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Section 3.0 Hydrology Analysis 3.4 Rainfall 3.4.1 Criteria 3.1 Subwatersheds The watershed was divided into 92 subwatersheds as shown on the map provided in Appendix A. The subwatersheds were limited to a maximum area of 130 acres. The average subwatershed area was 73 acres. Oak Gulch subwatersheds were numbered 101-116. Likewise, subwatersheds for the Unnamed Tributary and Lemon Gulch were numbered 201, 213 and 301,361, respectively. The design rainfall events modeled for the study area were developed using criteria from the Urban Storm Drainage Criteria Manual (USDCM) (UDFCD, 1969) and the Douglas County Drainage Criteria Manual (1986). Tables 4,1 and 4,2 of the Rainfall Chapter of the USDCM stipulate criteria for general storm durations and adjustments to rainfall based on watershed area. Table 3,1 provides a summary of area adjustments, based on Tables 4,1 and 4,2 of the USDCM, used for this study. 3.2 Streams Table 3-1 Summary of Area Adjustments Watershed Need for Area Storm Area Applicable Locations (sq.mi.) Duration Adjustment 0,5 2,Hour No ' all tributaries , mainstem Oak Gulch - mainstem Unnamed Tributary , upstream portion of mainstem Lemon Gulch 5,10 2,Hour Yes ' downstream 2 miles of mainstem Lemon Gulch Streams (major drainageways) draining approximately 130 acres or more of subwatershed area were defined as shown in Appendix D. To provide an orderly system of managing stream information and hydrologic results, streams (above each confluence point) were designated with an identification number. Streams were numbered consecutively from downstream to upstream. Oak Gulch was designated as Stream 100. Two tributaries were identified in the Oak Gulch watershed (Streams 101 and 102), The Unnamed Tributary was designated as Stream 200. Two tributaries were identified in the Unnamed Tributary watershed (Streams 201 and 202). Lemon Gulch was designated as Stream 300. Nine tributaries were identified in the Lemon Gulch watershed (Streams 301 through 309). One small stream draining directly to Cherry Creek was designated as Stream 400. In all, 17 separate streams were identified within the study area. Each stream shown in Appendix D was divided into a series of adjoining conveyance elements. The conveyance elements were designated with even numbers, increasing consecutively in an upstream direction. Design points (direct routing elements) were coded into the hydrologic models between each pair of conveyance elements. The design points were designated with odd numbers that fall between each pair of even conveyance element numbers. 3.4.2 Rainfall Depths 3.3 Hydrologic Models Rainfall depths for 1 ,hour durations were determined from the Douglas County Drainage Criteria Manual (1986). One,hour rainfall depths were used to produce 2-hour duration storms according to criteria shown in Table 3,1 of the Rainfall chapter ofthe USDCM (UDFCD, 1969). The 1 ,hour rainfall depths for the 500,year storm event were determined by plotting the 10" 50" and 1 OO'year depths on log,probability paper and extrapolating. A summary of the I-hour depths is shown in Table 3,2. The UDFCD's Colorado Urban Hydrograph Procedure (CUHP) (UDFCD, 1995a) model was used to develop runoffhydrographs for each subwatershed. The runoff hydro graphs were routed through the existing stream network using UDFCD's Storm Water Management Model (UDSWM2) (UDFCD, 1995b). The UDSWM2 model does not have a channel infiltration component; however, channel infiltration, even though it occurs, is not expected to significantly affect peak discharges during extreme flood events. Table 3-2 Summary of Unadjusted Rainfall Depths Unadjusted I-Hour Rainfall Depths Storm Event (inches) IO,Year 1.66 50, Year 2.26 100, Year 2.60 500, Year 3.20 3 Oak Gulch and Lemon Gulch FHAD.DOC