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CY\ 3-2 I I I I I I I I I I I I I I I I I I I 8 City of Littleton Massey Draw and SJCD jSouth) Major Drainageway Planning update Conceptual Design Report February 2006 3.7 UDSWM Routing Element Parameters The parameters for the UDSWM model conveyance elements east of the hogback were measured from the 2-foot contour interval mapping of the drainageways that was prepared for the project. West of the hogback, the USGS digital elevation models were used to measure the length and slope of the routing elements. Table 4 -100-Year Flows from Flood Insurance Stud Location 100-Year Peak Flow Masse Draw, North Tributa Garrison Street Wadsworth Boulevard 570 cfs 920 cfs Tarqet Flows, Ca I ration o e an o lie ase Ine o e OA Original Original Calibration Baseline Plan Plan Target Percent Design Location Model Model Design Subwater- 0100 (cfs) Difference Point 0100 (cfs) 0100 (cfs) Point sheds Massev Draw 123 13 F HOQback 2,020 2,070 2% 2,070 139 12 D2, F Garrison 2,610 2,763 6% 2,824 110 11 D1,D2,F Wadsworth 2,920 2,964 2% 3,027 North Tributary Massev Draw 142 10 C2 Garrison 572 549 -4% 549 151 9 C1, C2 Wadsworth 919 1,181 29% 1,181 SJCD (South) North Tributarv 104 C I A,B,C Ken Carvl 1,050 946 -10% 946 I SJCD (South) and Tributaries 166 8 E1 Garrison 708 729 3% 573 265 E2 E2 Garrison 283 284 0% 284 122 7 E1,E2 Garrison 990 955 -4% 694 169 6 B1, E1, E2 Wadsworth 1 ,484 1,372 -8% 1,041 259 B2 B2 Wadsworth 714 719 1% 719 154 5 B1, B2, E1,E2 Confluence 1,896 2,151 13% 1,821 273 3 A, B1, B2, E1, E2 S. Platte Canvon 2,441 2,572 5% 2,276 99 1 Confluence 3,314 3,166 -5% 2,879 Table 5 - Comparison of 100-Year Flows: I'b . M did M d'f dBI' M d I UDSWM model elements, including subwatersheds, design points, conveyance elements, and detention basins, are shown in Figures A-1 and A-2 in Appendix A. A schematic of the Massey Draw model network is shown in Figures A-7 and A-8, and a schematic of the SJCD (South) model network is shown in Figure A-9, Conveyance element parameters in the form of the UDSWM input files are shown in Tables A-3 and A-4 for Massey Draw and SJCD (South), respectively. 3.8 Model Calibration For the main stem of Massey Draw, the models were calibrated to approximate the future land-use conditions peak flow rates as reported in the UDFCD memorandum dated February 28, 1985, (Reference 7) and presented in Table 3. Table 3 -100-Year Flows Re orted in UDFCD Memorandum Location 100-Year Peak Flow Ho back 2,020 cfs Garrison Street 2,610 cfs Wadsworth Boulevard 2,920 cfs For the tributaries to Massey Draw, and for SJCD (South), the 1 OO-year peak flows were calibrated to approximate the flows as reported in the Flood Insurance Study (FIS) for Jefferson County (Reference 6) and presented in Table 4, 700 cfs 3.9 Results The 1 OO-year flows from the calibration model matched reasonably well with the target flow values, On the North Tributary of Massey Draw, the flow from the calibration model was 29 percent greater than the target value at Wadsworth Blvd. This difference was due, in part, to the larger watershed that was delineated and discussed previously in this report. The existing detention on Massey Draw, upstream of South Carr Street, and on SJCD (South) upstream of South Kipling Parkway at "The Meadows Sanctuary," was ?dd~d to the calibration .model to create the baseline model. These ponds are publicly owned and maintained and are recognized by UDFCD. They were added to the baseline model to reflect the existing conditions in the watershed. The baseline model for Massey Draw differs from the model presented in the April 2004 Hydrology Report, Modifications to the original baseline model contained herein were made to incorporate flow patterns observed during the June 27, 2004 flash flooding event. Refinement of the subwatershed delineations and routing affected the timing of the flows, resulting in slightly higher peak flows. SJCD South Wadsworth Boulevard South Platte Can on Road 1,895 cfs 2,440 cfs The calibration model included two existing detention basins that are located west of the hogback on Massey Draw. These detention basins were included in the model that was prepared for the 1985 UDFCD memorandum. The model was calibrated by varying the time to peak coefficient (C,) and the unit hydrograph peaking coefficient (Cp) to approximate the1 OO-year flow rates at select locations. Table 5 shows a comparison of the 1 OO-year flows generated from the FIS, the 1985 UDFCD memorandum and the calibration model.