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I I I I I I I I I I I I I I I I I I I This hydrologic computation was made uSing the appropriate Area Adjustments as the watershed increased in size from zero to 26.2 square miles, and el iminating the hydrographs from Reference 3 representing flow from the portion of the basin upstream from Interstate 25. TABLE VI SUMMARY OF PEAK FLOWS BIG DRY CREEK Hydrologic Uncontroll ed Peak Flow Design Drainage (cfs) Point Area (sq. mil 10-yr. 100-yr. Big Dry Creek at 11 ,9101 168th Avenue A 65.4 5550 Big Dry Creek d/s of 11 ,8101 Tributary Confluence B 63.2 5520 Big Dry Creek u/s of 10,6601 Tributary Confluence C 55.9 5190 Bi g Dry Creek at 10,8801 York Street D 52.2 5320 Bi g Dry Creek at 11 ,0502 144th Avenue E 46.4 5490 Bi g Dry Creek at 1-25 F 39.2 5640 11,1903 Peak flows computed in this manner were not as large as the peak flows computed from the enti re watershed storm. Thi s compari son 1 ed to the conclusion that the governing hydrologic condition is a single large storm occurri ng uniformly over the ent ire 65.4 square mil e watershed, wi th the appropriate Area Adjustments applied. Table VI presents peak flows computed in this manner. Figure 4 illustrates three typical hydrographs resulting from this analysis along the Study Reach. Hydraulic Analysis 1 Peak fl ow result i ng from a des i gn storm extendi ng over the enti re Uncontrolled Drainage Area tabulated. A hydraulic analysis was completed along Big Dry Creek within the Study Reach to determine the water surface elevations for the 10-year and the 100-year flood events. This hydraulic analysis was completed using the HEC-2 Water Surface Profile Computer Program developed by the U.S. Army Corps of Engineers (Reference 7). The peak flows (flood discharges) along the Study Reach for the hydraulic analysis were obtained from the results of the hydrologic study summarized in Table VI and Figure 3. 2 Peak flow resulting from a linear interpolation between peak flows at Hydrologic Design Points F and D. 3 Peak flow at 1-25 from Reference 3. Note: All 100-year peak flows include 480 cfs base flow from Standley Lake and Great Western Reservoir. The computed flood profiles and limits of the 100-year floodplain are shown on the Floodplain Maps (Sheets 2 through 16) and the Profiles (Sheets 17 through 26). Tabulated data regarding cross sections, stationing, flood elevations, and other pertinent hydraulic information are found in Table VII. More detailed information regarding the hydraulic calculations is available from the Urban Drainage and Flood Control District. The following paragraphs describe the data collection and adjustments, and the hydraulic assumptions used for the computer model in general and for speCific areas within the model. Cross section data for the hydraulic analysis were developed photo- grammetrically by the digitizing of sections of the channel and floodplain 10