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I I I I I I I I I I I I I I I I I I I planning study. Peak discharges were determined for floods having recurrence intervals of 2-, 5-, 10-, 25-, 50- and 100-years. The analysis included an evaluation of these discharges both for existing development and future development within the basins. The planning of improvement facilities was based on future devel- opment conditions. TABLE 2 Rainfall Losses Infiltration Previous Storage Impervious Storage Other Losses · 0.75 inches/hour 0.50 inches/hour 0.30 inches/hour 5% The hydrology was prepared using the Colorado Urban Hydrograph Procedure (CUHP) as outlined in the Urban Drainage Storm Criteria Manual (Reference 3). This is a unit hydrograph model which was specifically developed and calibrated for use on urban basins in the Denver Metropolitan area. Hydrographs were developed for each of the sub-basins and routed through reservoirs wherever available storage was large enough to affect flood peaks. These hydrographs were then lagged and combined in a downstream direction. Typical hydrographs are shown in Figures 1 and 2. The sub-basins used in this analysis are shown in Figures 2 and 3. Rainfall information was obtained from precipitation-frequency maps provided in the drainage criteria manual (Reference 3). Rainfall data varied for the different sub-basins. Maximum rainfall depths used in this report for each storm frequency during a two-hour duration storm event are shown in Table 1 for the 10- and 100-year events. Discharge profiles used in the subsequent hydraulic analyses are shown in Figures 3 and 4 for Sanderson Gulch and North Sanderson Gulch, respectively. Frequency discharges at selected reference points are presented in Table 3 for the 10- and 100-year events. TABLE 3 Frequency Discharges and Drainage Areas Drainage Area (cfs) (Square Miles) 10-Year (cfs) 100-Year TABLE 1 Rainfall Data Maximum Rainfall Depths Two-Hour Duration Sanderson Gulch At Confluence of South Platte At Federal Boulevard At Sheridan Boulevard At Wadsworth Boulevard 7.6 6.8 5.3 1.8 2,050 1,740 1,000 450 3,050 2,880 1,450 670 10-Year 100-Year Sanderson Gulch North Sanderson Gulch North Sanderson Gulch At Confluence with Sanderson Gulch At Wadsworth Boulevard 1,250 680 2.0 1.9 2.7 2.6 1.4 0.8 800 250 Infiltration and other losses used in this study were determined by calibrating the model using a recorded storm and runoff event in the Harvard Gulch basin, directly across the South Platte River from Sanderson Gulch. Values of these parameters are listed in Table 2. Hydraulic Analysis A hydraulic analysis for this project was completed to determine the water surface profile for the 100-year flood event throughout the study reach. Backwater calculations were completed using the HEC-II computer model (Reference 4), a steady state program developed by the Corps of Engineers. (4)