Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

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Sanderson Gulch Major Drainageway Plan Baseline Hydrology Report October 2012 The following reservoirs within the Mississippi Gulch hydrologic model were not included in the baseline hydrology model because there is no formal storage volume or agreement for detention: 1. Garfield Lake 2. Huston Lake Stage-storage-discharge curves were established using topographic mapping, design information and previous studies. No design information for Smith Reservoir could be found and stage-storage-discharge data was developed using 2012 aerial mapping and discharges from 1972 MDP. Kendrick Reservoir’s stage-storage-discharge information from the 1972 MDP was found to be consistent with existing conditions and was adopted into the Corrected (Baseline) Hydrology Model without change. Reservoir routing information for Jewell Park Reservoir, Green Gables Entrance Road and Bit-O-Sea Reservoir from the 1972 MDP were determined to be obsolete. The stage-storage-discharge curves were developed using the structure survey data provided by UDFCD, topographic mapping and as-built drawings of the outlet structures. Clairefield Reservoir information was derived from LOMR Case Number 03-08-0090P and modified to extend the stage-storage-discharge curve above the top of the control weir. The Green Gables Park Pond is an irrigation pond that has no outlet. This pond can contain 100-year runoff from the tributary basin because of the small tributary area. The reservoir routing information for this pond was derived from topographic mapping and field measurement. UDFCD's Detention Volume Estimating Workbook Version 2.03a was used to develop stage-storagedischarge information for input into the SWMM model. Overflow elements were added within SWMM where they were needed to convey the full 100-year storm event and ensure no inadvertent detention was being modeled at any of the detention ponds. A summary of the detention pond stage-storage-discharge information can be found in Table B-6 in Appendix B. 3.3.6 Hydrograph Routing Parameters for the SWMM model conveyance elements were determined using the mapping described in Section 1.4. Manning’s roughness values were determined based on the 2009 aerial image supplemented by field observation. Table 3-4 provides the Manning’s n values selected for use in the Corrected (Baseline) Hydrology Model and also provides the 1972 MDP study values for comparison. The Manning’s n values were increased by 25 percent in accordance with the UDFCD-USDCM. The underground storm sewer system was not modeled, except at locations where trans-basin inflows occur. This is the result of storm sewers contravening topography where flows in the pipes are routed down streets and do not follow surface flows based upon topography. There are two trans-basin inflows in the updated hydrologic model and occur from Mississippi Gulch and Federal Boulevard/Jewell Avenue. These storm pipes have less than 100-year capacity. Therefore, in the 100-year model, only the capacity of the pipe is used to determine the trans-basin inflow. Additionally, split flow design points were setup to analyze trans-basin outflows as shown in Appendix B, Figure B-7. These occur where streets or pipe are conveying stormwater runoff into or away from the Sanderson Gulch watershed. Rather than following surface topography, stormwater runoff can be contained in the curb, gutter and cross pans of the roadway in a major event which divert water from the natural watershed. Table 3-4 Manning's Roughness Coefficients & Comparison 2012 Sanderson Gulch MDP and FHAD Routing Element Description Original 'n' Increase 'n' by 25%* Reinforced Concrete Pipe (old) 0.015 0.019 Asphalt Pavement 0.016 0.020 Engineered Channel, Ditch 0.035 0.044 Natural Drainageway with No Brush 0.035 0.044 Natural Drainageway with Brush 0.044 - 0.052 0.055 - 0.065 Natural Drainageway with Trees 0.060 0.075 1972 MDP Initial Run with 0.035, final Run with "n" values determined from formula in SWMM manual using invert slope to approximate friction slope. *UDFCD-USDCM Chapter 4 EPA SWMM and Hydrograph Routing Section 4.2.2. The SWMM input parameters and 100-year future conditions output are included in Appendix B as Table B-7. SWMM model elements, including subbasins, design points and conveyance elements are illustrated within the interactive Hydrology Map of Figure B-1 in Appendix B. Figure B-5 of the Appendix B provides a SWMM routing schematic. 3.4 Mississippi Gulch Major Storm Hydrology The Mississippi Gulch drainage basin is adjacent to the Sanderson Gulch basin, but does not have a good outfall connection to the South Platte River. The watershed includes the existing Garfield and Huston Lakes owned by Denver Parks and Recreation. Although these lakes provide attenuation and inadvertent detention, there is no formal agreement to maintain any stormwater controls within these water bodies. Therefore, their impact to stormwater has not been included in the baseline hydrologic model. In the minor storm event on Mississippi Gulch, a 72-inch pipe transitioning to a 7-foot high by 4-foot wide box culvert constructed in 1981 will convey stormwater from Mississippi Gulch into Sanderson Gulch. In a major storm event, some water will flow easterly in Mississippi Avenue, some will flow down Mississippi Gulch and some will be imported into Sanderson Gulch. UDFCD’s UD-Inlet Workbook for Streets was used to determine the capacity of Mississippi Avenue during a major storm event where the flow in the gutter is 12-inches deep and the street slope is 2%. The amount of surface flow leaving the Mississippi Gulch watershed and flowing to the Sanderson Gulch watershed currently is limited by the capacity of the storm drain. Figure B-7 depicts major event storm flows from Mississippi where an estimated 90 cfs will be in Mississippi Avenue, 987 cfs in Mississippi Gulch and 307 cfs will be imported by pipe into Sanderson Gulch. The “duplicate effective” hydrologic model used to calibrate flows to the 1972 study did not include this transbasin inflow since the pipe did not exist at the time. However, the “corrected effective” model used in this study includes the 307 cfs imported Mississippi Gulch storm inflows. 30
Sanderson Gulch Major Drainageway Plan Baseline Hydrology Report October 2012 3.5 Results – Corrected (Baseline) Hydrology Model Corrected (Baseline) Hydrology Model peak discharges and volumes for the 2-, 5-, 10-, 25-, 50-, and 100- year storm events for selected key SWMM design points are presented in Table 3-5. Peak discharges and runoff volumes for all SWMM design points are provided in Tables B-8 in Appendix B. Figure B-5 in Appendix B shows hydrographs generated by EPA SWMM 5.0 for various storm events at key design points within the study area. Figure B-6 in Appendix B shows peak discharge versus location (station) for Sanderson Gulch and North Sanderson Gulch for all storm events. Table 3-5 Peak Discharge and Runoff Volume Summary (Selected Locations) Locations SWMM ID Total Drainage Area Future Conditions Peak Flow (cfs) Future Conditions Runoff Volume (acre-feet) (acres) (mi 2 ) Q 2 Q 5 Q 10 Q 25 Q 50 Q 100 V 2 V 5 V 10 V 25 V 50 V 100 Sanderson Gulch Confluence at S Platte River Sanderson_Out 4861 7.60 848 1413 1897 3015 3683 4548 179 286 378 535 646 762 W Florida Avenue 1106 4643 7.25 693 1191 1593 2621 3245 4040 153 245 328 474 578 687 Federal Blvd 1114 4334 6.77 663 1142 1522 2352 2909 3628 142 228 304 440 536 638 S Lowell Blvd 1124 4046 6.32 603 1016 1376 2109 2615 3329 123 200 270 396 485 579 Sheridan Blvd 1145 2933 4.58 482 783 1027 1611 1951 2450 87 143 193 284 347 414 Bit-O-Sea Reservoir - Out Bit-O-Sea_Out 2758 4.31 441 714 940 1487 1801 2272 0 0 0 0 0 0 Bit-O-Sea Reservoir - In Bit-O-Sea 2758 4.31 457 732 960 1519 1827 2307 79 129 176 261 320 382 Clairefield Reservoir - Out Clairefield_Out 2618 4.09 449 718 942 1487 1787 2256 0 0 0 0 0 0 Clairefield Reservoir - In Clairefield 2618 4.09 525 838 1096 1799 2196 2653 75 123 167 247 302 361 Confluence with N.S. G. - Downstream 1157 2192 3.42 453 710 923 1478 1773 2125 65 107 145 215 264 315 Confluence with N.S.G. - Upstream 1162T 1431 2.24 226 327 406 569 678 1017 42 67 92 136 165 196 Green Gable Entrance Road - Out GG_Entrance_Out 1335 2.09 206 296 379 530 639 947 0 0 0 0 0 0 Green Gable Entrance Road - In GG_Entrance 1335 2.09 252 399 511 801 976 1180 40 66 88 127 155 183 Wadsworth Blvd. 1167 1169 1.83 166 260 350 617 777 988 33 55 74 108 131 156 W. Jewell Avenue 1169 976 1.53 104 181 254 462 580 729 24 41 56 83 102 121 Jewell Park - Out Jewell_Out 746 1.17 68 120 172 325 413 519 0 0 0 0 0 0 Jewell Park - In Jewell 746 1.17 86 148 209 383 483 599 16 28 39 59 71 85 Kendrick Reservoir - Out Kendrick_Out 416 0.65 4 6 9 13 15 16 0 0 0 0 0 0 Kendrick Reservoir - In Kendrick 416 0.65 62 99 126 195 237 281 15 24 31 43 51 60 Smith Reservoir - Out Smith_Out 251 0.39 11 18 19 19 19 19 0 0 0 0 0 0 Smith Reservoir - In Smith 251 0.39 124 191 239 360 432 510 11 17 22 31 37 44 North Sanderson Gulch Confluence with Sanderson Gulch 1201 748 1.17 225 380 522 931 1167 1440 23 38 52 78 96 116 S Pierce Street 1202 699 1.09 220 367 504 886 1111 1364 22 36 49 73 90 109 Wadsworth Blvd. 1205 384 0.60 129 219 304 538 679 835 11 19 26 39 48 58 Florida Avenue 1206 277 0.43 93 158 221 392 492 605 8 13 18 28 34 42 Green Gables Park 1208T 113 0.18 37 63 87 152 192 234 3 5 7 10 13 16 31
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