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<br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />program. The basin was divided into 29 subcatchment areas (see Figure-2, Page 14) <br /> <br /> <br />which averaged 300 acres in size. The model computed the runoff from each sub- <br /> <br /> <br />catchment and routed the flood through the basin. Runoff hydrographs were <br /> <br /> <br />developed for each subcatchment based on a step by step accounting of rainfall <br /> <br />in~iltration losses. surface detention and flow concentration times. <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The rainfall values for various frequencies were obtained from the <br /> <br /> <br />National Oceanic and Atmospheric Administration's technical publication, <br /> <br /> <br />"Atlas 2; Volume III, Colorado" precipitation - frequency charts. A one <br /> <br /> <br />hour storm duration was chosen as the most representative thunderstorm <br /> <br /> <br />duration, for the area. <br /> <br />The "Boulder County Soil Survey", published by the Soil Conservation <br /> <br /> <br />Service, was used to estimate rainfall infiltration rates. Ten different <br /> <br /> <br />soil types are represented in the basin. The infiltration rates for these <br /> <br /> <br />soils vary from moderate to slow with the greatest percentage being in the <br /> <br /> <br />moderate range. Minimum infiltration rates of 0.4 to 0.5 inches per hour <br /> <br /> <br />were selected for each subcatchment based on the soils data. <br /> <br />The SWHM program also has the capability to route floods through <br /> <br /> <br />reservoirs. This was considered to be a key element in the hydrologic <br /> <br /> <br />analysis, since a number of large reservoirs are present in the upper <br /> <br /> <br />basin. Basic operation data, spillway sizes, and potential storage volumes <br /> <br /> <br />were obtained for five major reservoirs. Actual design and construction <br /> <br /> <br />plans from the Colorado State Engineer's files were reviewed to determine <br /> <br /> <br />the potential for flood storage in each reservoir. In the final analysis <br /> <br /> <br />each reservoir was assumed to be full to the level of the normal operation <br /> <br /> <br />spillway. Floods were routed through the reservoirs based on the surcharge <br /> <br /> <br />storage available above that elevation. The reservoir storage was found to <br /> <br /> <br />have a substantial effect in reducing peak flows. <br /> <br />I <br />I <br />I <br /> <br />Detailed computations for the hydrologic analysis were submitted to the <br /> <br /> <br />technical committee as part of the technical addendum. The selected peak <br /> <br /> <br />flows for Dry Creek No. I at specified design points are presented in <br /> <br /> <br />Table-4. <br /> <br />I <br /> <br />3 <br />