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Item 4C - Stacy Drive Flood Study ReportMaterial Copyrighted ©All Rights Reserved STACY DRIVE FLOOD STUDY Unnamed Tributary Southlake, TX Prepared by: TBPE Firm No. F -11976 Prepared for: November 28, 2023 i Holmes Builders Attn: Jon Dostert 225 E Hwy 121, Suite 120 Coppell, TX 75019 Re: Flood Study – Unnamed Tributary Project Name – Stacy Drive, Southlake, Texas Dear Mr. Dostert: Enclosed herewith is the Flood Study Report for the Unnamed Tributary to support the proposed fence construction at three residential lots on Stacy Drive in Southlake, Texas. This report includes hydrologic analysis and hydraulic modeling and results for the existing and proposed conditions for fully developed flows. Should you have any questions or concerns, please do not hesitate to contact me at (214) 986-2283 to discuss any items related to this Report. Sincerely, John W. Foose, P.E. CFM Cardinal Strategies Engineering Services, LLC TBPE Firm Registration No. F-11976 Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 ii Tables of Contents 1.0 Project Description ...................................................................................................................................... 1 1.1 Purpose ................................................................................................................................................................ 1 1.2 Site Location and Proposed Improvements ............................................................................ 1 1.3 FEMA Flood Insurance Rate Map ................................................................................................... 1 1.4 Data Collection ........................................................................................................................................... 1 2.0 Hydrologic Analysis .................................................................................................................................... 4 3.0 Hydraulic Analysis ........................................................................................................................................6 3.1 Pre-Project ...................................................................................................................................................6 3.2 Post-Project .................................................................................................................................................6 4.0 Conclusion ........................................................................................................................................................9 List of Tables Table 1 – Rational Method Parameters & Calculation ......................................................................... 4 Table 2 – Comparison of 100-YR Fully Developed Water Surface Elevations .......................6 List of Figures Figure 1 – Vicinity Map ............................................................................................................................................. 2 Figure 2 – FEMA FIRM ............................................................................................................................................. 3 Figure 3 – Drainage Area Map ............................................................................................................................ 5 Figure 4 - Hydraulic Workmap ......................................................................................................................... 8 List of Appendices Appendix A – Collected Data Appendix B – Hydraulic Data Appendix C – Digital Data Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 1 1.0 Project Description 1.1 Purpose The purpose of this flood study is to evaluate the 100-year fully developed floodplain for an unnamed tributary to Tributary BB-8 in the vicinity 904 Stacy Drive, 900 Stacy Drive, and 300 Sanctuary Circle in Southlake, TX. A hydrologic and hydraulic evaluation was performed for the watershed and stream. This study will establish the ultimate 100-year floodplain for the unnamed tributary and will evaluate the impacts of proposed fences along the eastern boundary of these three lots. 1.2 Site Location and Proposed Improvements The three lots are located in Southlake, TX, north of Southlake Boulevard and east of Shady Oaks Drive. An unnamed tributary to Tributary BB-8 flows along the eastern boundary of these three lots. The site location can be seen in Figure 1. 1.3 FEMA Flood Insurance Rate Map The unnamed tributary is unstudied and currently mapped within FEMA Special Hazard Area, designated Zone X on the Flood Insurance Rate Map (FIRM) 48439C0085K dated September 25, 2009, as shown in Figure 2. 1.4 Data Collection Data obtained in support of the flood study includes: • Texas Natural Resources Information System (TNRIS) 1-m LiDAR Digital Elevation Model dated 2019 • Texas Parcel Data • Record Drawings for Southridge Lakes Phases II, V, & VI The horizontal datum utilized for the project is the NAD83 Texas State Plane North Central Zone FIPS 4202. The vertical datum is NAVD88. Brazos Dr SouthridgeLakesPkwySouthridgeLakesPkwySouthridgeLakesPkwySabineCt 1709 Sabi neCt Normandy Dr B razosDrBlancoCir Ster l i n g C t S t a c y D r 1709 1709GingerCtStacyDr ParkVistaCirW S out hl a k e B l v d W S ou t hla k e B l v d Stacy D r 1709 TimberLakePlLegacyCtCrescent Heights Dr ParkVistaCirT ri b utary B B -8 Justin White Settlement Saginaw Colleyville Flower Mound Keller Haltom City Grapevine Lewisville Coppell Carrollton Irving I:\Projects\2023_0695_HolmesBuilders_StacyRoad_Southlake\Data\ArcGISPro\69_HolmesBuilders_StacyRoad_Southlake.aprx 11/28/2023 jfoose Maxar, Microsoft, Esri, CGIAR, USGS, Esri Community Maps Contributors, Tarrant County College, City of Fort Worth, City of Southlake, Texas Parks & Wildlife, © OpenStreetMap, Microsoft, Esri, HERE, Garmin, SafeGraph, GeoTechnologies, Inc, METI/ NAD 1983 StatePlane Texas North Central FIPS 4202 Feet NAVD88 1" = 200-Feet Prepared For: Figure ± 2770 Capital Street | Wylie, TX 75098 | 469-547-1281 TBPE Firm No. F-11976 | www.cardinalstrategies.com Project Site Boundary FEMA NFHL Water Lines Flood Hazard Zones 0.2% Annual Chance Flood Hazard 0 200100 1Vicinity Map Stacy Drive Southlake, TX Unnamed Tributary Flood Study 48439C0080KEffective: 9/25/200948439C0085KEffective: 9/25/200948439C0085K Effective: 9/25/2009 48439C0090LEffective: 3/21/201948439C0095KEffective: 9/25/200948439C0095K Effective: 9/25/2009 DallasTarrant Denton I:\Projects\2023_0695_HolmesBuilders_StacyRoad_Southlake\Data\ArcGISPro\69_HolmesBuilders_StacyRoad_Southlake.aprx 11/28/2023 jfoose NAD 1983 StatePlane Texas North Central FIPS 4202 Feet NAVD88 1" = 1,000-Feet Prepared For: Figure ± 2770 Capital Street | Wylie, TX 75098 | 469-547-1281 TBPE Firm No. F-11976 | www.cardinalstrategies.com Project Site Boundary FIRM Panel Boundary 0 1,000500 2FEMA FIRM Stacy Drive Southlake, TX Unnamed Tributary Flood Study Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 4 2.0 Hydrologic Analysis A hydrologic analysis was performed for the unnamed tributary to determine the peak discharges in the stream. Because this watershed is fully developed, the existing and fully developed peak discharges are the same. This analysis was done be investigating the development upstream of the site. This site called the Southridge Lakes has a pond outfall is the headwaters of the unnamed tributary. Therefore, the discharge from this pond is a large portion of the flow that is in the tributary. According to the drainage study in the record drawings, the development and pond associated with it discharges 327.7 CFS in the 100-yr fully developed conditions. Downstream of the Southridge Lakes development, 7.4 acres contribute to the tributary up unto the downstream limit of the site. The rational method was used to calculate the flow that this acreage contributes. A time of concentration of 15 minutes was used for the basin. The intensity was correlated to this time of concentration and taken from the Tarrant County table in the iSWM hydrology manual. The contributing acreage is completely comprised of residential backyards which have a high amount of pervious space. Therefore, a coefficient of 0.35 was used. Table 1 shows the calculation. Figure 3 shows the basin. Table 1 – Rational Method Parameters & Calculation Basin Area (acres) Coefficient ‘C’ Intensity (in./hr) Q (CFS) 7.5 0.35 7.98 20.9 By combining the flow ascertained from the record drawings with the additional flow downstream of Southridge Lakes, a total flow in the tributary of 348.6 CFS was calculated. This is the fully developed 100-yr peak flow in the unnamed tributary at the project site. Brazos Dr SouthridgeLakesPkwySouthridgeLakesPkwyHoustonCt SabineCt C rockettCt1709 WaterfordDrSabi neC t Normandy Dr W S o u t h l a k e B l v d W S o u t h l a k e B l v d B razosDrB r a z o sDrBlancoCirPr e si di oCt Ster l i n g C t S t a c y D r 1709 1709 Stratford DrGingerCtN o r mandyDr StacyDrParkVistaCirW S o u t h la k e B lv d W S out hl a k e B l v d Love He n r y C t Stacy D r 1709 TimberLakePlLegacyCtCrescent Heights Dr ParkVistaCirW S o ut h l a k e B lv d W S ou t hlake B l v d 6 9 0 680670650 640 660650640630650 64066065068067066065066 0 6506606506 5 0 6406 5 0 64 0 6 7 0 66 0 660 650 660660670 630660640630650640650 6 9 0 680670650 66066 0 6506606606506 4 0 6506406506 5 0 640640 640 6306306506506 5 0 6506407.47 Ac I:\Projects\2023_0695_HolmesBuilders_StacyRoad_Southlake\Data\ArcGISPro\69_HolmesBuilders_StacyRoad_Southlake.aprx 11/28/2023 jfoose Maxar, Microsoft, Esri Community Maps Contributors, Tarrant County College, City of Fort Worth, City of Southlake, Texas Parks & Wildlife, © OpenStreetMap, Microsoft, Esri, HERE, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, NAD 1983 StatePlane Texas North Central FIPS 4202 Feet NAVD88 1" = 250-Feet Prepared For: Figure ± 2770 Capital Street | Wylie, TX 75098 | 469-547-1281 TBPE Firm No. F-11976 | www.cardinalstrategies.com TNRIS_20 Contours 10-FT 2-FT Drainage Area Project Site Boundary 0 250125 3Drainage Area Map Stacy Drive Southlake, TX Unnamed Tributary Flood Study FLOW AT THIS JUNCTION = 327.7 CFS PER RECORD DRAWINGS (DRAINAGE STUDY, SOUTHRIDGE LAKES PHASE 2) FLOW AT PROJECT SITE IS EQUAL TO FLOW FROM SOUTHRIDGE LAKES DEVELOPMENT PLUS FLOW FROM THE 7.5 ACRE BASIN. TOTAL FLOW = 348.6 CFS Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 6 3.0 Hydraulic Analysis A hydraulic model was created for the unnamed tributary to establish the fully developed 100-year floodplain and to evaluate impacts of the proposed fencing on water surface elevations. A hydraulic model was created using HEC-RAS version 6.4.1. The following sections will provide more detail for both the pre-project and post-project analysis. 3.1 Pre-Project Cross-sections were cut along the unnamed tributary from downstream of the Southridge Lakes development to downstream of the project site. The cross sections were cut off the most recent TNRIS LiDAR data. Recent aerial imagery was used to determine the roughness coefficients for the channel and overbanks of the model. The Manning’s n-values range of 0.035 to 0.055 in the channel and the overbanks respectively. The flows from Section 2.0 were used for the hydraulic model. A normal depth was used for the downstream boundary condition. 3.2 Post-Project The pre-project model was used as the starting point for the post-project hydraulic analysis. The pre-project model was modified to model the proposed fencing along the back of the properties. The fences were modeled as blocked obstructions that are roughly 8’ tall and 0.5’ wide. An ineffective flow area was placed behind the fence. The geometry was left unchanged apart from these fence modifications. The existing flow file was used for post-project as well. The post project model went through several iterations until a fence configuration was achieved that did not affect water surface elevations. A comparison of pre- and post-project water surface elevations can be seen below in Table 2. A hydraulic workmap is included as Figure 4. Table 2 – Comparison of 100-YR Fully Developed Water Surface Elevations Cross-Section Pre-Project WS Elevation (ft) Post-Project WS Elevation (ft) Difference (ft) 610 639.39 639.39 0.00 458 638.03 638.03 0.00 334 637.27 637.27 0.00 214 636.73 636.73 0.00 64 635.61 635.61 0.00 As can be seen in the comparison table there is no rise in water surface elevations due to the proposed fences. In this configuration the proposed fence for 900 Stacy Drive and 300 Sanctuary Circle were able to be placed along the eastern boundary of the Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 7 lots. For lot 904 Stacy drive however, the fence must be placed further inside the property to mitigate its impact. The fence must be placed approximately 46 ft from the eastern boundary at a minimum. Figure 4 shows the exact fence offset from the boundary. GingerCtStacyD rStacyDr ParkVistaCirTrib utary B B-8 658652648646644642638632658 656 654 652648644 654 652 648 646 652 648 646 644 642 64464264 8 64 6 64664464263 6636634644 642 636634636634648 646 642 642638656638634642 658656636 636 6 5 26446326 5 4 644 64864664 6 6446 3 8 63 8648652 646 6466426486446466446 3 463663663 6 644642 646 6346366346 4 6 6486486 4 8 646644 646646 646 646646642642644642 642 638 638638638 634634 658660650640650 640 650640630 640640640640 660640 6 3 0 64 0 6 4 0 640Tr ib 1 Trib164 458 214 334 610 I:\Projects\2023_0695_HolmesBuilders_StacyRoad_Southlake\Data\ArcGISPro\69_HolmesBuilders_StacyRoad_Southlake.aprx 11/28/2023 jfoose Maxar, Microsoft, Esri Community Maps Contributors, Tarrant County College, City of Fort Worth, City of Southlake, Texas Parks & Wildlife, © OpenStreetMap, Microsoft, Esri, HERE, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, NAD 1983 StatePlane Texas North Central FIPS 4202 Feet NAVD88 1" = 100-Feet Prepared For: Figure ± 2770 Capital Street | Wylie, TX 75098 | 469-547-1281 TBPE Firm No. F-11976 | www.cardinalstrategies.com TNRIS_20 Contours 10-FT 2-FT Proposed Fence Proposed_Floodplain Floodplain Existing_Floodplain Floodplain Cross Sections Stream Centerline Project Site Boundary FEMA NFHL Water Lines Flood Hazard Zones 0.2% Annual Chance Flood Hazard 0 10050 4Hydraulic Workmap Stacy Drive Southlake, TX Unnamed Tributary Flood StudyPROPOSED FENCE LOCATION OFFSET FROM EASTERN BOUNDARY 46.0' AT THIS LOT (900 STACY DR) PROPOSED FENCE MAY BE ON BOUNDARY ON THESE LOTS (904 STACY DR & 300 SANCTUARY CIR) Stacy Drive Flood Study Unnamed Tributary – Southlake, Tx November 28, 2023 9 4.0 Conclusion The proposed placement of fencing for the 3 lots causes no increases in 100-year water surface elevations along the unnamed tributary. To achieve this the furthest downstream lot must have its fence offset from the eastern boundary. The two upstream lots may have their fences on the eastern boundary. With this configuration the fences will not affect the floodplain adversely. All collected data is provided in Appendix A and hydraulic output is in Appendix B. Appendix C contains the digital models, GIS data, and an electronic copy of the report and appendices.