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I I I I I I I I I I I I I I I I I I I 3.0 3.1 3.2 3.3 HYDROLOGY The Urban Drainage and Flood Control Design Criteria and the City of Uttleton Storm Drainage & Technical Criteria were used as a basis for all hydrologic analysis. Basin Parameters Sub-basin areas, lengths, centroids, and slopes were determined from 1"=200', 2-foot contour interval topographic mapping. Times of concentration were also determined for each sub-basin. Detention storage depths were taken as the same for all basins -- 0.35 inches and 0.05 inches for pervious and impervious areas, respectively. Infiltration rates, which are based on soil type, varied depending on location. However, most soils in the area were Type C with initial and final infiltration rates of 3.0 in./hr. and 0.50 in./hr., respectively. An infiltration decay coefficient of 0.0018 was used for all soils. Imperviousness was determined for each sub-basin with the aid of the Littleton Zoning Plan and Zoning Regulations. Refer to Appendix A, Table A-1 for a list of all sub-basins with their associated parameters and Figures A-2a and A-2b for the sub-basin boundaries. 3.4 3.5 Desian Rainfall The 1-hour rainfall depths for the 10-,50-, and 1 OO-year events were determined from the Littleton Drainage Criteria and are shown in Table 3-1. TABLE 3-1 1-Hour Rainfall Depths FREQUENCY RAINFAll DEPTH Inches 1 O-yr 1.65 50-yr 2.32 100-yr 2.67 Hvdroaraohs Runoff hydrographs were developed for each sub-basin using the Colorado Urban Hydrograph Procedure (CUHP). The hydrographs were based on fully- developed land use conditions. For basins under 90 acres, a modified time to peak was used in accordance with Drainage District policy. Since all basins were under 160 acres, an estimated peak flow was calculated using the Rational Method. The only basins not analyzed by the CUHP method were those upstream of Mclellan Reservoir. The hydrographs from these basins were determined from previous studies. Flood Routina The hydrographs developed from CUHP were routed using a modified portion of the Environmental Protection Agency's Storm Water Management MDdel (UDSWMM). The storm routing networks represent the existing drainage system, but do not recognize any detention features unless publicly owned. This includes detention behind railroad embankments, non-recognized detention facilities, and privately owned detention ponds. Flood discharge profiles were developed using the UDSWMM output and are shown in Figures A-3, A-4 and A-5. Uooer Dad Clark Gulch Basin The initial review of the background hydrology for Upper Dad Clark Gulch revealed that a 100-year composite inflow hydrograph for Mclellan Reservoir was not available. The following studies were reviewed: 1. Master Plan of Drainaae. Dad Clark Gulch, prepared by Jack G. Raub Company for Mission Viejo CDmpany and UD&FCD, April 1980, revised November 1980. 2. Final Reoort on the Investiaation of the 100-Year Flood Plain on Dad Clark Gulch Across Santa Fe Park Develooment, prepared by Sellards & Grigg, Inc., for the Hardin Company, March 1985. In order to obtain a composite 100-year inflow hydrograph for McLellan Reservoir, seven previously developed 100-year hydrographs were routed using the UDSWMM computer model. Utilizing the results of the CUHP analysis for other sub-basins in the FHAD study, ratios of the 10- and 50-year peak flows to the 1 OO-year peak flow were determined. These ratios were applied to the seven 100-year hydrographs and were routed to obtain 10- and 50-year composite inflow hydrographs for McLellan Reservoir. The hydrology for the Upper Dad Clark Gulch basin was combined with the lower basin hydrology to obtain peak flows downstream of McLellan Reservoir. 4