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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Atlas (Reference 7). Soil loss rates were determined from County <br />Soil Maps developed by the SCS (Reference 8). <br />The Douglas Reservoir hydrology study completed for the Irri- <br />gation Ditch Company used methods described in "Design of Small Dams" <br />(Reference 9) which is a modification of the SCS unit hydrograph <br />method. This study determined inflow design hydrographs at Douglas <br />Reservoir. <br />The Douglas Reservoir hydrology study completed for the Colorado <br />State Engineer by the Corps of Engineers examined only the Maximum <br />probable Flood at Douglas Reservoir. Loss rates for this study were <br />average rates used by the Corps in the Front Range area. <br /> <br />- HYDROLOGIC PROCEDURE - <br /> <br />SWMM Model. The SWMM model, a very detailed hydrologic computer pro- <br />gram, was selected for use in this study because of the nature of the <br />project (a project that may lead to the expenditure of capital funds), <br />and because of the complexity of the basin. The procedures for use <br />of this model are discussed in the following paragraphs. <br /> <br />Model Setup. The Dry Creek drainage basin was delineated on D.S.G.S. <br />7~ minute quadrangle maps at a scale of 1"=2000' (Reference 10) <br />(Figure 2). The Park Creek Reservoir was added to the maps from <br />aerial photography. The basin was then divided into 190 subbasins <br />draining into 139 channel sections. Basins were selected so that <br />hydrographs would be available at critical design points, particu- <br />larly at irrigation canal crossings and at on-stream reservoirs. <br />A time increment of five minutes was selected for computation of <br />the hydrographs and basin sizes were determined accordingly so that <br />travel times through each subbasin are less than the computation in- <br />crement. In this manner, hydrographs are properly lagged through the <br />study reach. <br /> <br />Input Data. Rainfall information for this study was taken from the <br />NOAA Atlas, Volume III, Colorado (Reference 7). The SOO-year rainfall, <br />not available from the Atlas, was extrapolated assuming a Gumbel dis- <br />tribution. The total rainfall depths for each frequency for a l-hour <br />duration storm were adjusted by an areal reduction factor, and these <br /> <br />-6- <br />