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Freeport NIP(7.6)-SY001207Analysis of an Alternate Outfall for the Storm Water Drainage and Detention Study For 144 Acre Tract Duke-Freeport Addition City of Coppell, Texas Prepared for Duke-Weeks Corporation December, 2000 Executive Summary The purpose of this study was to design an alternate outfall structure for the southern detention ponds previously designed on the Duke-Freeport Addition. This outfall structure will allow Duke Weeks to pass the storm water generated from Freeport Parkway as well as the storm water generated on the Champion development site to the down stream creek. The outfall structure is to be designed in a way to only allow a flow equal or less than currently exists (undeveloped conditions) Design Parameters: In order to quantify the amount of water that currently flows across the Duke Freeport Addition and the Champion Tract, a topographical survey was performed. This Survey indicated that a common discharge point was located in the Southwest Comer of the Duke-Freeport Addition. With this information the expected time of concentration as well as the drainage area contributing to this point were determined. Utilizing the Modified Rational Method, an existing flow rate could be determine for the 2, 5, and 100 year storm, as can be seen in Table 1-1 below. TABLE 1.1 - Existing Drainage Areas DRAINAGE AREA C CA Tc 12 Q2 15 Q5 11oo Q~oo AREA (acres) (minutes (in/hr)(cfs) (in/hr)(cfs) (in/hr) (cfs) ) EX DA(A) 146.6 0.31 45.77 43 1.73 79.19 2.20 100.70 3.56 162.95 In order to determine the proposed conditions, the following assumptions were made: )~ The only development that is to take place on the Duke-Freeport Addition is the construction of Freeport Parkway and the excavation of the main detention ponds. )~ Champions is allowed to fully develop their site provided that they satisfy their storm water release requirements dictated by the City of Coppell. Based on this information as well as the Construction Documents for the extension of Freeport Parkway prepared by Pacheco Koch, a proposed drainage area was delineated. Both the existing as well as the proposed drainage areas can be seen on the following page (Figure 1-1). Next Pacheco Koch determined the effective volume that could be utilized in the detention ponds. By studying the topographical survey it was determined that the Duke-Freeport Addition would be able to release their water at a 508.6 elevation. After designing a channel that would interconnect the discharge point to the detention pond, it was determined that the normal water surface elevation would have to be at a 509 elevation with a 100 year water surface elevation of 512. It should be noted that although the normal water surface elevation has increased from the original drainage study, the 100 year water surface elevation has not changed. Another component required to design the outfall structure consisted of picking a rainfall distribution pattern that would effectively model this watershed. As was done in the previous drainage study the Modified Rational Method is only effectively for small watersheds, Pacheco Koch took the approach of using a unit hydrograph. By utilizing the trait hydrograph the same input values such as the rainfall intensity, runoff coefficient, and drainage areas could be utilized. The only values that would change would be the rainfall distribution. This was achieved by taking a ratio of the peak rainfall, which is based on the storm duration, from time index zero and increasing the amount of rainfall as to reach the peak, time index 1.0, then lagging the storm out five times the peak time. The ratio used in this exercise can be seen in Table 1-2. TABLE 1.2 - Unit Hydrograph Ratios Time Discharge Time Discharge Ratio Ratio Ratio Ratio (t/tp) (q/qp) (tJtp) (q/qp) 0.0 0.000 1.7 0.460 0.1 0.030 1,8 0.390 0.2 0.100 1.9 0.330 0,3 0.190 2.0 0.280 0.4 0.310 2.2 0.207 0.5 0.470 2.4 0,147 0.6 0.660 2.6 0.107 0.7 0.820 2.8 0.077 0.8 0.930 3.0 0.055 0.9 0.990 3.2 0.040 1.0 1.000 3.4 0.029 1.1 0.990 3.6 0.021 1.2 0.930 3.8 0.015 1.3 0.860 4.0 0.011 1.4 0.780 4.5 0.005 1.5 0.680 5.0 0.000 1.6 0,560 Analysis of the Watershed The analysis of the watershed involved picking the shape of the outfall structure. It was determined that a V-notched weir would be utilized. The reason for using the V- notched weir was that its shape would aid in designing a release rate needed to comply to City's requirements of the 2, 5, and 100 year storm events. Due to the fact that it was previously determined that the normal water surface elevation would need to be at a 509 elevation and that the 100 year water surface elevation would be at 512, the only variable that needed to be determined was the angle at which the V-notched weir needed to be. Now that the shape of the outfall was determined, Pacheco Koch utilized a program called "Pond Pack" to finalize its design. A trail and error approach was used to insure that a storm water release rate, for any of the three given events, would be equal or less than the storm water flow in existing conditions. The first step in this trial and error approach was to pick a starting angle for the weir. Once this angle was set, the watershed was analyzed by testing the outfall structure at various storm durations in efforts to determine which storm duration would control the design as well as to see if the configuration of the weir was adequate in handling the flows. When it was determined that too much water was passing through the weir, the angle of the V- notched weir was decreased. Once the angle was decreased, the revised outfall structure was then re-tested to ensure that the same storm duration controlled the design as well as to ensure that the amount of flow across the weir was equal or less than in existing conditions. It was determined through these iterations that a 90 minute storm would control the design of the weir and that a V-notched weir with an internal angle of ninety degrees would be sufficient. Provided in Table 1-3 is a summary of the resulting analysis of the various storm events. The inflow as well as the outflow hydrographs generated for the main detention pond can be seen in Appendix Al. TABLE 1.3 - Southern Detention Pond Outfall Design Design Storm Allowable Expected Storm Evenl Event Discharge Discharge Elevation (cfs) (cfs) (ft) 2 Year Event 79.19 55.7 510.93 5 Year Event 100.7 78.4 511.21 100 Year Event 162.95 133.7 511.74 Appendix A1 Southern Detention Pond Design Parameters: · The 2, 5, and 100-year storm events were modeled in this exercise. · The 90-minute storm duration controlled the design. · V-notched weir was utilized, · = 90 degree angle opening, Cw = 2.5 Hydrogmph SOUTH DET OUT 2 yr ! Currently Plotted Curves o~ SOUTH DET IN 2 yr ~ SOUTH DET OUT 2 yr 0 1000 2000 3000 · me (min) Hydrograph SOUTH DET OUT 5 yr SOUTH DET IN $ yr SOUTH DET OUT 5 yr 40 20 0 1000 2000 3000 Time (rain) Hydrograph SOUTH DET OUT 100 yr 150 100 Currently Plotted Curves SOUTH DET IN 100 yr SOUTH DET OUT 100 yr 0 1000 2000 3000 · me (min) Elev. vs. Tin~ SOUTH DET 512.5- 512.04 511.0- Currently Plotted Curves - SOUTH DET OUT 510.$' SOUTH DET OUT - SOUTH DET OUT §10.04 509.5- 0 1000 2000 3000 'Sine (rain)