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DR1801-CN190118 HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 January 18, 2019 Kent Collins, P.E. Director of Public Works City of Coppell 255 E. Parkway Blvd. Coppell, TX 75019 RE: Woodridge Channel Drainage Feasibility Study Dear Mr. Collins, This letter report presents an assessment of the flooding problems along the Woodridge Channel, specifically, at the Wellington Place Apartments (Wellington) and Universal Academy (Universal) located on Sandy Lake Road in Coppell, Texas. These properties have experienced flooding during several rain events, most recently in May and June 2015. The goal of this study is focused on evaluating existing conditions to verify flooding areas and developing two (2) conceptual level alternatives utilizing two-dimensional hydraulic analysis to alleviate flooding at these two properties. STUDY AREA The Woodridge Channel is a tributary of Denton Creek that spans approximately 1 mile from its confluence with Denton Creek to Redcedar Way Drive in the Woodridge subdivision in the City of Coppell. The study portion of the channel drains parallel to Sandy Lake Road and extends from its confluence with Denton Creek to downstream of MacArthur Drive. The channel is not part of a Special Flood Hazard Area (SFHA) according to the FEMA Flood Insurance Rate Map (FIRM). However, the channel is controlled by backwater from Denton Creek which is also controlled by the Elm Fork Trinity River. Universal and Wellington are located south of the channel and contain storm water systems that outfall into the channel. A location map is provided in Figure 1. See attached Exhibit 1 for overall location map. Figure 1. Project Area HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 FLOODING HISTORY In May and June 2015, heavy sustained rainfall throughout North Texas resulted in record flooding in the Denton Creek and Elm Fork watersheds. Residents of the Wellington Place Apartments reported knee high ponding and the City of Coppell initiated pumping of the existing wet well near Universal to reduce flood depths. The USGS gage located at Sandy Lake Road and the Elm Fork Trinity River reported a peak water surface elevation of approximately 444.5 feet, almost 3 feet higher than the lowest ground elevation at Wellington. The storm was reported to be equivalent to a 10-year frequency storm event. As a result, the City of Coppell contracted Halff Associates, Inc. to perform a drainage feasibility study to evaluate alternatives to reduce flood depths at these two properties. PREVIOUS STUDIES The following studies have been previously conducted including the Woodridge Channel. • Woodridge Channel Improvement Concept Study – Teague Nall and Perkins, Inc. (TNP), April 2014 • Denton Creek Drainage Study – Halff Associates, Inc. (Halff), October 2018 The TNP study was used as the basis for this evaluation and was further modified to incorporate local impacts to storm sewer systems within Universal and Wellington properties. The Halff study performed a two- dimensional (2D) analysis for Denton Creek which incorporated the Woodridge Channel for calibration to reported flooding during the May/June 2015 floods. DATA COLLECTION The data collected for this analysis included recorded flood complaints, previous hydrologic and hydraulic studies, and digital spatial data. Digital spatial data was obtained from the City of Coppell provided in ERSI ArcGIS format including the following data: • 2018 City of Coppell Storm Drain Lines and Storm Inlets • 2009 Texas Natural Resources Information System (TNRIS) Light Detection and Ranging (LiDAR) Topography • 2018 City of Coppell Zoning • Drainage Improvements for Lot 1 Wellington Place Addition Design Plans • Wellington Place Apartments flood limit Channel and structure surveys were conducted by Halff in August 2018 at the six culvert crossings within the project limits including Sandy Lake Road, the Christus St. Joseph Village Driveway, the three driveways to Universal Academy, and MacArthur Boulevard. Field survey of the existing storm drains and inlets within Wellington and Universal was performed to obtain ground elevation, inlet size, and inlet flowlines. The datum used was the North American Datum 1983 (NAD 83), and the project coordinate system used was State Plane Texas North Central FIPS (4202). The North American Vertical Datum 1988 (NAVD 88) was used. HYDROLOGIC ANALYSIS The existing hydrologic model that was used to generate storm event discharges was based on the TNP study which utilized HEC Hydrologic Modeling System (HEC-HMS) version 3.5. Utilizing LiDAR topography and storm sewer information, the drainage areas and runoff curves numbers were revised to reflect the added detail required to evaluate the storm sewer system within Wellington called the revised existing conditions model. The revised existing conditions model was based on fully developed land use conditions for the 2-, 10- , 50- and 100-year storm events. The revised existing conditions HEC-HMS model was used to provide input hydrographs at the upstream limit of the study located at the downstream face of MacArthur Boulevard. Hydrology for the remaining drainage areas were computed using 2017 XPSWMM in conjunction with the 2D hydraulic analysis. Table 1 shows a comparison between the previous TNP study and the revised existing conditions model 100-year discharges. HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Table 1. 100-Year Discharge Comparison Location Element Name Drainage Area (mi²) 100 YR (cfs) TNP Halff TNP Halff TNP Halff Difference Upstream of MacArthur Blvd J_MacArthur J_MacArthur 0.43 0.45 1,523 1,686 163 Downstream of MacArthur Blvd J_Mac_DS J_Mac_DS 0.44 0.45 1,566 1,686 120 Downstream of Universal Academy Crosssing J2 WDR_03 (J2) 0.47 0.48 1,656 1,756 100 Sandy Lake Road J3 J3 0.50 0.53 1,733 1,859 126 The differences in discharge can be attributed to additional subbasin delineation necessary to simulate the storm sewer system and inlet capacity within Wellington and Universal hydraulically, minor differences in total drainage areas, and updated Curve Numbers. Curve Number attributes were selected based on the City of Coppell zoning files. Lag times were kept consistent with the TNP study, except for inlet basins within the study area computed using the TR-55 method while maintaining a minimum time of concentration of 10 minutes per the City of Coppell Code of Ordinances Appendix C. Design Criteria and Standards. Channel routing through the Woodridge Channel for the reaches upstream of MacArthur Boulevard utilized the same storage vs area curves determined from the TNP study. See attached Exhibit 2 for drainage area map. HYDRAULIC ANALYSIS Existing Conditions Existing conditions hydraulic analysis was performed in XPSWMM version 2017.1.1. The storm sewer network within the Wellington Place Apartments and Universal Academy were simulated using links and nodes based on as-built design plans and the City of Coppell GIS storm sewer file. Inlets were modeled using inlet capacity curves generated from field reconnaissance measurements. The Woodridge Channel was modeled using cross sections obtained from the TNP study. The hydrograph from node J_Mac_DS (at MacArthur Boulevard) in the revised existing conditions HEC-HMS model was used as the input for the upstream limit of the study. The downstream limit for the study was the confluence with Denton Creek, where a stage-hydrograph from the Denton Creek Drainage Study performed by Halff in 2017 was used to simulate tailwater conditions. Figure 2 displays the XPSWMM model layout. See attached Exhibit 3 for existing floodplain mapping of the 2-, 10-, 50- and 100-year storm events. HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Figure 2. XPSWMM Model Layout Model Calibration The existing conditions model was calibrated to the May/June 2015 storm event. The rainfall during that period correlated to approximately a 10-year storm event. Therefore, the 10-year event was chosen as the calibration frequency storm. Flooding information from the May/June 2015 event, provided from the City of Coppell, was used to facilitate model calibration. As a result, the model was able to reproduce similar flooding limits and depths in Wellington as shown in Figure 3. The City provided survey points for the flooding extents, which were used in the calibration. HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Figure 3. Calibration Results ALTERNATIVE ANALYSIS Two storm sewer improvement alternatives were analyzed to determine impacts on the Wellington Place Apartments. The first alternative includes a pump station located east of Universal where the Wellington system outfalls into the Woodridge Channel. It is currently under design by Vasquez Engineering for the apartment complex. The second alternative includes a redirection of the existing storm sewer system. Three, new four-grate inlets located within the courtyard of Wellington are included in both alternatives to collect areas of ponding. Installation of flap gates at the downstream face of Christus St. Joseph Village Driveway are also included in both alternatives to minimize flooding from Denton Creek within Wellington and Universal. The results are discussed below, along with a summary of the conceptual level construction costs estimates for both alternatives. Detailed conceptual level construction cost tables can be found as attachments to this letter report. Alternative 1 The first alternative selected consists of a proposed force main and wet well for Wellington Place. The proposed plans include two 65 horsepower pumps within a 10-foot by 10-foot wet well and discharge via an 18-inch RCP with a flap gate installed at the outfall. The existing 60-inch is proposed to remain however the existing headwall and flap gate is to be removed and replaced to accommodate the new 18-inch RCP. New flap gates are proposed for both outlets. The pumps were modeled using a multi-link feature in XPSWMM, which defines pump features such as the size and capacity curve. This allows the runoff to discharge completely from the storm sewer system without causing the existing storm sewer to surcharge and flood the apartments. Figure 4 and Figure 5 show flood depths for the 10- and 100-year events. As shown in Table 2 and Table 3, Alternative 1 reduces flood depths by a maximum of 1.5 and 0.7 feet for the 10- and 100-year storm events. There are no adverse impacts on the Woodridge channel. HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Figure 4. Alternative 1 10-year Storm Flood Depth Figure 5. Alternative 1 100-year Storm Flood Depth HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Table 2. Alternative 1 10-Year Depth Comparison Node Name 10 Year Depth (ft) Difference (ft) Existing Alt 1 IN_006 1.4 1.1 -0.3 IN_007 0.6 0.3 -0.3 IN_008 1.4 0.2 -1.2 IN_009 1.7 0.2 -1.5 IN_010 0.4 0.4 0.0 IN_011 0.3 0.2 -0.1 Table 3. Alternative 1 100-Year Depth Comparison Node Name 100 Year Depth (ft) Difference (ft) Existing Alt 1 IN_006 2.0 1.4 -0.6 IN_007 1.0 0.5 -0.5 IN_008 1.8 1.1 -0.7 IN_009 1.9 1.3 -0.6 IN_010 0.6 0.6 0.0 IN_011 0.4 0.3 -0.1 Advantages i. Pump and wet well already designed ii. Relatively quick and easy construction iii. No land acquisition or easement dedication required iv. Minimal impact to surrounding properties v. Reduces flooding within Wellington Disadvantages i. Increased Operating and Maintenance (O&M) Costs The engineer’s estimate of probable construction cost for this alternative is $1.66 million. Alternative 2 The second alternative selected consists of constructing a new storm sewer system to drain the runoff from Wellington into an existing Riverchase Golf Club pond that eventually flows through a series of ponds that outfall into the Elm Fork Trinity River. Constructing a new storm sewer system consists of capping or plugging the existing 42-inch RCP located at the northeast corner of Wellington and installing a new 42-inch RCP that discharges into the Riverchase Golf Club pond. The proposed system spans approximately 1,700-feet on 0.7% slope extending to the pond and traverses through six separate property owners including Wellington. Due to the discharge into the Elm Fork Trinity River floodplain, a coincidental peak was used in accordance with the North Central Texas Council of Government Integrated Stormwater Management Technical Manual Table 1.10 Frequency for Coincidental Occurrences. Table 4 provides the data utilized for the coincidental peak along the Elm Fork Trinity River. Known water surface elevations from the FEMA effective model were used as the tailwater condition for the proposed outfall. Figure 6 and Figure 7 show flood depths for the 10- and 100-year storm events. As shown in Table 5 and Table 6, alternative 2 reduces flood depths by a maximum of 1.6 and 0.9 feet for the 10- and 100-year storms. Table 4. Coincidental Peak Frequencies Frequency Event (YR) Tailwater* (ft) Woodridge Elm Fork 100 10 438.8 *Tailwater elevation from FEMA effective model HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Figure 6. Alternative 2 10-Year Storm Flood Depth Figure 7. Alternative 2 100-Year Storm Flood Depth HALFF ASSOCIATES, INC. 1201 NORTH BOWSER ROAD TEL (214) 346-6200 WWW.HALFF.COM RICHARDSON, TX 75081-2275 FAX (214) 739-0095 Table 5. Alternative 2 10-Year Storm Flood Depth Comparison Node Name 10 Year Depth (ft) Difference (ft) Existing Alt 2 IN_006 1.4 1.1 -0.3 IN_007 0.6 0.3 -0.3 IN_008 1.4 0.4 -1.0 IN_009 1.7 0.1 -1.6 IN_010 0.4 0.4 0.0 IN_011 0.3 0.4 0.1 Table 6. Alternative 2 100-Year Storm Flood Depth Comparison Node Name 100 Year Depth (ft) Difference (ft) Existing Alt 2 IN_006 2.0 1.1 -0.9 IN_007 1.0 0.5 -0.5 IN_008 1.8 1.1 -0.7 IN_009 1.9 1.3 -0.6 IN_010 0.6 0.6 0.0 IN_011 0.4 0.3 -0.1 Advantages i. No O&M Cost ii. Reduces flooding within Wellington iii. Solves backwater issues from Denton Creek Disadvantages i. Requires easement dedication ii. Construction disruption to Wellington residents iii. Diverts runoff onto another property The engineer’s estimate of probable construction cost for this alternative is $1.64 million. COST ESTIMATES Conceptual level construction cost estimates were prepared for each alternative, reflects 2018 unit prices and are provided as an attachment to this letter. All costs presented in this letter report are based on average unit prices per the TxDOT average low bid unit prices and bid tabulations from recent projects constructed within the DFW Metroplex. An allowance for contingencies was included based on 40% of the total construction cost. Engineering service fees are estimated at 18% of the total estimated construction cost. A summary of the conceptual level construction cost estimate is provided in Table 7 below. Table 7. Conceptual Construction Cost Estimate Summary Alternative Alternative Description Cost1 1 Pump and Wet Well $ 1,657,000 2 Storm Sewer Realignment $ 1,641,000 1. Includes 40% Construction Contingency, 18% Engineering Fee D ENTO N C R E E K SANDY LA KE ROAD RIVERCHASEELEMENTARYSCHOOLSMACARTHURBLVD Stream Wellington Place Apartments City Boundary ExhibitDenton CreekExhibit 1Woodridge Channel Drainage Feasibility Study Project Project Area MapTitle Watershed 0 500 1,000 Scale in Feet Aerial Imagery from 2018 NearMap KEY TO FEATURES COPPELL CARROLLTON D ENTONCRE E KSAN DY L AKE ROAD SMACARTHURBLVDWDR_01183.9 Acres WDR_02107.2 Acres WDR_0416.1 Acres IN_0053.4 Acres WDR_0311.2 Acres IN_0021.6 Acres IN_0030.6 Acres IN_0040.8 Acres IN_0080.6 Acres IN_0072.6 Acres IN_0063.0 Acres IN_0091 Acre IN_0101.2 Acres IN_0111.2 Acres IN_0120.6 Acres IN_0011.5 Acres Revised Existing Drainage Area Existing Drainage Area Wellington Pla ce Apartments Stream City Boundary ExhibitDenton CreekExhibit 2Project Drainage Area MapTitle Watershed Aerial Imagery from 2018 NearMap KEY TO FEATURES 0 600 1,200 Scale in Feet Woodridge Channel Drainage Feasibility Study DENTONCREEKExhibitDenton CreekExhibit 3Project Existing Floodplain MapTitle Watershed Aerial Imagery from 2018 NearMap KEY TO FEATURES COPPELL 0 200 400 Scale in Feet Woodridge Channel Drainage Feasibility Study CARROLLTON Stream Wellington Pla ce Apartments City Boundary 10 Year 50 Year 100 Year 2 Year Floodplains DENTONCREEKExhibitDenton CreekExhibit 4Project Alternative 1Title Watershed Aerial Imagery from 2018 NearMap KEY TO FEATURES COPPELL 0 200 400 Scale in Feet Woodridge Channel Drainage Feasibility Study CARROLLTON SANDYLAKEROAD SMACARTHURBLVDWellington PlaceApartments Universal Academy ProposedFlap Gate Nodes Stream City Boundary Alternative 1 - 100 Yr Max Water Depth 0 - 0.25 0.25 - 0.5 0.5 - 1.5 1.5 - 3 3 - 6 6 - 9 9 - 12 12 - 15 Links ProposedWet Well Replacement DENTONCR E EK ExhibitDenton CreekExhibit 5Project Alternative 2Title Watershed Aerial Imagery from 2018 NearMap KEY TO FEATURES COPPELL 0 400 800 Scale in Feet Woodridge Channel Drainage Feasibility Study CARROLLTON SANDYLAKEROAD SMACART HURB L V D ProposedFlap Gate Wellington PlaceApartments Universal Academy Nodes Proposed StormSewer Realignment Stream City Boundary Alternative 2 - 100 Yr Max Water Depth 0 - 0.25 0.25 - 0.5 0.5 - 1.5 1.5 - 3 3 - 6 6 - 9 9 - 12 12 - 15 Links Item Unit Quantity Unit Price Rounded Cost Wet Well Mobilization LS 1 $91,000 $91,000 Care of Water LS 1 $5,000 $5,000 Storm Water Pollution Prevention Plan LS 1 $5,000 $5,000 Implement Erosion Control Plan LS 1 $5,000 $5,000 Flap Gates EA 4 $65,000 $260,000 Wet Well and Pump1 LS 1 $600,000 $600,000 4-Grate Inlet (30" x 18")EA 3 $7,000 $21,000 Storm Sewer (18")LF 210 $75 $15,750 Estimated Cost $1,003,000 SUBTOTAL $1,003,000 40% Construction Contingency $401,200 TOTAL ESTIMATED CONSTRUCTION COST $1,404,200 18% Engineering Services2 $252,756 Total $1,657,000 Assumptions: 1. Wet Well plans and cost were developed by Vasquez Engineering. The estimated cost does not consider O&M. 3. All cost are in 2018 US Dollars as of November 1, 2018. These estimates were prepared utilizing standard cost estimating practices. These statements exclude "soft" costs including, but not limited to, administrative costs, financing costs, USACE permitting, geotechnical investigations, and construction materials testing. It is understood and agreed that this is an estimate only, and that Engineer shall not be liable to Owner or to a third party for any failure to accurately estimate the cost of the project, or any part thereof. Woodridge Channel Drainage Feasibility Study ALTERNATIVE 1 Engineer's Opinion of Probable Cost BY HALFF ASSOCIATES, INC Wet Well and Pump 2. Cost of the Engineering Services is based on 18% of the estimates construction cost, including contingency. Item Unit Quantity Unit Price Rounded Cost Storm Sewer Realingment Mobilization LS 1 $73,000 $73,000 Care of Water LS 1 $5,000 $5,000 Storm Water Pollution Prevention Plan LS 1 $13,000 $13,000 Implement Erosion Control Plan LS 1 $21,000 $21,000 Flap Gates EZ 4 $65,000 $260,000 Storm Sewer (42")LF 1,700 $220 $374,000 Drainage Easement LS 1 $21,000 $21,000 4-Grate Inlet (30" x 18")EA 3 $7,000 $21,000 Storm Sewer (18")LF 210 $75 $15,750 Pavement Remove and Repair SY 1,800 $105 $189,000 Estimated Cost $993,000 SUBTOTAL $993,000 40% Construction Contingency $397,200 TOTAL ESTIMATED CONSTRUCTION COST $1,390,200 18% Engineering Services1 $250,236 Total $1,641,000 Assumptions: 1. Cost of the Engineering Services is based on 18% of the estimates construction cost, including contingency. 2. All cost are in 2018 US Dollars as of November 1, 2018. These estimates were prepared utilizing standard cost estimating practices. These statements exclude "soft" costs including, but not limited to, administrative costs, financing costs, USACE permitting, geotechnical investigations, and construction materials testing. It is understood and agreed that this is an estimate only, and that Engineer shall not be liable to Owner or to a third party for any failure to accurately estimate the cost of the project, or any part thereof. Woodridge Channel Drainage Feasibility Study ALTERNATIVE 2 Engineer's Opinion of Probable Cost BY HALFF ASSOCIATES, INC Storm Sewer Realignment