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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 DFA 0054 Basin, the various land uses for present and future fully developed conditions were plotted from 1978 aerial photography, CUlTent zoning maps and land use plans of the counties and municipalities 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.a.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 22 of the drawings. Loss es 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 Users Manual CUHP Computer Prowam. A decay coefficient of .0018 per second was used. Maximum depression storage values were obtained from the Urban Storm Drainag-e Criteria Manual. Uniform values of .4 and .1 were used for pervious and impervious retention, respectively, under future 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 8 through 12. 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 13, 14, and 15. Detailed information on the hydrologic analysis is available at the office of the Urban Drainage and Flood Control District. - 11- HYDRAULIC DETERMINATIONS Analysis of the hydraulic characteristics of DF A 0054 and its tributaries was per- formed to provide estimates of the water surface elevations of each of the design floods. A total of approximately 6.3 miles of channel was included in the analysis. Water surface profiles were computed using the U. S. Army Corps of Engineers HEC-2 computer program. Cross-sections were digitized when the topographic maps were compiled and have been augmented by field surveys at culverts. Locations of the cross-sections are shown on the Flood Hazard Area Delineation Plan and Profile Sheets. Channel roughness coefficients (Manning's "n" values) for these computations were assigned on the basis of field inspection of the floodplain areas. Typical values for Manning's 'n' used in this study are: natural channel sections natural overbank areas obstructed overbank areas maintained grass channel sections- concrete or asphalt channel or overbank areas 0.04 to 0.066 .045 to 0.055 .055 to 0.115 0.03 to 0.04 0.015 to 0.025 Hydraulic losses through culverts on DFA 0054 have been determined with the use of nomographs published by the Bureau of Public Roads, Reference N, and input into the HEC-2 program. Culvert data is listed in Table 3. Computed flood profiles for the 10- and 100- year floods and the floodplain de- lineation for the 100-year flood for future basin conditions are plotted on the Flood Hazard Area Delineation Drawings for the Study Reaches. Floodplain and floodway reference data are tabulated in Tables 4, 5, and 6. More detailed information on the hydraulic calculations is available at the office of the Urban Drainage and Flood Control District.