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I I I I I I I I I I I I I I I I I I I Derivation of Hydrographs Because the present drainage patterns of Little Dry Creek are diverse and resulting routing procedures numerous, the Runoff Block of the Environmental Protection Agency's "Storm Water Management Model", commonly called "SWMM", was selected with the approval of the Urban Drainage District to model the drainage basin runoff character- istics. The Runoff Block of the "Storm water Management Model" simulates both the quantity and quality of runoff and routes flows and contaminants through drainage systems; however, in the case of Little Dry Creek, only the quantity of runoff and routing of flood flows was simulated. The basin is represented by an aggregate of idealized subareas and channel sections. Given the appropriate rainfall hyetograph, a step-by-step accounting of rainfall, infiltration losses in pervious areas, surface detention, overland flow and channel flow is made which leads to the calculation of hydrographs at specified points along the channel. The program was modified by the Missouri River Division of the Corps of Engineers in June of 1973 and expanded to include modeling of overbank floodway sections used in conjunction with channels or pipes and detention dams. These modifications allow the simultaneous routing and combining of flood flows through a number of different system elements, greatly simplifying routing problems in a system such as exists on Little Dry Creek. Input requirements are rainfall hyetographs, watershed parameters and channel segment characteristics. The drainage basin, subareas and channel configuration are shown on the Drainage Basin Map, Sheet 40 of 41 of the drawings. Values for the Little Dry Creek Drainage Basin parameters are listed in Table 2. Channel segment characteristics have been taken from cross-sections used in the hydraulic analysis. -8- Land Use To estimate the percentage of impervious area in the Little Dry Creek Basin, the various land uses for present and future fully developed conditions were plotted from 1977 aerial photography, current zoning maps and land use plans of the counties and municipalities involved. Percent of impervious area for each subarea was obtained by taking a weighted average of the percent imperviousness of the different land uses in the subarea. Subareas were determined from U.S.G.S. mapping and major basin features such as the Croke Canal, U. S. 36, the Colorado and Southern Railroad, and so forth. The future land use map is shown on Sheet 41 of the drawings. Losses Losses due to infiltration and depression storage were estimated from U.S.D.A. Soil Conservation Service Soil Surveys and the Urban Storm Drainage Criteria Manual. Because of the insufficient and conflicting data on typical infiltration decay rates, a zero decay rate was assumed and an average of the minimum and maximum infiltration rates was used as a constant. Maximum depression storage values were obtained from the Urban Storm Drainage Criteria Manual. Values for these parameters are listed in Table 2. Flood Hydrographs Selected flood hydrographs for the 100-year flood, future basin conditions, as determined using the "SWMM" program are shown in Figures 6 through 10. Peak flood flows for the design storms considered in the study have been plotted against the channel reference stationing in the Peak Flood Discharge Curves, Figures 11, 12, and 13. Detailed information on the hydrologic analysis is available at the office of the Urban Drainage and Flood Control District.