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I I I I I I I I I I I I I I I I I I I Depth-Duration-Frequency Maps in the criteria manual (Reference 4). These depths are summarized in Table 1. The depths were then ad- justed to account for infiltration al1d detention losses in the per- vious and impervious areas of the basins. Infiltration losses, 0.5 inches/hour, and detention losses, 0.3 inches for pervious areas and 0.1 inches for impervious areas, were taken from the recommended values in the criteria manual (Reference 4) and were assumed uniform throughout the basin. A 2-hour storm duration was used for calculating frequency discharges. TABLE 1 Rainfall Depths this discharge. The resulting hydrograph was lagged downstream and combined with the hydrograph for the basin below Logan Street. Peak discharges for each design point are summarized in Table 2. Discharge profiles for Harvard Gulch, Dry Gulch and West Harvard Gulch are shown in Figures 1, 2 and 3. Typical hydrographs are shown in Figures 4, 5, 6, 7, 8 and 9. TABLE 2 Discharge Summary Drainage Area (mi.2) Peak Discharges (cfs) 10-Year 50-Year 100-Year Location 10-Year (inches) Harvard Gulch Colorado Boulevard Race Street Downing Street Logan Street Platte River 1.9 3.8 4.3 6.3 7.7 100-Year (inches) 1340 1750 2290 2950 2530 3300 3360 4400 1400*(2200) 2600*(2200) 1900 3230 3600 4860 3150* (2200) I-Hour Rainfall 6-Hour Rainfall 24-Hour Rainfall 1. 75 2.60 3.35 2.30 3.60 4.00 Dry Gulch Harvard Gulch 900 1200 1.8 1332 Runoff Computations Discharge hydrographs were developed for the 10- and 100-year fre- quency storms for the design points shown on the basin boundary maps (Sheets 1 and 2). Drainage areas, stream lengths and channel slopes necessary to develop the hydrographs were measured on U.S.G.S. 1:24,000 sclae topographic maps (Reference 5). Hydrographs were added at each design point after lagging by appropriate travel times determined for each study reach. West Harvard Gulch 680 1090 1300 870 1350 1630 Federal Boulevard Zuni Street Platte River * peak overland flow ( )peak flow through Logan Street culvert Hydraulic Analysis A hydraulic analysis was completed on Harvard Gulch, West Harvard Gulch and Dry Gulch to determine water surface elevations during the 100-year flood event. These elevations were computed using the HEC-II Water Surface Profile Computer Program developed by the U.S. Army Corps of Engineers (Reference 6). 0.6 0.9 1.4 960 1485 1785 Along Harvard Gulch in Logan Park, a spill structure was designed to divert drainage flows to a detention pond when flow magnitude exceeds 1,600 cfs. Routing of the 100-year and the 10-year flood events through this reservoir showed no significant reduction in peak flows. Cross section information was obtained photograrnmetrically by digitizing sections marked on aerial photographs of the stream channel and floodplain (Reference 2). This was the same photo- graphy used to develop the 2-foot contour interval, 1"=100' scale topographic mapping (Reference 2) used in this study to plot floodplain information, The digitized cross section information was supplemented with cross sections taken from these maps, and The outflow hydrographs were combined with those from the Harvard Gulch Tributary to determine flows at Logan Street. The Logan Street box culvert was designed to convey a maximum flow of 2,300 cfs, and therefore, the overflow hydrograph at Logan Street was determined by subtracting out the portion of the hydrograph below -4-