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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 DFA 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. drainage way grades. In the extremely wide portions of the floodplain, the procedure also maintains flow concentration, which with conventional channel routing procedures would be disseminated in channel storage. Lag times were calculated from the velocity estimate computed in the HEC-2 backwater program which was used to determine water surface profiles and floodplain delineations. Routing through existing and proposed detention ponds was completed using the routing and upstream features of the MITCAT catchment simulation model. Flood Hydrographs Losses Selected flood hydrographs for the 100-year flood, future basin conditions, as determined using the CUHP program are shown in Figures III-2 through III-4. The 100-year hydrographs for historical basin conditions are also included at Holly Street and East 112th Avenue on Figures 1I1-3 and 1I1-4, 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 III-5, and 1I1-6. Detailed information on the hydrologic analysis is available at the office of the Urban Drainage and Flood Control District. 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 on Sheet 11 of the drawings. 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 Program. 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 Drainage 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 Routing Hydrographs were combined by appropriately lagging upland sub-basin hydro- graphs before temporally summing flow quantities. This procedure neglects channel storage but is reasonable because of small channel sections and unusually steep ID-3