72 CHAPTER 11: PLAN ADOPTION & PUBLIC ENGAGEMENT 76 STORMWATER MASTER PLAN POLICY STATEMENTS SUMMARY 78 GLOSSARY OF TERMS ORDINANCE 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. Without effective management, these conditions can cause localized flooding, accelerate channel erosion, and degrade water quality as sediment and pollutants are transported into creeks and lakes. Stormwater runoff can carry sediment, nutrients, bacteria, oils, and debris into the storm drainage system and local waterbodies. These contaminants can diminish drainage performance (for example, clogging inlets and depositing sediment in pipes and channels), impair water quality, degrade aquatic habitat, contribute to nuisance conditions, and increase drinking water treatment costs. Stormwater management includes the planning, control, collection, storage, conveyance, and filtering 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. This Stormwater Master Plan provides a long-range framework for: • Protecting life safety and emergency access by reducing stormwater-related hazards. • Sustaining reliable system performance as infrastructure ages and the community approaches buildout. • 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 infrastructure and property. STORMWATER MASTER PLAN • Clarifying public versus private stormwater responsibilities. • Establishing a strategic foundation for a future dedicated Asset Management Plan. • Improving customer response, public education, and community understanding. 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 it primarily involve public infrastructure, private maintenance, or both? • What is the appropriate response pathway (maintenance, inspection, study, ordinance tool, education)? • How does the City communicate what happened and what happens next? 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. In fact, stormwater drainage appears within the survey's highest-satisfaction group of City services, alongside utilities and key public safety measures. This combination of very high importance and high satisfaction supports the plan's emphasis on sustaining system performance through disciplined maintenance, clear service expectations, and long-term reinvestment planning to preserve outcomes that residents already value and notice. ORDINANCE For stormwater specifically, where concerns are often event-driven and emotionally charged, this underscores the importance as to why the Stormwater Master Plan includes a stronger customer response and public education framework in the Plan: consistent intake and triage, clear public vs. private responsibility messaging, and reliable "closure" communication after 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. RELATIONSHIP TO THE SOUTHLAKE COMPREHENSIVE PLAN The Stormwater Master Plan is a supporting element ofthe City's comprehensive planning framework and is intended to translate community-wide goals into practical 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/redevelopment can occur, how streets function during major rain events, and how reliably residents and visitors 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 SOUTHLAKE shared expectations for stormwater levels of service, identifying where watershed constraints should shape Comprehensive Plan 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 decades. Prior studies provide valuable information on critical drainage structures, watershed behavior, floodplain mapping, localized flood risk, and channel erosion and stability. The City also maintains program documents supporting regulatory compliance, including MS4 program documents and floodplain management practices. This master plan does not reproduce technical reports; it translates study findings into policy and program direction and identifies where updated studies will strengthen future decisions. The STORMWATER MASTER PLAN City's study library includes critical structure evaluations, watershed studies, citywide screening and modeling memos, erosion engineering reports, culvert reports, downstream assessments, and mapping update documentation. Studied/DefinedYear Study/ Report 2012 2030 Drainage Masterplan Citywide baseline; basin con- Planning-level improvements; Report (07-11-2012) straints long-range priorities 2014 Drainage Study - Zena Rucker Crossing hydraulics Culvert sizing/configuration Rd Culvert (TNP) 2014 NCTCOG CTP FY14 TSDN - Risk MAP mapping technical Flood mapping documentation Southlake basis support 2019 Highland & Dove Creek Trail Crossing constraints/ Conceptual improvement Culvert Report (JIC) overtopping risk options 2020 Drainage Plan - TNP Critical structures screening Concept improvement framework 2020 Citywide 2D Rain-on-Mesh 2D depth grids; hotspots Hotspot maps; follow-up Memo - Halff priorities 2021 Kirkwood/Higgins Watershed Watershed H&H; alternatives Concept alternatives; next Study (Halff) steps 2021 Downstream Assessment - N Project downstream check Confirms impacts/mitigation Peytonville (Letter) 2022 CLOMR Report Floodplain impacts of project CLOMR documentation 2022 Erosion Control Improvements Erosion/stability risks Stabilization concepts/phasing PER (J&C) 2022 Stormwater Master Plan Study - Citywide baseline; prioritization Roadmap: studies/data/tools Phase 1 (Halff) 2022 Final Southlake Stormwater MS4 program structure/BMPs Compliance actions/ Management Plan Report documentation 2022 Stormwater Management Plan - Permit-term BMPs/goals Tracking/reporting structure 2nd Permit Term (v1.2) 2023 Open Space Drainage Open space drainage concepts Priority corridors/ Enhancement Study opportunities These sources confirm an important message stormwater issues can be highly localized even when citywide outcomes are strong. Localized issues may arise from unique site constraints, debris blockages, channel geometry changes over time, or private drainage and maintenance conditions. The City's program is most effective when it uses consistent processes to diagnose what is driving an issue, apply the right response (maintenance, study, capital improvement concept, or private maintenance guidance), and communicate clearly. ORDINANCE Y 1 ► Am t CHAPTER2: SYSTEM AND WATERSHED OVERVIEW The following chapter establishes the shared baseline for how stormwater behaves across Southlake and why a watershed-based view is essential for making defensible decisions. 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 chapter explains the "system" in plain terms, natural creeks and floodplains, streets and overland flow paths, inlets and storm drains, culverts and bridges, detention features where present, and the way these components interact during routine storms and larger events. It also introduces the City's watershed planning approach: using basin boundaries to organize information, identify where cumulative impacts concentrate, and sequence study updates and investments over time based on risk and changing conditions. This framework aligns with the City's prior work to prioritize watershed studies and capital improvements using objective inputs such as complaint patterns, monitoring locations, and documented risk indicators. This chapter also 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 conceptual 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 Stormwater Master Plan can communicate clearly both internally and to the public 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, 10 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN 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. a - Trash left on sidewalks _ g gets swept into sewers Pet waste washes and contaminatesInto pipes and leaked from cars 91 y� • stormwater. n / n n n r , pollutes parks and washes down 41 recreation areas drains and Into downstream. o oa® Lawn fertilizer and pesticides can Carwashlng� wash across pavement and down chemicals can harm drains,taking toxics with them. fish and animals. Where Stormwater Flows, Everything Goes V EPA ol (� ) Figure 2-1:EPA-Stormwater Flow Infographic roofs, pavement, compacted soils, and curb-and-gutter streets reduce infiltration and move water quickly. 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. WATERSHEDS, SUB-BASINS, AND JURISDICTIONAL CONTEXT Stormwater is a watershed issue. Runoff generated within Southlake ultimately leaves the City through connected creek systems and receiving waters that are part of larger basins. Water does not recognize jurisdictional boundaries, and it does not stop at the edge of a subdivision or roadway corridor. This means stormwater outcomes in Southlake are influenced by upstream conditions . - .INANCE NO. 1030-A I ADOPTED MONTH .A ■ t1 �hlwa�r1 it 1 •�■la • �lll � s I• t Am Ah lo Be P ■I� Tributary • • r • �`� . • � .�.._.dry I-I�y■17 ' ,1 •� '� �I==_�i:il�_•�'• Cl■1.'•1��I`� ■ � ��'�1,��I'irif 110.��7 q 7 r Inl'plllill_11 „?I I'-� !�..Ir��;� , ►� :r�!,�1► �- ! y 1 t ' >. �IRIw �L uuniiil_� Ltnillll.■....�`r�•' /�11_I�i�I�I:::IY> 'obi p► rl h .# ��. err ruu/Jl7 �.rv'.2+`i� mr e v.t 1.• 4 �`k._.i► F*�<+✓ti ��'��` d r I I:Jl� �L��1'?i:NE _ �, 1 �� I�■ ��• yt:=�f1 �� . ■:I`-_- I C:'J, '�:':��'�S vW� y� '�R }:� -t ai, 12 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN (runoff volumes, timing, and sediment loads entering the City) and downstream constraints (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. A watershed-based framework provides the structure needed to understand these cause-and- effect relationships and to avoid isolated solutions that reduce a symptom in one location while unintentionally shifting risk elsewhere. Sub-basins are a practical way to break the community into manageable planning units that align with how 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. 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 improves communication by helping 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 upstream or downstream conditions. It also supports regional coordination where creek corridors cross municipal boundaries or where local drainage behavior is influenced by regional floodplain mapping updates and receiving water conditions. Southlake's MS4 program, the City's required stormwater pollution prevention and stormwater quality program for its Policy Municipal Separate Storm Sewer System, documentation further anchors this watershed context by identifying WS1. Ground watershed-based receiving waters and reinforcing that stormwater runoff planning and stormwater from across the community ultimately discharges to those investment decisions waters. This is why public education, construction controls, studies illicit discharge prevention, post-construction controls, and downstream impacts . nd and municipal good housekeeping practices are essential overall system behavior. components of the City's stormwater approach not separate tasks. In short, 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 its creeks, tributaries, drainageways, floodplains, and low-lying storage areas provide the foundational framework that the built stormwater system works within. During typical rainfall events, 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. In other words, 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 receiving "spine" for outfalls and local drainage systems across multiple neighborhoods, which means their stability, ORDINANCE 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 Stormwater Master Plan treats creek corridors and floodplain function as long-term assets that require both proactive stewardship and clear standards for how adjacent development and maintenance activities occur. Floodplains are often misunderstood as "failures,"when in reality they exist because water has always had 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 rules. 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 SO 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. 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-758/May 2070) (https://www.fema.gov/sites/default/ files/documents/fema nfip substantial-improvement-substantial-damage-desk-reference.pdf) 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 conveyance elements that move flow downstream, and the crossings and controls that govern how STORMWATER MASTER PLAN water passes through constrained locations. In practical terms, this system is what residents most often associate with "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 Cam=sediment accumulation accumulation that blocks inlets, fills catch basins, reduces pipe conveyance, or creates localized backwater conditions. -� Long-term condition of pipes �� r 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 ORDINANCE 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 the Stormwater Master 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. 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 j Y 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 16 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN 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. Citywide evaluations such as the 2020 Drainage Master Plan critical structures work 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 Policy Statement infrastructure can be exceeded during extreme events. The City needs clear procedures for storm - manage stormwater monitoring, including identifying locations mostpublic susceptible to overtopping, establishing triggerproperty proactive points for closures, and coordinating barricade performance placement and public alerts. A predictable practices. operational plan helps reduce risk during the event and supports stronger post-event documentation capturing what overtopped, for how long, and under what conditions so the City can determine whether the response should be routine maintenance, targeted inspection, study update, or capital planning consideration. Equally important is consistent public messaging: roadway closures and barricades are safety actions intended to protect motorists and emergency responders, not an indication that the City is ignoring the issue. When communicated consistently and backed by documented monitoring and follow-up practices, this approach builds credibility and supports the Stormwater Master Plan's overarching goal of proactive risk reduction and sound system performance practices. REGIONAL FACILITIES AND INTER-JURISDICTIONAL COORDINATION Watershed systems and flood risk do not stop at city limits, and many of the conditions that influence local stormwater performance are shaped by regional context. Creeks and drainageways often extend across municipal boundaries, and floodplain behavior reflects basin-wide hydrology and downstream controls not just what happens within one jurisdiction. As a result, some stormwater challenges cannot be fully understood, communicated, or addressed through a purely local lens. Regional coordination 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. This is one of the core reasons the Stormwater Master Plan emphasizes watershed-based planning: it provides the structure needed to evaluate impacts upstream and downstream and to pursue solutions that are effective and defensible at the basin scale. ORDINANCE Regional coordination also improves the City's ability to use consistent data and assumptions when discussing flood risk. Floodplain mapping is a prime example. FEMA Risk MAP updates and Cooperating Technical Partner (CTP) technical support materials document the datasets, hydrology/hydraulics 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 CLOMR/LOMR. Consistency in mapping inputs and assumptions supports more credible communication and reduces confusion when residents compare flood risk information across neighboring jurisdictions. In addition, consistent criteria and standards across a region such as integrated stormwater management concepts or aligned drainage criteria can reduce cumulative impacts over time by ensuring that redevelopment and infrastructure decisions are not working at cross purposes across the same watershed. Regional partnerships also create practical implementation benefits. Many grant programs and external funding opportunities are more competitive when they demonstrate basin-wide benefit, multi-jurisdictional coordination, or documented alignment with regional plans. Coordination can also reduce duplication: shared models, shared mapping updates, and shared understanding of constraints can accelerate project development and improve the quality of technical analysis. Finally, regional coordination provides a pathway for addressing "shared-risk" locations areas where a local issue is driven by downstream or upstream conditions outside the City's sole control. In those cases, coordination allows the City to frame the problem honestly, evaluate 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, conveyance pinch points, and how the system has evolved over time. Chokepoints at 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 issues 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/outlet controls that may not be maintained consistently. These conditions do not necessarily mean the citywide system is inadequate; rather, they reflect that stormwater is highly location-specific and that a small number of constrained locations can create outsized community visibility during intense storms. A central purpose of this Plan is to ensure that these recurring issues are handled consistently and transparently over time. That begins with a documented inventory of known concern locations that is grounded in objective information, not only anecdotal perception. 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 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 complaints. Maintaining this inventory serves several essential functions. First, it creates continuity: staff can see what has happened at a location over time, what maintenance actions were taken, and whether a problem is repeat or worsening. Second, it supports strategic prioritization by allowing the City to distinguish between issues driven primarily by public infrastructure capacity, maintenance conditions, natural exceedance behavior, and private property responsibilities. Third, it improves STORMWATER MASTER PLAN N e _ q� _ Wi'�jC�17 F. ` 1 o Lak Grapevine � � • ' )i T `tit `�= , y, , Kirkwood y. 1 -�, Branch Unnamed_' Tribut ry to w KirkwoodTBranch n V 4 South Fork iirkwood j ranch C 1 -Dove Creek Higgins '`�1 ; • Branch West Jones M Branch ' Jones G Branch O .a ;1 Tributary • 1 - Tributary r BB-9 BB-8 Timarron Unnamed Tributary Creek Tributaryjq .,Jt _ _ fir, `v I i� 4 - Tributary °+ To Big Be r- 3� BB. `t S 4 1pp `ri r$• Watershed Study Basin Prioritization: Flood Risk Priority Raw Data Points Southlake Stormwater Master Plan , , ' September 2021 -AVO # 42929 Legend SOUTHLAKE ■o■ ^,=} j Southlake City Limits 9 mom HALFF Major Watersheds Notes: 1) Imagery Source: Nearmap Emergency Response Locations 2) Green hatched watersheds indicate local basins recommended for future detailed 2D a Low Water Crossings analysis as needed. 0 0.5 1 2 Miles 0 Drainage Complaints Figure 2-3:HalffAssociates Inc.Recommended Watershed Basin Prioritization and Complaint Origins customer service because it creates a repeatable response pathway maintenance, inspection, monitoring, study input, or capital planning consideration and ensures residents receive consistent explanations and next steps. Finally, it supports transparency and trust. Residents should be able to see that the City tracks issues, uses clear decision rules, and is willing to explain why certain locations are prioritized for action and why other concerns 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 appears similar. For example, a site may maintain a similar amount of paved area but change grading, curb alignments, drainage patterns, and surface connectivity in ways that move runoff more quickly to inlets and pipes. Redevelopment can also alter overland flow paths, sometimes in unintended ways by raising a portion of a lot, adding walls or fences, reconfiguring driveways, or changing how water exits a property. These changes can shift where water concentrates during storms, potentially creating new localized issues or increasing pressure on downstream inlets and crossings. 20 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN T v�'�tat STORMWATER MASTER PLAN Redevelopment can also increase runoff volume and peak flow during short-duration storms because newer surfaces are often more connected and less porous than older site configurations. Finally, redevelopment can affect detention and post-construction controls. If outlet controls are modified, if maintenance is deferred, or if a facility gradually loses storage due to sedimentation or vegetation, the intended long-term benefit can erode over time often without being noticed until repeated neighborhood impacts occur. 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 Stormwater Master Plan's policy direction to treat buildout as the correct long-term planning condition for stormwater decisions and to require development and redevelopment to demonstrate responsibly managed stormwater impacts under buildout conditions, including upstream and downstream considerations where applicable. It also supports the policy direction to explore 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 CITY PROPERTY OWN& 0-4 RESPONSIBILITY ■ • RESPONSIBILITY PROTECTING INFRASTRUCTURE MAINTAINING ALL, OTHER AREAS OF EE EE -THE DRAINAGE EASEMEN MAINTAINING _ s-PUBLIC INFRASTRUCTURE - :_ WITHIN DRAINGAGE EASEMENTS A consistent stormwater program requires clear, repeatable definitions of responsibility both to manage public risk effectively and to communicate with residents fairly.Southlake's role is to maintain and improve public stormwater infrastructure within public rights-of-way and improved easements, to manage stormwater planning and capital improvements, and to administer required regulatory programs (including MS4 stormwater quality requirements and floodplain administration). At the same time, private property owners and HOAs are responsible for maintaining private drainage features on their property, including yard drains, private swales, private channels located on private or HOA property, and privately owned detention facilities and outlet controls. These private systems can materially influence neighborhood drainage performance and, in some cases, can affect public risk when failures cause runoff to leave a site in unintended ways, obstruct downstream conveyance, or create recurring flooding and erosion impacts. This distinction is important because many stormwater concerns reported by residents are not caused by deficiencies in the public stormwater system alone. They are often driven by private maintenance conditions clogged yard drains, blocked swales, modified grading, fences or landscaping that disrupt historic flow paths, HOA channels that accumulate debris, or detention outlets that become partially blocked. When these conditions persist, residents may understandably assume the City should "fix drainage," even when the City does not have ownership authority or legal responsibility to maintain private facilities. Without consistent responsibility messaging, the City can find itself pulled into repetitive complaint cycles where expectations are misaligned, and where time is spent revisiting the same issue without a path to resolution. AM3 directly addresses this: clearly defining public versus private stormwater responsibilities and strengthening how the City communicates, supports, and verifies private maintenance obligations where those facilities influence public risk. Setting expectations early is one of the most effective ways to reduce conflict and improve outcomes. The best time to communicate responsibilities is before a problem becomes a crisis through HOA outreach, plain-language guidance materials, and consistent messaging during development and redevelopment review when post-construction controls are established. A robust public education program also supports helping residents understand stormwater basics, common private maintenance issues, and practical steps they can take to prevent problems (such as keeping inlets clear near their property, maintaining yard drains, and ensuring HOA facilities are inspected after storms). Finally, the City's customer response process should reinforce this clarity. When a complaint is received, staff should consistently document whether the driver appears to be public infrastructure, private conditions, or a combination, and then provide a clear response pathway: what the City can do, what the private party must do, and what follow-up steps will occur. "a��,r./��(•��t:,`�`,�\ ,�1 . .`r '1 � !� ,rA ��,-i'�aigr{a}r.''; ,�ItM1�� .•}1�,4��••�"��A 'S ,1 _.�.• ar� ,rat� ,ti��•+"�� ,r� - .�,, „�;�a , M AN Aff N t. `,•° --- `lii lal - r IP . _ o mpr I�. . n siv 3 r;ILA lip i Chapter 3 establishes the technical "translation layer" for the Stormwater Master Plan without turning the plan into an engineering report. Stormwater decisions often become confusing to the public because the system's performance is event-driven and because storm events vary significantly in intensity, duration, and frequency. This chapter explains the foundational concepts that the City uses to evaluate stormwater behavior and communicate performance expectations in a consistent, factual way. It introduces the language of design storms and rainfall frequency, the runoff drivers that influence how much water enters the system and how quickly, and the hydraulic factors that determine whether water stays within channels and pipes or spills into streets and floodplains. These concepts create a shared baseline for what "system performance" means and why certain locations experience ponding or overtopping during larger storms even when the overall system is functioning as designed. This chapter also supports a core objective of the Stormwater Master Plan: establishing clear and strategic expectations for levels of service and risk management. Stormwater systems are not designed to eliminate all flooding under all storms. Instead, 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 occur in extreme events. By explaining these concepts upfront—and by linking them to Southlake's technical study library and flood risk mapping—the City can communicate more clearly with residents about what the stormwater system is intended to do, why certain outcomes occur during rare events, and how the City uses data and technical evaluation to guide 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. These statistics allow engineers and planners to compare storm events on a consistent basis such as a "2-year storm," "10-year storm," or "100-year storm" which are simply shorthand ways of describing probability. In simple terms, larger-frequency storms represent rarer, more intense events, while smaller-frequency storms occur more often. This distinction is important because stormwater infrastructure is designed and evaluated against defined storm events, and those design expectations help the City communicate what the public can reasonably expect the STORMWATER MASTER PLAN system to manage routinely versus what may exceed the system during extreme conditions. In Texas, NOAA Atlas 14 precipitation frequency data is widely used as the current standard reference for rainfall depths across different durations and storm frequencies. Atlas 14 matters because rainfall assumptions drive nearly every downstream conclusion in a stormwater evaluation: 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. Updated rainfall data supports consistent evaluation of infrastructure performance under defined storm events and improves public communication by grounding discussions in a recognized standard rather than subjective impressions of whether a storm was "big." It also strengthens transparency if the City and its consultants are using the same rainfall reference across studies, then performance comparisons between watersheds, corridors, and projects are sustained by fact rather than impressions. This Plan reinforces a consistent message: no Precipitation Frequencies - 24-Hour Duration practical stormwater system can prevent all Recurrence Interval Depth (inches) flooding under all possible storms. Even well- designed systems can be exceeded during rare, 50% ACE (2-Year) 3.9 high-intensity rainfall especially short-duration 10% ACE (10-Year) 5.8 storms that overwhelm local capture capacity 1% (100-Year) 9.1 and create rapid street ponding before runoff can Figure 3-1:PDS-based precipitation frequency estimates with enter inlets and pipes. Responsible stormwater 90%confidence intervals management focuses on risk reduction, life safety, infrastructure protection, and defined levels of service where feasible, while maintaining clear exceedance pathways and operational readiness for storm events that exceed design conditions. This chapter's purpose is to provide the shared language that supports those decisions and communications, including how Southlake distinguishes nuisance impacts from higher-consequence hazards such as roadway overtopping at key crossings. 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 a set of assumptions about soils, land cover, slopes, drainage connectivity, and storage features such as detention or natural depressions. The assumptions matter because they drive the calculated peak flows and volumes that then 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 increases and flow paths become more connected and efficient. A central hydrologic driver is imperviousness, but it 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 pathways to streets, inlets, and pipes, reducing infiltration, and shortening the time it takes water to concentrate. Hydrologic methods also consider soil infiltration potential; more clayey soils infiltrate less and generate more runoff than sandy soils. When these factors combine with local slopes and flow path connectivity, the result is a runoff hydrograph the timing and magnitude of flow that the stormwater system must convey. For Southlake,this supports two practical expectations for studies and development review. First,the City should continue requiring clear documentation of hydrologic assumptions in watershed studies ORDINANCE and development drainage submittals so outputs can be understood, compared, and validated over time. Second, evaluation should continue to consider both existing conditions and reasonable future conditions, including buildout where applicable, because stormwater performance is ultimately influenced by cumulative change across a watershed. This directly supports the policy direction that development and redevelopment demonstrate responsibly managed stormwater impacts under buildout conditions, including upstream and downstream considerations where applicable (Po/icy Statement DPI). Clear assumptions also support fairness: consistent methods ensure applicants and neighborhoods are being evaluated using comparable standards, rather than inconsistent or shifting criteria. 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 discharge. The system behaves ' as a connected network, so an upstream improvement may not perform as expected if a downstream constraint governs water z surface elevations during larger events. •� � '' Hydraulic evaluation is therefore essential for understanding chokepoints and for ensuring that investments reduce risk rather than shifting it. One of the most important hydraulic influences in Southlake as in many communities is tailwater, which occurs when downstream water levels limit discharge and create backwater effects. Tailwater 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 function as designed but still experience elevated upstream water levels because the system cannot release water efficiently. This is why localized flooding or roadway overtopping can appear "upstream" of a constraint and why watershed-based evaluation is necessary to understand cause-and-effect. It is also why some projects must be evaluated not only at the immediate site but in the context of downstream conveyance. Another critical concept especially for roadway crossings and key corridors is freeboard. Freeboard is a safety margin between an anticipated water surface elevation and a roadway surface, structure, or critical elevation. Freeboard is used as a practical risk-reduction buffer to account for uncertainty (debris, sedimentation, model limitations, and event variability) and to protect life safety and emergency access. In crossing evaluations, freeboard is commonly 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 extremes. Several of Southlake's study approaches and criteria references treat freeboard and overtopping frequency as key indicators of public safety risk at crossings and critical drainage structures. This supports a central policy direction of the Stormwater Master Plan: higher-risk areas warrant enhanced evaluation,and the City should maintain clear performance expectations and risk language. STORMWATER MASTER PLAN When residents experience roadway overtopping or rapid ponding, the City's response must be grounded in objective evaluation what storm event was experienced, what the system is expected to manage, what constraint governs performance, and what options exist to reduce risk. Clear hydraulic criteria and consistent evaluation practices allow the City to make those determinations transparently and to prioritize improvements in a way that is defensible and repeatable. LEVEL OF SERVICE: CLEAR EXPECTATIONS FOR SAFETY, RELIABILITY, AND RESPONSE Levels of service translate community expectations into practical, repeatable benchmarks for stormwater performance and stormwater service delivery. In many communities, stormwater expectations can become unclear because the system is tested only during storm events and because experiences vary widely one neighborhood may see minimal impacts while another sees temporary ponding, overtopping, or creek corridor erosion during the same storm. A clear level-of- service framework provides a shared definition of what the City is managing for and why. It does not promise "no flooding." Instead, it establishes what outcomes the City is working to deliver, how performance will be evaluated across different contexts, and how staff will communicate consistently when impacts occur. Levels of service are also an important transparency tool: they help residents understand the difference between outcomes the system is designed to manage routinely and exceedance outcomes that may occur during extreme rainfall events. A practical stormwater level-of-service framework clarifies three things. First, it identifies what performance is expected for different asset types and locations. A neighborhood inlet and local street conveyance network serve a different function than a major trunk line, a major creek crossing, or a corridor adjacent to critical facilities. Second, 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. This distinction is essential to fair decision-making and effective public communication. Third, it provides a basis for how maintenance and reinvestment decisions are guided. When the City defines performance expectations clearly, it can evaluate where the system is meeting expectations, where performance gaps exist, and whether the appropriate response is routine maintenance, targeted renewal, additional study, standards refinement, or capital improvement planning. Because risk is not uniform across the city, a reasonable level-of-service framework differentiates between Policy Statement common stormwater contexts. Local street drainage SP2. Define and maintain clear typically manages frequent storms and relies on inlets, stormwater levels of service for safety, pipes, and safe surface pathways; shallow ponding reliability, and response, using these during intense rainfall can occur even when the system expectations to consistently guide is functioning as intended, particularly during short- planning, maintenance, and investment duration high-intensity storms. Trunk system corridors decisions. affect larger drainage areas; constraints in these corridors can influence multiple neighborhoods, so performance expectations emphasize reliability and system-wide consequence rather than isolated symptoms. Roadway crossings and low-water crossings are treated as high-consequence assets because overtopping can create immediate safety hazards and restrict emergency access; levels of service in these locations may prioritize life safety and operational readiness (monitoring and closure protocols) in addition to structural performance. Finally, areas near critical facilities require heightened consideration because access and service continuity carry greater consequence; even temporary loss of access can materially affect emergency response and community function. In short, levels of service help the City apply a consistent logic: different contexts justify different performance targets, monitoring practices, and investment priorities. Establishing clear levels of service directly supports Policy Statement SP2 by allowing the City to use community expectations for safety, reliability, and response to guide planning, maintenance, and investment decisions. It also strengthens customer service by improving consistency in how concerns are evaluated and explained. When residents ask "why does this happen here," staff can reference the applicable service context, explain what the system is intended to manage, describe what exceedance looks like, and outline what actions the City will take (maintenance, monitoring, study input, or capital planning). This reduces confusion, improves 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 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 Policy Statement evaluated carefully to ensure they measurably reduce risk without shifting impacts to other Prioritize by life safety, followed by consistent, locations. objective criteria that support defensible and This hierarchy does not require a rigid scoring repeatable decision-making. formula in the master plan. Instead, it establishes the consistent policy basis for how staff evaluates and communicates why certain actions occur first. Staff can still use objective criteria and project-specific factors (feasibility, cost-effectiveness, coordination opportunities, and readiness), but those factors are applied within a clear framework that prioritizes safety and resilience. This supports Policy StatementSP3 by ensuring decisions remain consistent across time and across storm events reducing the likelihood that the City is perceived as arbitrary or inconsistent in how it directs stormwater resources. DEFINING "DAMAGE" AND COMMUNICATING PROPERTY IMPACTS OBJECTIVELY Stormwater concerns often involve strong resident perceptions and significant disruption, but effective communication requires the City to be consistent and factual about how risk and "damage" are defined. STORMWATER MASTER PLAN The City's communications must distinguish between three different concepts that are often conflated: 1. Disruption (such as street ponding, temporary access issues, or yard flooding) 2. Nuisance impacts (repetitive but typically non-structural impacts that affect convenience and quality of life) 3. Objective regulatory definitions used for floodplain administration and compliance The purpose of maintaining these distinctions is not to minimize resident experience; it is to ensure the City uses accurate and consistent standards when describing building impacts, floodplain requirements, and compliance triggers. FEMA floodplain management guidance defines substantial damage as damage where the cost _ 4 to restore a building to its pre-damage condition 1 equals or exceeds 50 percent of the building's market value before the damage occurred. This definition matters because it is the standard used %' ►!,� r� '�9 in floodplain management to determine when a structure must be brought into compliance with current floodplain regulations following damage, and it provides a consistent basis for .. 3,:_�•- _ making determinations that have significant - regulatory and financial implications. Using this federal definition supports consistent floodplain administration, provides a factual standard that is not based on sentiment, and clarifies when compliance upgrades are required. (FEMA Desk Reference:Substantiallmprovement/Substantial Damage Desk Reference FEMA P-758/May 20 70) The City recognizes that nuisance flooding and disruption can still be significant quality- of-life issues even when they do not meet a federal "substantial damage" threshold. Those experiences matter for customer service, for maintenance targeting, and for identifying �; Y locations that may warrant further evaluation. The role of objective definitions is to ensure that when the City communicates about building damage risk, floodplain compliance, and regulatory determinations, it does so accurately and consistently. This clarity protects residents by reducing misinformation, supports fair and lawful administration of floodplain requirements, and strengthens public trust by ensuring the City's messaging is grounded in established federal standards while still acknowledging and responding appropriately to real- world impacts residents experience during storms. ORDINANCE NO Iry ip >�- - CHAPTER4: COLLECTINGDATA AND UPDATING MODELING Chapter 4 explains how Southlake builds and maintains the information foundation needed to make stormwater decisions that are consistent,strategic,and easy to communicate. Stormwater performance is shaped by a connected system of public infrastructure, natural drainage corridors, and private drainage features, and it is tested most visibly during storm events. Because of that, the City's ability to respond effectively and plan wisely depends on having reliable data knowing what assets exist, where they are located, how they connect, and what their condition and performance history look like overtime. This chapter establishes the Stormwater Master Plan's approach to data management as a core program function, not an optional support activity. It describes the City's "system of record" expectations, including GIS inventory standards, documentation practices, and how asset information should be maintained so it supports day-to-day service delivery as well as long-range planning. This chapter also describes how the City uses monitoring, field reconnaissance, and modeling as decision-support tools that work together. Field observations and storm event documentation help staff distinguish between issues driven by maintenance conditions, infrastructure constraints, natural exceedance behavior, and private maintenance responsibilities. Modeling and mapping whether at the citywide screening level or within detailed watershed studies help quantify system behavior under defined storm events, test improvement concepts, and understand cumulative impacts as redevelopment occurs over time. By setting clear expectations for data quality,validation,and scenario planning (existing, mitigated, buildout, and exceedance conditions), the chapter supports the Stormwater Master Plan's policy direction to use watershed-based decision-making, maintain a regular study cycle sequenced by risk, strengthen decision-support tools, and ensure major investments are supported by appropriate technical evaluation and documented evidence. 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 critical elements are out of sight such as underground pipes, • STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN culverts, junction structures, and private connections that influence performance. Because of that, the City's GIS inventory functions as more than a map. It is a foundational decision- support tool and the core "system of record" that allows staff to manage stormwater infrastructure as an integrated network of public assets. When inventory data is complete and consistently maintained, the City can respond faster during storms, target maintenance more efficiently, plan reinvestment more defensibly, and communicate more clearly with residents about what is known and what is being evaluated. A consistent system of record supports several essential outcomes. It improves 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 system function and risk. It strengthens capital planning by allowing project concepts to be tied to verified asset locations, sizes, constraints, easements, and connection points reducing uncertainty and improving cost estimation. It supports regulatory documentation because many MS4 program requirements involve mapping, inspection documentation, tracking of assets and outfalls, and demonstration that the City is implementing required activities in a measurable way. Finally, it supports continuity across staff transitions. Stormwater knowledge often becomes "tribal knowledge" without a strong system of record. A maintained inventory ensures that knowledge stays with the organization and does not rely on a single person's experience. Inventory expectations should be explicit enough to produce consistent information over time. At a minimum, the system should include unique asset identifiers and standardized attributes such as asset type,size/material where known,ownership/responsibility(public vs. private vs. HOA), and right-of-way/easement status. Where as-built drawings and historical documentation exist, the inventory should link to those records so staff can quickly retrieve design details when needed. Over time, maintenance history and inspection findings should be tied back to asset IDs 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 ongoing reactive work. These elements directly support the policy direction to manage stormwater infrastructure as a portfolio of public assets using inventory, condition/criticality, and risk-based reinvestment practices, and to combine data-driven analysis with field knowledge to act early in emerging problem areas. MONITORING, HIGH-WATER MARKS, AND FIELD RECONNAISSANCE Stormwater planning improves significantly when models and maps are supported by real- world observations. Targeted monitoring strengthens the City's understanding of storm behavior and improves both technical credibility and public communication. Even where full-scale monitoring is not feasible across the entire system, well-chosen monitoring locations and consistent event documentation can provide meaningful evidence to validate assumptions and identify emerging risks before they become chronic problems. Monitoring can take several forms. Rainfall tracking provides event context and helps staff describe storms in objective terms rather than impressions. Water level (stage) or flow ORDINANCE Storm SewerUSOUTHLAKE ITY OF Infrastructure Box Culvert Channel �'~d Pipe ---_ Drainage Basin Divide o,. 100-Year Flood Plain 4. g „i rwq cs.n l I� i ................................................................ ..I. ... _ ...............................�:. ........... .. ....................................... Denton Couniv ... Q� tom.. .... QI •��••�• s \ Tarrant County I 1 I I N_ t r WA—\/ I S IV ------- Scale 1:54000 I , 1 inch = 4500 feet IPA r Li Q r� i �EHo O� j ♦ u16. ' v • % � i fJ • ; 7Si ♦d it _ {n �• •r <as b + aw �23, DISCLAIMER This data has been compiled for The (ity of SoUhlake and is for informational purposes,various official and unofficial ,ources 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, Out it was not prepared for and may not be sudaw for legal,engineerin ury g,or seying purposes. As such,no guarantee is given Last Storm Sewer Revision- 1/1/2025 Office of Performance Excellence or mplied as to the accuracy of this data. Last Basema Revision-1/1/2026 P Geographic Information Systems 32 STORMWATER MASTER PLAN I SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN observations at selected locations especially near known crossings, chokepoints, or sensitive channel segments can help confirm where overtopping begins, how long conditions persist, and how quickly the system recovers. High-water marks after significant events provide additional evidence for verifying flood extents and understanding whether modeled results align with observed conditions. Field reconnaissance is equally important because many stormwater drivers are maintenance- and condition-related: debris blockages at inlets, sediment deposition that reduces pipe or channel capacity, outfall undermining, erosion progression, and private drainage conditions that redirect flow paths. Field observations help distinguish between "capacity" problems and "maintenance/condition" problems, which is essential for choosing the right response. Maintaining a consistent storm event documentation process is one of the highest-value improvements a city can make without becoming overly technical. A standard process improves the City's ability to: 1. Explain what occurred in clear terms; 2. Compare observed outcomes to expected performance; 3. Identify whether maintenance actions,study updates,or capital planning is appropriate; and 4. Maintain continuity of evidence over time. For example, if a crossing overtops in multiple events, documented observations support defensible prioritization and help staff evaluate whether the issue is driven by structural constraint, tailwater, debris loading, or approach grade. Likewise, repeated field evidence of erosion progression at a location supports timely intervention before infrastructure is threatened. 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" in a way that is consistent and defensible. The City's studies demonstrate that stormwater evaluation occurs at multiple scales. Corridor-level and watershed-level studies help 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 helps identify where localized flooding risk concentrates and where follow-up study or concept development may be warranted. Integrated modeling approaches can be particularly valuable in built-out communities because stormwater behavior is often influenced 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 "what if" scenarios to be tested consistently. That capability directly supports Po/icy Statement WS5. ORDINANCE A key planning value of modeling is that it supports transparency. When residents Policy Statement experience localized flooding or roadway WSS. Regularly explore opportunities to overtopping, model outputs and mapped results improve the City's stormwater decision- when used carefully can help communicate what support tools evaluating, procuring, is happening and why. That communication participating advanced must remain responsible: models are decision- watershed and infrastructure modeling support tools that rely on assumptions. But when platforms and regionalcollaboration paired with field observations and documented studies as warranted, • staff can storm event evidence, models help reduce consistently quantifyand - uncertainty and guide consistent, basin-scale development im• - system decision-making. performance ' " defined ' and support knowledgeable, data-driven CALIBRATION, VALIDATION, AND CREDIBILITY capital planning, and standards updates. 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 Policy indicates that system performance is generally strong while still acknowledging localized WS3. Combine data-driven concerns that residents observe. staff field to act early in emerging problem Majorcapital recommendations should reference major investments are supported by the technical basis for the recommendation in appropriate technical evalu- • 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. WS3 supports this combined approach: data-driven analysis paired with staff field knowledge so the City can act early in emerging problem areas while ensuring major investments are supported by appropriate evaluation. SCENARIO DEVELOPMENT: EXISTING, MITIGATED, BUILDOUT, EMERGENCY Scenario planning is essential because stormwater performance depends on both physical conditions and future change. Typical scenarios include: 1. Existing conditions; 2. Mitigated conditions reflecting proposed improvements; 3. Buildout/redevelopment conditions reflecting cumulative long-term change; and 4. Emergency or exceedance conditions that illustrate how the system behaves when storms exceed design expectations. • • . • ••• •- • • • • • • - • -• Policy Statement Policy Statement WS2. Establish a regular program of technical WS4. Explore enhanced planning and stormwater and watershed studies, conducting design evaluation in higher-risk flood at minimum one basin-level evaluation every areas based on technical study findings, other year sequenced by risk, to refresh recognizing that risk is not uniform across priorities, validate assumptions, and support the city and standards may need to vary long-term capital planning. by context. • • • • •• • • • - • 9 roll • • -• • --• • • • • •• • i r" "S •ti 7" �rlyrt«11h� 41�'y J.=1,' Ir 1,1t � �• k' } � $= #� - e •: � 4... 01. INC- �. � . ORDINANCE NO. 1030-A ADOPTED MONTH DAY, 2026 35 4 t' t -• � ` � _ ....._. �, -- 1._ ` _ "� � _,ram t. C` ♦ ♦ • Chapter 5 summarizes Southlake's current understanding of stormwater system performance and identifies the types of conditions that most often drive resident impacts, maintenance needs, and future investment decisions. This chapter does not function as 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 into clear themes that support the Stormwater Master Plan's policy direction. The purpose is to describe how the system behaves, where stormwater risks tend to concentrate, and how the City distinguishes between routine, manageable outcomes (such as temporary street ponding during intense rainfall) and higher-consequence hazards (such as roadway overtopping that affects public safety and emergency access). The chapter also provides a practical "diagnostic framework" for staff and decision-makers. Stormwater concerns are often reported as a single 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. By describing these drivers in a consistent way, Chapter 5 supports consistent and repeatable decisions: when maintenance is the appropriate response, when targeted field investigation is needed, when a study update should be scheduled, and when a capital improvement concept may be warranted. This evaluation framework directly supports System Performance & Public Safety policy direction and strengthens transparency and consistency in how the City communicates with residents about what is happening and what happens next. 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. Southlake's technical studies and field experience show that many stormwater concerns arise from a limited set of recurring mechanisms: STORMWATER MASTER PLAN • 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 chokepoints at crossings. Crossings are natural pinch points. 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. 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. 3. Determine the appropriate response: 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 supports policy within this Plan by emphasizing proactive risk reduction and sound system performance practices and improves trust by making response decisions repeatable and transparent. ORDINANCE FLOODPLAIN ENCROACHMENTS AND OUT-OF-BANK CONDITIONS Out-of-bank flow is a natural function of creek systems during larger events. Floodplains convey and store floodwater when channel capacity is exceeded, and they provide space for floodwaters to spread in a way that can reduce downstream peak intensity. A key point for system evaluation is that out-of-bank flow does not automatically indicate a deficiency in the storm drain network. In many cases, floodplain inundation is expected during rare events because floodplains are the natural relief areas for basin-wide storm flows. 500 Year Floodplain _ 0.2%Annual Chance FEMA 100-Year Floodplain 1%Annual Chance FEMA Flood Fringe FEMA Floodway oot _______ Encroachment Area �� Encroachment Area Base flood elevation after encroachment on floodplain. Channel Base flood elevation before encroachment on floodplain. One Foot Increase in Base Flood Elevation-Section View However, floodplain performance can be degraded when conveyance is reduced by encroachments fill, structures, fences, landscaping, or other obstructions placed within flood hazard areas. Encroachments can increase water surface elevations, shift risk to adjacent properties, and reduce the reliability of upstream conveyance by increasing tailwater effects. That is why floodplain administration is a core element of system evaluation and risk management. It is also why the Stormwater Master Plan aligns floodplain stewardship with both public safety and regulatory stewardship. Floodplain administration supports the City's participation in the National Flood Insurance Program, which provides residents access to federally backed flood insurance and establishes consistent standards for development in flood hazard areas. FEMA guidance provides objective standards for substantial damage and substantial improvement determinations, including the "50 percent rule." These objective definitions matter because they determine when regulatory compliance upgrades are triggered and they ensure that building damage risk is discussed factually and consistently rather than based on anecdotal perception. (FEMA substantial damage guidance) From an evaluation standpoint,this master plan supports a clear message:the City manages flood risk through a combination of mapped flood hazard understanding, consistent floodplain administration, maintenance of creek corridor function, and targeted mitigation where it measurably reduces risk while recognizing that floodplains are a normal part of basin behavior during large storm events. STORMWATER MASTER PLAN 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/HOAs have responsibility. Where erosion threatens public assets, the City must be able to access and address the issue; where erosion is solely private, the City's role is often guidance and ensuring that private 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. DETENTION/RETENTION ADEQUACY AND RELEASE CONTROLS Detention and retention can reduce peak flows, manage release rates, and protect 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, and downstream neighborhoods may experience increased nuisance flooding without a clear single "failure event." This is why detention evaluation must include both technical performance and ownership/ 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. This directly supports policy within this plan clarifying and verifying private maintenance responsibilities and ensuring stormwater impacts are responsibly managed under buildout conditions. A practical evaluation approach includes: ORDINANCE • 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. ROADWAY/BRIDGE PERFORMANCE AND OVERTOPPING FREQUENCY Roadway crossings are high-consequence assets because overtopping can create hazards for motorists and disrupt emergency access. Crossings are also hydraulically sensitive because they often control upstream water levels during large events. Overtopping risk is influenced by opening size, roadway approach profile, debris susceptibility, downstream tailwater, and channel stability around the structure. The City will manage crossing risk through: • Storm event documentation (where, when, duration, observed conditions). • Monitoring and closure protocols (operational readiness). • Maintenance actions that reduce avoidable blockage and preserve effective capacity. • Capital planning where improvements provide measurable risk reduction and do not create downstream harm. This approach aligns with policy in this plan that focuses on proactive risk reduction and sound system performance practices and supports transparent communication that closures are safety actions taken to protect motorists and responders. 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 consequence beyond property impacts—such as isolating neighborhoods, restricting access to public safety facilities, or disrupting routes used during emergencies. This master plan supports maintaining: a current inventory of critical routes and facilities, a list/map of known closure locations and monitoring points, clear procedures for barricade placement and public alerts, and post-event documentation that records closures and supports follow-up evaluation. ENVIRONMENTAL AND HABITAT CONSIDERATIONS Creek corridors and open space provide community and environmental value and are also functional stormwaterassets.System evaluation should recognize that stability,conveyance reliability,and water quality outcomes can be improved through strategies that protect long-term channel function and maintainability.Stabilization projectsand channel improvementsshou Id consider habitatand long-term maintenance access. Where feasible, multi-benefit projects can align stormwater performance with open space goa Is—i m proving creek corridor safety and f unction wh i le enhancing com mu nity amenities. 40STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN Water quality protection is also tied to system evaluation. Sediment transport, erosion control, illicit discharges, and construction runoff all influence receiving water quality. Evaluating stormwater performance therefore includes recognizing where operational practices and best management practices can reduce pollutant transport and improve system reliability. Southlaku,Texas Legend u Southlake Drainage Master Plan _ EXHIBIT 1 �_�sou make car limns CRITICAL STRUCTURES Southlake Drainage Basin Sa0tamber I019 tnp --- Strearn C Drainage Complaints / .`. Critical Structures y, �K n�1_ • Deficient \(�I • Improved ' ` •, • Inaccessible \ • Need Data i -• • NonS[ructurs ti • Sufficient 1 T • under Consimcn- t 9 114 EV W •J _ cn ®gym c7 d w • i OD v, r ■ ■ tn■ - r as ••�� - u,�,�._cn:�,�r, - Figure 5-2:TNP Critical Structures Map Exhibit r r.. MC' CHAPTER6: WATER QUALITY ANDREGULATORY 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. MS4 / water quality compliance. Southlake operates a regulated small Municipal Separate Storm 42 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN 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 1S, 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 SO percent rule (damage equals or exceeds SO% 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 Policy Statement minimum control measuresthatdefinewhatregulated cities must implement.The Environmental Protection W02. Sustain compliance with MS4 and Agency's (EPA) Phase II framework describes these floodplain-related r- . - measures and emphasizes measurable goals and using the policy flexibility to exceed ongoing implementation. TCEQ's TXR040000 minimum standardswhen permit is described as comprehensive and requires improves outcomes - . - specific best management practices and measurable goals by MS4 level. For Southlake, these requirements translate into a program that must be both implemented and ORDINANCE 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). The six minimum control measures 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 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, Policy Statement training records, inspection logs, IDDE investigations, enforcement actions, W01. Protect - quality municipal facility practices). through consistent implementationof • Use performance tracking to show progress regional stormwater • est management and support continuous improvement (this practices across municipal operations, is where the Stormwater Master Plan's construction activity, and • •st- responsibilities. recommended dashboards and annual construction "program snapshot" reporting fit). l ff y� �r/ L� 'lty5.. 44 STORMWATER MASTER PLAN SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN 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 matterjust as much because development impacts do not end when construction ends. Post- construction requirements address long- e_- term runoff and pollutant transport and ti establish maintenance responsibilities for permanent controls (detention, water quality features, drainageways, outlet controls). Clear ordinances, predictable submittal requirements, and enforceable maintenance expectationsIt are essential for long-term compliance ,_`ram „ • ,i"` and performance. This is also where the F - - �•,w ,��, ' '''�� Stormwater Master Plan's ordinance modernization direction is most { ; i 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 A 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. STORMWATER MASTER PLAN 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. ORDINANCE .�t �' r Iv 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, park projects, and targeted upgrades rather than large- scale system expansion. Green infrastructure provides a practical toolbox for 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. This chapter frames these strategies as complementary not a replacement for conveyance and storage so the City can apply the right solution to the right location and achieve measurable risk reduction and system reliability benefits. This chapter also emphasizes that successful green infrastructure depends on feasibility, maintainability, and clear responsibility for long-term operation. Many green infrastructure strategies can deliver strong performance when they are incorporated early in project planning and when maintenance expectations are established from the start. Conversely, features that are difficult to maintain or lack clear ownership can lose effectiveness and create long-term liabilities. By grounding green infrastructure and resilience approaches in regional integrated stormwater management guidance, and by focusing on strategies that match Southlake's built environment and maintenance realities, Chapter 7 supports a realistic, scalable approach to runoff reduction and creek corridor stewardship that improves stormwater outcomes while also delivering community co-benefits where appropriate. This chapter also supports an important communication message: green infrastructure is not "either/or" with traditional drainage improvements. It is a complementary toolbox that can reduce pressures on the existing system, improve water quality outcomes, support channel stability, and create community co-benefits when applied in the right places. The goal is not to implement green infrastructure everywhere; the goal is to apply the right strategy to the right context especially where solutions can deliver measurable benefit and long-term maintenance responsibilities are clear. This approach aligns with Southlake's policy direction to advance iSWM-aligned practices where feasible and to prioritize strategies that mimic natural hydrology by soaking it in, slowing it down, and spreading it out. STORMWATER MASTER PLAN 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 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 Stormwater Master 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 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 • . • it in, conveyance, and flood control issues associated with slowing it down, and spreadingout development and redevelopment, and it is intended through feasible low-impact and green 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. ORDINANCE 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. � z i 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 Stormwater Master 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, but certain sites may offer multi-benefit opportunities when operational feasibility and maintenance responsibilities are clear. 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 • STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN 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. ADAPTIVE RESILIENCE AND CLIMATE CONSIDERATIONS Resilience recognizes uncertainty related to rainfall intensity, infrastructure aging,and redevelopment patterns. Over time, stormwater performance can be influenced by updated rainfall standards, cumulative impervious changes, and aging infrastructure that loses capacity or reliability. In this context, resilience is not a single project—it is an approach to managing uncertainty through disciplined planning, monitoring, and staged investment decisions. 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. ORDINANCE • r IN-,iL 3- �� _ ai. - tea. _ ... - y, '•+ Nib � �1 v��.'�'Y `p.+. k' `T _.�.y - .^� r pny�`.Z4 y� '%6 if`•q,-d,_�y�y,�.a bf.':.�5'i '• ��.,• `• <'y�� f � ...aR _i ��' k�. e� p'i r�.ti;� (.}J Tr-�t". _ ...r_. ti ,+3a �� t+i a .�4' le 1 '�e .''.ffe��.+, �'yN' a✓:4.- _ • 1 1 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 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" A complete and current inventory is the foundation for managing stormwater as a public asset portfolio. The inventory must be GIS-based and must serve as the City's "system of record." This means 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, work history,and supporting documentation. If stormwater information is fragmented across spreadsheets, paper files, or staff memory, the City will inevitably lose continuity, create inconsistent responses, and struggle to defend investment recommendations. A system of record prevents that by creating a single authoritative foundation that staff maintain over time. The inventory must include unique asset identifiers with consistent naming conventions so assets can be tracked across inspections, maintenance, and capital projects. It must include standardized attributes such as asset type and function, size/material where known, ownership/responsibility (public, private, HOA,shared), right-of-way/easement status, and access constraints. Where feasible, connectivity (upstream/downstream linkages) should be captured so staff can quickly understand ORDINANCE how an issue at one location relates to upstream contributions and downstream constraints. The inventory should also include condition indicators and known issues such as repeat blockage locations, erosion hotspots, and overtopping history. Finally, it must link to work history inspection dates, maintenance actions, photos, and related documentation because work history is often the key factor that distinguishes a one-time issue from a chronic performance problem. 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 in a way that improves continuously. Staff must standardize fields and ID conventions, prioritize completeness for critical assets and known hotspots, expand coverage through field verification and project updates, and integrate work order history and inspection notes so the system becomes more valuable every year. This is a central building block for AM1 because the City cannot manage assets as a portfolio without a reliable inventory foundation. 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) STORMWATER MASTER PLAN 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) 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. reliablePolicy Statement Policy Statement AM1. Manage stormwater infrastructure as AM2. Sustain system portfolio . - maintenance,an inventory, understanding . renewaland criticality, . _ . . channels, - facilities. maint- - - stment. J L_ 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 ORDINANCE 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 be immediateand 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. 04 OPY 56 f ` y . STORMWATER MASTER PLAN SOUTHLAKECOMPREHENSIVE STORMWATER MASTER PLAN 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, particularly where overtopping occurred, where erosion is likely, and where debris accumulation is common. This practice directly supports policy in this section with emphasis on proactive inspection, maintenance, and renewal in rights-of-way and improved easements. 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 be consistent and meaningful—such as inlet blockage, localized ponding, erosion, culvert/ crossing concern, private drainage maintenance issue, suspected illicit discharge, and construction runoff concern. Standardization allows the City to identify patterns, justify proactive maintenance targeting, and use customer concerns as an input to planning while maintaining risk and feasibility as primary drivers. This is the operational backbone that supports CS1 and ensures customer service improves decision-making rather than destabilizing it. 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. Staff must begin building a renewal forecast structure as part of asset management maturity. A renewal forecast connects what the City owns, what condition it is in, what it costs to preserve service outcomes, and how funding needs evolve over time. This is essential for transparent budgeting and aligns directly with policy within the Plan because sustainable funding evaluations must be supported by appropriate financial analysis tied to documented needs. Renewal forecasts provide the documented "why" behind long-term obligations and prevent funding discussions from becoming reactive after failures occur. ORDINANCE 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 program efficiency and fairness; it is also a matter of legal and fiscal accountability. 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 Policy Statement with the Texas Constitution's prohibition on granting AM3. Clearlydefineversus public money for private benefit without a valid private _ r responsibilities and public purpose and appropriate controls. strengthenhow the City communicates, supports, and verifies private Because of these constraints, staff must treat maintenanceons where private responsibility clarity as a core programfacilities influence 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. r: �Y2 r r Y1 5 a.y , FAI , �r� C�-��_ - -:-'�+•n � -. jam' zo '' dip Com dix V _ b- UL CHAPTER 9 : DEVELOPMENT STANDARDS ANDFRAMEWORK Chapter 9 explains how Southlake uses development standards, ordinances, and technical criteria as a primary stormwater management tool especially as the community approaches buildout and most change occurs through redevelopment and reinvestment. In a built-out city, long-term stormwater performance is shaped less by large new infrastructure expansion and more by whether each project, parcel, and corridor change is completed in a way that responsibly manages runoff and does not shift risk to neighbors or downstream systems. Clear, modern standards provide the mechanism for doing that. They establish expectations before construction begins, when stormwater solutions are least costly to incorporate, and they create predictability and fairness for applicants, staff, and residents by ensuring similar projects are evaluated consistently using transparent requirements. This chapter also explains why Southlake benefits from aligning with regional best practices particularly the North Central Texas Council of Governments integrated stormwater management framework where it improves clarity, consistency, and long-term outcomes. Stormwater is a watershed issue, and regional alignment helps ensure redevelopment across the basin reduces cumulative impacts rather than compounding them over time. At the same time, this chapter emphasizes practicality: standards must be scaled to project risk, maintenance responsibilities must be clear and enforceable, and documentation must support long-term accountability for post-construction controls. By tying together buildout evaluation, iSWM-aligned runoff reduction, standards modernization, floodplain administration, and private system accountability, this chapter provides the policy and implementation framework needed to protect system performance while supporting reinvestment and redevelopment in a predictable, defensible way. 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. STORMWATER MASTER PLAN 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 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 master plan supports modernization of standards in a way that preserves Southlake's local priorities while aligning with regional best practices where appropriate. Policy Statement Policy Statement DR1. Administer and enforce stormwater DR3. Explore ordinance and development regulations requiring new development - related buildoutredevelopment to demonstrate responsible coverage and runoff mitigation to reduce management of impacts under full - time, while conditions, consistent with applicable state maintaining fairnessadministrative law and City oupstream. downstream . - where k�_ applicable. -A BUILDOUT, DOWNSTREAM PROTECTION, AND CUMULATIVE IMPACT MANAGEMENT Po/icy StatementDR7 requires new development and redevelopment to demonstrate that stormwater impacts are responsibly managed for full buildout conditions, including upstream and downstream considerations where applicable. 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 the combined effect of many projects overtime. If each project is evaluated only under immediate conditions, the City risks approving incremental changes that, in aggregate, increase peak flows, worsen downstream constraints, or accelerate erosion in channel corridors. Buildout-based evaluation provides a long-term, equitable lens that ensures projects contribute to responsible watershed outcomes rather than creating "future 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 Po/icy 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. ORDINANCE 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 Policy Statement reference established regional practices, the City's requirements are easier to explain and justify. DR4. Updateordinances and technical standards to align with Third, iSWM supports practical runoff reduction regional strategies where appropriate, strategies that directly address cumulative impacts. recognizing stormwater DR2 emphasizes feasible practices that mimic and basin issue that benefits natural hydrology soaking it in, slowing it down, and coordinated practices. 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 Po/icy Statement DP4 emphasizes modernizing stormwater ordinances and technical standards to align with regional strategies where appropriate, recognizing that stormwater is a watershed and basin issue that benefits from coordinated practices. Standards modernization is not primarily about adding complexity; it is about improving clarity, fairness, and enforceability while ensuring technical evaluation is scaled to risk. 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 s. JOr 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. Po/icy Statements WS4 and WS5 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 NFIP 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 Erosion control standards and channel protection practices reduce long-term maintenance burdens ORDINANCE t _ /r ♦ yt�-Z':�w�i � LIr 'r"' l� +� ��, err�.�h _ -• +p��s "' r.�t�L i t • f 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. This section is also important for setting expectations. 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. 64 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN fly sa H�74.. ..{ .� , ♦ - - - - - r4 ? 4 T S r ' °` - t ,�� 4 -- � � ` )�ii5>� �� aWi rI'n f� • �� ` i � �,�'r•h� ..o --. +- -.. .., .T r+F . ty '.• M. yi F�.l drPlll��l�7�1� � 'rf., . r ! ��' 1 ,2• s a'.'�Ri�4CC_ *�,��.`�' � J � -, `y Y !' .'�. L [V3� ,"- "?>' i 't ' �`• y �`u x'd s: e • '�a t�'�} `9''iK ?'.'.�\rA�"+� i'„.i '? � S=�.yL�,e,tk,. .I 4,yafy,�;h ,�, .: i wk?'�YS a� Sg��f�!•,�rU?„�� f 'r u rw''y,�r'`� !b w1.,$a �`a`r_' ah f 14� �� > fK�`'�. ��, Vria�^s&�' 3 `:r,C is s• '��'j '�'> + t hr y ,•i > X' ... 4 i �t .' Ye 't• `�Zy�k Y11 po - +..� .F V1r .''� y �:: _ j`; � > �z'�:+` rl?ice,,,is 1.�a..1 r �'-z� ` �t i ��\ a. ♦'.'(=. _IM ro 1'.... .�J`r- .. • I.'f Y- !tee ..._.-�k _ 01 V CHAPTER 10: COMMUNITY PARTNERSHIPS, ANDFUNDING Chapter 10 explains how the Stormwater Master Plan becomes day-to-day practice through consistent communication, clear program pathways, and sustainable resources. Stormwater is one of the City services residents experience most directly during stressful moments like 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 HOA's may need to do when private maintenance responsibilities are involved. This chapter is designed to strengthen that trust by establishing a consistent customer service and communication framework, building a robust public education approach, and clarifying how resident concerns are documented and used as an input to planning while objective risk and feasibility remain the primary drivers of prioritization. This chapter also connects implementation to partnerships and funding because stormwater performance is 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 across departments and, at times, across jurisdictions. By defining how stormwater concerns move from intake to response, from recurring hotspot to investigation, and from study finding to capital planning, the chapter provides the practical structure needed to implement the policy direction of the Stormwater Master Plan. It also frames funding and partnerships as essential tools for achieving the plan's outcomes, ensuring the City can align resources with documented needs, pursue grants and interlocal coordination where beneficial, and maintain transparency and accountability in how stormwater services are delivered over time. 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. STORMWATER MASTER PLAN 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, 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, Policy Statement safety,and property.A consistent customer response framework improves trust and reduces repeat issues CS1. Incorporate documented - • - by ensuring similar concerns receive similar handling, drainage concerns as an input to documentation, and communication. This framework prioritization through clear, objective also protects the integrity of prioritization. Policy criteria ensuring customer Statement CSl does not require that customer informs decisions while data-driven concerns become the primary driver of investment and - • 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 B - 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. ORDINANCE 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. For Southlake, communications and education should be structured around consistent messages delivered through predictable channels. The objective is not to overwhelm residents with technical J . Ab 68 STORMWATER MASTER PLAN SOUTHLAKE COMPREHENSIVE PLAN sr - STORMWATER MASTER PLAN 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, . . violations, . awareness gaps and improve community supportedpractical maintenance steps, . . achieved.by fair and consistent enforcement where prevention. L_ voluntary compliance is not 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. 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. ORDINANCE 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. STORMWATER FEE - BENCHMARK ( RESIDENTIAL) $11.58 $11.68 $12.00 $10.00 $8.00 $6.25 $6.50 $7.00 $7.08 $5.00 $4.00 $4.50 $4.50 $1 0 J`eye P\` �e�.er yoy�e� J a `t ao Ja mooA c o c aim y�o �� N Oe yo P` rya Go Figure 10-1:Southlake Benchmark Cities Comparison of Monthly Residential Stormwater Fee The Stormwater Master Plan ties funding discussions to documented needs so conversations remain transparent and defensible. Policy Statement FNl supports evaluating sustainable funding strategies, including rate/fee structure options every five years supported by appropriate financial analysis, so resources align with documented risk and long-term obligations. Policy Statement FN2 supports equitable funding approaches that reflect demand drivers and system impacts while maintaining transparency and administrative feasibility. Policy StatementFN3 supports pursuing grants, interlocal partnerships, and cost-sharing opportunities especially where privately owned facilities materially influence public risk while maintaining clear standards and accountability for outcomes. 70 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN LU Policy Statement • Explore sustainable funding ■ strategies LL1 structure options years supported by appropriate Lu financial analysis, • resources align with documented needs, obligations.sk, and long-term ■ • AUTHORITY& INFRASTRUCTURE& CONSIDERATIONS z LEGAL SYSTEM POLICY&COMMUNITY GUARDRAILS MAINTENANCE Policy Statement Funding discussions should be grounded in the obligations FN2. Evaluate tiered and equitable documented throughout this master plan: the asset funding approaches - inventory and renewal forecast foundation (Chapter 8), stormwater demand documented hotspot maintenance needs, watershed system impacts, while maintaining study cycle commitments, known high-consequence transparency, predictability, vulnerabilities (crossings, erosion threats), and the staffing administrative and implementation needs required for stormwater quality compliance. This framing is important because it clarifies that stormwater funding is not simply about "building projects." It is about sustaining service levels and protecting public safety and infrastructure reliability over time. Policy Statement Pursue grants, interlocal Partnerships are a key implementation strategy, not an partnerships, and cost-sharing afterthought. Regional watershed studies and shared opportunities especially modeling can reduce duplication and improve basin-wide privately owned facilities that outcomes. Interlocal coordination for creek corridors can materially influence public address shared constraints that no single city can solve alone. while maintaining clear Grant partnerships can support mitigation and stabilization and accountability for outcomes. 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. J• / lie J CHAPTER 11 : PLANADOPTION & 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. 72 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN 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 is the SPIN Open House / Town Hall Forum on March 2, 2026 at S:00 p.m. in the Southlake Town Hall Council Chambers. The City's Comprehensive Planning page noted that this open house is 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 S,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 ORDINANCE 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. 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. 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. 74 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN '. �. l- _ Y�.w•. � I I w... /III J r ' r< "'ra„�..-r..mow►—, ,rl.' tea► �r� f � ' . NI • �. y I :t / ate. �,,,/"_� � � am � 1 ~W, Ty pf, f' Compre' nensivE.: d STORMWATER MASTER PLAN POLICY STATEMENTS SYSTEM PERFORMANCE & PUBLIC SAFETY (SP) Convey,store,and manage stormwater to 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 - . - 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 WS5 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 & - - Administer and enforce stormwater regulations requiring new development and DR1 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. 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 DR3 coverage and runoff mitigation to reduce cumulative impacts over time, while maintaining fairness and administrative clarity. Update stormwater ordinances and technical standards to align with regional DR4 strategies where appropriate, recognizing stormwater is a watershed and basin issue that benefits from coordinated practices. WATER QUALITY & REGULATORY STEWARDSHIP (WQ) Protect surface water quality through consistent implementation of regional W01 stormwater best management practices across municipal operations, construction activity, and post-construction responsibilities. Sustain compliance with MS4 and floodplain-related requirements while using W02 the 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. FUNDING, COST RESPONSIBILITY & PARTNERSHIPS (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 risk while 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 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 78 STORMWATER MASTER PLAN I SOUTHLAKE COMPREHENSIVE PLAN STORMWATER MASTER PLAN LULC — Land Use / Land Cover ROM — Rain-On-Mesh (21D 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 TCAD — Tarrant County Appraisal District Accuracy 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. 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. ORDINANCE