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I I I I I I I I I I I I I I I I I I I Land Use To estimate the percentage of impervious area in the DF A 0054 Basin, the various land uses for present and future fully developed conditions were plotted from 1978 aerial photography, current zoning maps and land use plans of the county and municipality involved. Since the City of Thornton does not project any annexation east of Holly Street, the Adams County Zoning Office indicated that it is unlikely development more dense than the 2.5 acres per unit currently allowed by the Agricultural Zoning of this area would occur. This assumption was therefore incorporated into the percent of impervious area estimates. 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 Colorado Agricultural Canal, Lower Clear Creek Canal, Riverdale Road, Holly Street, and so forth. The future land use map is shown in Figure II-3. Losses A variable infiltration rate based on Horton's equation was assumed. For future basin conditions, initial and final infiltration rates and the decay coefficient were based on recommendations in the Urban Drainage and Flood Control District's User Manual - CUHP Computer Profl,'ram. Initial and final infiltration rates under historic conditions were estimated from U.S.D.A. Soil Conservation Service Soil Surveys for Adams County, with minimum and maximum infiltration rates used to estimate final and initial infiltration rates, respectively. A decay coefficient of .0018 per second was used for both historic and future basin conditions. Maximum depression storage values were obtained from the Urban Storm Drainafl,'e Criteria Manual. Uniform values of .4 and .1 inches were used for pervious and impervious retention, respectively, under future conditions and .4 and .05 inches under historic conditions. Flood Hydrog'I'aphs Selected flood hydrographs for the 100-year flood, future basin conditions, as determined using the CUHP program are shown in Figures IIT-2 through IIT-4. For in- formational purposes only, 100-year hydrographs for historical basin conditions are also included at Holly Street and E. 112th Avenue on Figures IIT-3 and IIT-4, respectively. 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 1II-5, and 1II-6. Detailed information on the hydrologic analysis is available at the office of the Urban Drainage and Flood Control District. III-2