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I I I I I I I I I I I I I I I I I I I III. SUMMARY OF HYDROLOGIC ANALYSIS A detailed hydrologic analysis was performed during the Phase 'A' study to define the flood plain according to existing and future basin conditions with the present channel chlU'acteristics. This was also done as the basis for deriving and evaluating alternative solutions to the flooding problems. The Phase 'B' hydrologic analysis consists of defining the magnitude and volume of the storm runoff that is used for preliminllI'Y design of the facilities included in the selected plan. For design purposes, flood flows derived from future basin conditions are used because future development increases the potential runoff. Derivation of Hydrographs There is little recorded information regarding past floods on Little Dry Creek. Reliable rainfall, soils, and land use data are available for Little Dry Creek, however, and these data have been applied to the derivation of synthetic flood hydrographs for the study area. A hydrologic analysis was performed to determine flood hydrographs based on runoff from the 2-, 5-, 10-, 25-, 50-, and 100-year st{)l'ms, and all design hydrographs are based on future basin development, assuming fully developed conditions. Because the present drainage patterns of Little Dry Creek llI'e 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 char acteris tics. The Runoff Block of the "Stormwater 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 Little Dry Creek drainage basin is represented by an aggregate of idealized subareas and channel sections. Given the appropriate rainfall hyetograph, a step-by-step accounti'1l' of rainfall, infiltration losses in pervious llI'eas, surface detention, overland flow and channel flow is made which leads to the calculation of hydrographs at specified points along the channel. m - I The program was modified by the Missouri River Division of the Corps of Engineers in September of 1974 and expanded to include modeling of detention dams and overbank floodway sections used in conjunction with channels or pipes. 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 pllI'ameters and channel segment characteristics. The drainage basin, subareas and channel configuration are shown on the Land Use Maps, Sheets 29 and 30 of the Drawings. Values for the Little Dry Creek Drainage Basin parameters are listed in Table III-2. Channel segment chllI'acteristics have been approximated using cross-sections obtained for the hydraulic analysis. Rainfall Data Flood hydrographs have been derived by routing runoff from design rainfall through the channel system of Little Dry Creek. Design rainfall values for the 2-, 5-, 10-, 25-, 50-, and 100-year return period have been derived from the "Precipitation-Frequency Atlas of the Western United States, Vol. Ill, Colorado" published by the National Weather Service and llI'eally reduced according to Weather Bureau Procedures described in the Atlas. Rainfall values adjusted for the Little Dry Creek Basin are listed in Table III-I. A two-hour design storm was used in the development of flood hydrographs for this study. The hyetograph for the 100-year storm is shown in Figure I1I-4. Land Use To estimate the percentage of impervious area in the Little Dry Creek Basin, the vllI'ious land uses for present and future developed conditions were plotted from 1977 aerial photography, cUlTent zoning maps and land use plans of the counties and municipalities involved. The land uses estimated for runoff calculations lU'e illustrated on Sheets 29 and 30. Percent of impervious llI'ea for each subbasin was obtained by taking a weighted average of the percent imperviousness of the different land uses within the subbasin. Subbasins were outlined on U.S.G.S. 7-1/2' minute quad maps enlarged to 1"- 1000'.