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DR1502-ST160206Stream Stabilization Report Stream G6 Prepared for: City of Coppell February 2, 2016 NDM Job No. 15041 Stream G6 Stream Stabilization Report 2 TABLE OF CONTENTS 1.0 INTRODUCTION ............................................................................................................................................ 3 1.1 PURPOSE ......................................................................................................................................................... 3 1.2 BACKGROUND ................................................................................................................................................. 3 1.3 OVERVIEW ....................................................................................................................................................... 4 2.0 DATA COLLECTION AND FIELD RECONNAISSANCE ........................................................................................ 4 3.0 HYDROLOGIC AND HYDRAULIC ANALYSIS ..................................................................................................... 5 3.1 HYDROLOGIC ANALYSIS ................................................................................................................................... 5 3.2 HYDRAULIC ANALYSIS ...................................................................................................................................... 5 4.0 DESIGN ANALYSIS AND CONCEPT PLAN ........................................................................................................ 8 5.0 PERMITTING ............................................................................................................................................... 18 6.0 CONCLUSION AND RECOMMENDATION ..................................................................................................... 18 7.0 REFERENCES ............................................................................................................................................... 19 EXHIBITS Figure 1: Location Map .................................................................................................................. 3 Figure 2: Alternative 1 Site Overview ............................................................................................. 9 Figure 3: Alternative 1 Site Overview ........................................................................................... 10 Figure 4: Alternative 1 Stream Profile .......................................................................................... 11 Figure 5: Alternative 2 Site Overview ........................................................................................... 13 Figure 6: Alternative 2 Gabion Structure ...................................................................................... 14 Figure 7: Alternative 2 Gabion Structure ...................................................................................... 15 Figure 8: Alternative 2 Typical Cross Section................................................................................ 16 APPENDICES A Integrated Environmental Solutions Waters of the United States Delineation B Blackland Prairie Channel Slope Discharge Curve C Opinion of Probable Cost Calculations Stream G6 Stream Stabilization Report 3 1.0 INTRODUCTION 1.1 PURPOSE The purpose of this report is to summarize results from the field analysis, hydraulic evaluation and developed concept plan for the Stream G6 stabilization project in the City of Coppell. This report provides technical data to support the proposed stream stabilization concept plan. 1.2 BACKGROUND The project site is located between East Belt Line Road and East Sandy Lake Road in Coppell, TX, see Figure 1. The project scope begins at South Moore Road and continues downstream approximately 1600-feet to Lot 12 of Northlake Woodlands East, Phase 3. Existing conditions on Stream G6 include bank erosion, exposed utility lines and channel degradation. Slopes of the existing channel range from mild to adverse (3.59% to -0.10%) and the existing channel alignment is mostly straight with minor bends. The existing channel is also positioned between multiple neighborhood developments with homes and pools close to either side of its banks. Figure 1: Location Map Stream G6 Stream Stabilization Report 4 1.3 OVERVIEW This study is based on the concept plan described in the City-Wide Storm Water Management Study for the City of Coppell, Texas, by Albert H. Halff Associates, Inc. (Halff Associates 1991). Data collection, Field Survey and Reconnaissance was the first task performed on Stream G6. During this task Nathan D. Maier (NDM) collected existing topographic maps, property and easement information, subdivision plates, utility plans, storm drain plans, sanitary sewer plans and noted several items of concern in the project site. Hydrologic and hydraulic analyses were then performed to determine the baseline flood elevations and channel velocities within Stream G6. Based upon these results a concept plan was developed, coupled with a hydraulic analysis, to assess the impacts of the proposed improvements on water surface elevations and channel velocities. 2.0 DATA COLLECTION AND FIELD RECONNAISSANCE NDM, with assistance from the City of Coppell, performed data collection and field reconnaissance for Stream G6. Data collected for this study site included existing topographic maps, property and easement information, subdivision plates, utility plans, storm drain plans, sanitary sewer plans and aerial imagery data. Two sets of topographic maps were received from the City of Coppell collected in 2001 and 2007. Both of these were originally obtained from North Central Texas Council of Governments (NCTCOG). The 2001 topographic data set was collected using an Airborne LIDAR providing a vertical accuracy of 5.9-7.8 inches and was provided as 2-foot contours. The 2007 topographic data set was produced using stereo images to automatically generate a digital elevation model which was used to produce 2-foot contours. Property and easement boundaries, subdivision plates, utility plans, storm drain plans and sanitary sewer plans were all obtained from the development plans for Northlake Woodlands East No. 1, 3, 4. Aerial Imagery data from 2013 was provided from the City of Coppell. NDM performed a limited field survey from South Moore Road to Lot 12 of Northlake Woodlands East, Phase 3. Five cross-sections were surveyed along with locations of any utilities and trees six inches and larger from South Moore Road to Lot 7. Stream flowline data was also taken from Lot 7 to the end of the project area. Lastly a field visit was made from several NDM employees. During this visit the stream was examined for adverse conditions such as large bank erosion zones, existing stream protection and exposed utilities. Stream G6 Stream Stabilization Report 5 3.0 HYDROLOGIC AND HYDRAULIC ANALYSIS 3.1 HYDROLOGIC ANALYSIS There currently are no available hydrologic models to provide flow data for this study area. However, the 1-percent-annual-chance flood event discharge was provided at select locations in the City-Wide Storm Water Management Study for the City of Coppell, Texas, by Albert H. Halff Associates, Inc. (Halff Associates 1991). The discharge points from the 1991 Halff study were located at the Moore Road culvert and Bethel School Road. Both discharge locations were outside the scoped region so a gage transfer method was implemented. The equation below was used to transfer peak discharge data from Bethel School Road upstream to the end of Stream G6s scope. 𝑄1 =𝑄2 √𝐴1 𝐴2 (1) Where, Q1 = Estimated discharge at ungaged watershed 1 Q2 = Known discharge at gaged watershed 2 A1 = Area of watershed 1 A2 = Area of watershed 2 Using this approach a 1-percent-annual-chance flood event discharge of 1090 cfs was calculated and used in the hydraulic analysis. Lastly a bank full flow condition was developed for the study area. This was completed by running multiple flows through the baseline condition model until water surface elevations (WSEL) were at approximately bank full levels. A discharge of 350 cfs was determined to be representative of bank full conditions. 3.2 HYDRAULIC ANALYSIS During the hydraulic analysis of Stream G6 four separate HEC-RAS models were developed. These included a baseline condition and three other proposed alternative models. A fourth alternative is provided in this section but no modeling was conducted as it did not evaluate modifying the channel slope or cross section geometry. Baseline Condition: The baseline condition model covers the study area from downstream of Moore Rd to Lot 12 of Northlake Woodlands East, Phase 3. In order to create a stream centerline alignment for this model a combination of topography and imagery maps were utilized to trace the existing Stream G6 flowline. Next 11 cross sections were placed at locations where NDM survey was completed along this alignment. From Moore Rd to Lot 7 of Northlake Woodlands East, Phase 3, five cross sections Stream G6 Stream Stabilization Report 6 were placed aligning with the detailed channel survey. The survey elevation data were then combined with 2001 topographic elevation data to produce station elevation data for these five cross sections. Further downstream from Lot 7 to Lot 12 of Northlake Woodlands East, Phase 3, six cross sections were placed aligning with six channel flowline points recorded by the NDM survey. Since detailed channel survey was not completed at these locations and the 2001 topographic elevation data did not provide accurate channel delineation, an average cross section from surveyed cross sections was developed and placed into each cross section station elevation data set. Alternatives: Once a baseline model was established for this study region it was then used to create three additional alternative models. These models were modified as necessary to capture the proposed channel modifications while preserving cross section locations to allow for comparison between the baseline and alternatives. Tables 1 to 4 below display results from the baseline and all three alternative models under bank full and 100 year flow conditions. It should be noted that all data used for this conceptual analysis was best available and results presented herein do not represent final design values. Table 1: Water Surface Elevations for Base flow Conditions Bank Full Flow Conditions (Q = 350 CFS) Station Water Surface Elevations (ft) Difference (Alternative - Baseline) Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3 1686.85 488.15 487.62 487.62 487.27 -0.53 -0.53 -0.88 1534.38 485.88 485.54 485.49 485.55 -0.34 -0.39 -0.33 1426.89 485.31 484.73 485.32 485.32 -0.58 0.01 0.01 1264.80 482.35 483.79 484.09 484.09 1.44 1.74 1.74 983.53 477.48 478.01 477.97 477.97 0.53 0.49 0.49 802.70 476.78 476.54 476.54 476.54 -0.24 -0.24 -0.24 705.24 476.22 475.65 475.65 475.65 -0.57 -0.57 -0.57 563.96 475.1 474.48 474.48 474.48 -0.62 -0.62 -0.62 432.04 474.15 474.03 474.03 474.03 -0.12 -0.12 -0.12 179.28 471.55 471.34 471.34 471.34 -0.21 -0.21 -0.21 0.00 469.54 469.84 469.84 469.84 0.3 0.3 0.3 Stream G6 Stream Stabilization Report 7 Table 2: Average Channel Velocities for Base flow Conditions Bank Full Flow Conditions (Q = 350 CFS) Station Average Channel Velocity (ft/s) Difference (Alternative - Baseline) Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3 1686.85 7.95 7.03 7.03 8.25 -0.92 -0.92 0.30 1534.38 5.15 6.51 6.64 4.95 1.36 1.49 -0.20 1426.89 3.46 6.68 4.51 4.00 3.22 1.05 0.54 1264.80 8.66 5.22 6.31 6.31 -3.44 -2.35 -2.35 983.53 5.42 2.97 7.31 7.31 -2.45 1.89 1.89 802.70 4.84 4.96 4.96 4.96 0.12 0.12 0.12 705.24 5.94 6.75 6.75 6.75 0.81 0.81 0.81 563.96 7.58 6.68 6.68 6.68 -0.90 -0.90 -0.90 432.04 4.72 4.83 4.83 4.83 0.11 0.11 0.11 179.28 8.85 8.92 8.92 8.92 0.07 0.07 0.07 0.00 6.68 5.64 5.64 5.64 -1.04 -1.04 -1.04 Table 3: Water Surface Elevations for 100 year Flow Conditions 100 yr Flow Conditions ( Q = 1090 CFS) Station Water Surface Elevations (ft) Difference (Alternative - Baseline) Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3 1686.85 490.47 489.94 489.94 489.94 -0.53 -0.53 -0.53 1534.38 487.41 487.5 489.01 489.04 0.09 1.6 1.63 1426.89 487.35 487.24 489.18 489.18 -0.11 1.83 1.83 1264.80 485.17 485.36 485.86 485.86 0.19 0.69 0.69 983.53 480.09 481.08 481.8 481.8 0.99 1.71 1.71 802.70 478.58 478.61 478.61 478.61 0.03 0.03 0.03 705.24 477.82 477.61 477.61 477.61 -0.21 -0.21 -0.21 563.96 476.62 476.56 476.56 476.56 -0.06 -0.06 -0.06 432.04 476.15 476.05 476.05 476.05 -0.1 -0.1 -0.1 179.28 473.74 473.87 473.87 473.87 0.13 0.13 0.13 0.00 471.49 472.03 472.03 472.03 0.54 0.54 0.54 Stream G6 Stream Stabilization Report 8 Table 4: Average Channel Velocities for 100 year Flow Conditions 100 yr Flow Conditions (Q = 1090 CFS) Station Average Channel Velocity (ft/s) Difference (Alternative - Baseline) Baseline Alternative 1 Alternative 2 Alternative 3 Alternative 1 Alternative 2 Alternative 3 1686.85 8.85 9.46 9.43 9.43 0.61 0.58 0.58 1534.38 8.73 9.68 6.01 5.34 0.95 -2.72 -3.39 1426.89 3.53 5.38 3.03 2.97 1.85 -0.5 -0.56 1264.80 9.84 9.38 12.76 12.76 -0.46 2.92 2.92 983.53 9.71 4.71 9.2 9.2 -5 -0.51 -0.51 802.70 9.26 8.48 8.48 8.48 -0.78 -0.78 -0.78 705.24 9.97 9.9 9.9 9.9 -0.07 -0.07 -0.07 563.96 10.33 8.85 8.85 8.85 -1.48 -1.48 -1.48 432.04 5.03 5.18 5.17 5.17 0.15 0.14 0.14 179.28 10.98 10.5 10.5 10.5 -0.48 -0.48 -0.48 0.00 9.44 7.99 7.99 7.99 -1.45 -1.45 -1.45 4.0 DESIGN ANALYSIS AND CONCEPT PLAN When comparing each topographic map to existing ground information, it was found that the 2001 data matched the best. However, the 2007 map provided a channel alignment that agreed with existing conditions the best. Overall, the final channel alignment used during the design analysis was based upon a combination of each topographic map along with aerial imagery. Alternative 1: Alternative 1 was developed as a single solution to prevent additional stream degradation. This consists of placing grade control structures at key locations along the channel to create a stabilized slope condition, as shown in Figures 2 and 3. In order to determine an appropriate stable slope for Stream G6 the Blackland Prairie (Appendix B) and GED No. 54 (Watson 1999) slope methods were applied to this stream. Once calculated, the two were then averaged producing a stable slope of 0.45%. Each drop structure will provide a hard control point in the stream keeping the channel bottom from incising or eroding while providing a stabilized channel slope. Due to dynamic stream conditions at this site each grade control structure must be designed in a way that does not create adverse impacts to the upstream, downstream or adjacent property owners. Figure 4 shows a profile view of Stream G6 with a conceptual design of the size and location of all proposed drop structures. The opinion of probable cost for this alternative is $172,190. Figure 2: Alternativ e 1 Site Overview Figure 3: Alternati ve 1 Site Overview Stream Stabilization Report 11 Figure 4: Alternati ve 1 Stream Profile Stream Stabilization Report 12 Alternative 2: Alternative 2 includes all components from in Alternative 1 along with the addition of gabion walls in two reaches. These four gabion walls have been proposed at key locations where the existing banks have eroded to steep and near vertical slopes. The first structure will begin at lot 5 and end at lot 6 of the Northlake Woodlands East, Phase 3 neighborhood, Station 11+04 to 13+17. As shown in Figures 5 and 6, this proposed structure consists of two gabion gravity walls separated by a gabion mattress to provide channel protection. The second proposed gabion structure will run from Lot 6 to Lot 7 of Northlake Woodlands East, Phase 3, Station 09+03 to 10+02. This proposed alternative will included a gravity wall on the southwest side of the stream along with a gabion wall on the northeastern side, as shown in Figures 5 and 7. Due to the height and weight of this second proposed wall, tie-backs will be required at two tiers along the back of the northeastern wall. Each tie-back will be placed at approximately 10-foot intervals along the wall requiring 20 individual tie- backs for the completed wall. Figure 8 shows typical cross sections for both gabion structures and Figure 9 shows the stream profile. The opinion of probable cost for this alternative is $434,360. Alternative 3: Alternative 3 includes all components from in Alternative 2 along with a slope lay back from the channel centerline through the right over bank facing downstream. This slope was cut at 4:1 (H:V) to provide a stable bank slope and aims to provide flooding relief and reduced velocities at the upstream end of the site. The slope lay back begins at station 16+00 and continues downstream to station 14+13. Refer to Figure 2 for stationing reference. The opinion of probable cost for this alternative is $455,360. Alternative 4: Alternative 4 consist of lining approximately 2,050-feet of channel with 18-inch think rock riprap from the existing gabions upstream of Bethel Rd. to the existing rock riprap downstream of the S. Moore Rd. culvert at station 16+34. No other channel improvements or bank stabilization improvements are included in this alternative. The opinion of probable cost for this alternative is $447,890. Summary: Table 5 compares the opinion of probable cost for the 4 alternatives evaluated. Opinion of probable cost calculations do not include engineering and permitting costs and are provided in Appendix C. Table 5: Opinion of Probable Cost for each Proposed Alternative Proposed Design Total Price Alternative 1 $172,190 Alternative 2 $434,360 Alternative 3 $455,360 Alternative 4 $447,890 Stream Stabilization Report 13 Figure 5: Alternati ve 2 Site Overview Stream Stabilization Report 14 Figure 6: Alternativ e 2 Gabion Structure Stream Stabilization Report 15 Figure 7: Alternativ e 2 Gabion Structure Stream Stabilization Report 16 Figure 8: Alternati ve 2 Typical Cross Section Stream Stabilization Report 17 Figure 9: Alternati ve 2 Stream Profile Stream Stabilization Report 18 5.0 PERMITTING In order to determine the status of Stream G6 as a part of the waters of the United States or an area prone to wetland development, Integrated Environmental Solutions, LLC (IES) was contracted by NDM to perform an assessment of the study site. This included evaluating U.S. Geological Survey (USGS) topographic maps, Soil Survey of Dallas County and the Natural Resources Conservation Service (NRCS) digital soil databases, the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) and aerial photographs of the project site. In addition IES performed a field visit to the site and documented local information including vegetation and stream properties. Once all information was carefully evaluated, IES determined that Stream G6 can in fact be considered as a waters of the United States according to their professional opinion and guidelines provided in 33 Code of Federal Regulations (CFR) 328.3. This was determined based upon the fact that Stream G6 is a tributary to a Traditional Navigable Water (TNW). The tributary for Stream G6 is the main drainage for the project site that empties into Grapevine Creek, which ultimately discharges into Elm Fork Trinity River, a TNW. The complete report from IES is provided as Appendix A. 6.0 CONCLUSION AND RECOMMENDATION As requested by the City of Coppell, NDM has completed a conceptual design and analysis to stabilize conditions on Stream G6. During this process NDM has collected survey information for the study site, developed four alternative design options, produced conceptual HEC-RAS models to assess alternatives and developed an opinion of probable cost for each alternative. All alternatives presented in this report provide different levels of protection for Stream G6 by applying several common stream restoration and stabilization techniques. Alternative 1 provides protection to channel degradation using drop structures as hard control points in the stream. This will keep the channel bottom from incising further and help to maintain a stable slope of 0.45%. However it will not stabilize the existing steep bank slopes. Alternative 2 will provide channel bottom protection through drop structures while also providing channel bank slope protection with the four proposed gabion walls. Alternative 3 will provide the benefits from Alternatives 1 and 2 with the addition of a slope lay back to provide bank stabilization and flooding relief at the upstream end of the study site. This helps to reduce velocities at the upstream side of the stream and provides additional conveyance. Alternative 4 will cover the entire stream from the existing gabions upstream of Bethel Rd. to the existing rock riprap downstream of the S. Moore Rd. culvert at station 16+34’ with 18-inches rock riprap. This will armor the channel bottom but not stabilize the bed slope or channel banks. Since best available data was used for this study and only a limited survey was conducted, results from the HEC-RAS models do not represent final design values, but are intended to identify Stream Stabilization Report 19 potential impacts from the construction of each alternative. Detailed survey and hydraulic modeling should be used to refine the design elements such that no adverse impacts result from the final design. NDM recommends the City of Coppell to consider moving forward with Alternative 3. This design will provide the greatest protection and help reduce future damages to individual properties and City utilities. 7.0 References Halff Associates, Inc., Albert H. City-Wide Storm Water Management Study. Rep. 1991. Print. Watson, CHESTER C., and DAVID S. Biedenharn. "Design and effectiveness of grade control structures in incised river channels of north Mississippi, USA." Incised River Channels: Processes, Forms, Engineering, and Management, SE Darby and A. Simons (Editors). John Wiley and Sons, West Sussex, United Kingdom (1999): 432.