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I I I I I I I I I I I I I I I I I I I The average slopes of the sub-basins varied from 0.7% to 3.9% with an average of about 2%. The average imperviousness for the basin is about 28% for the existing development, which will increase to about 43% for the future development. A summary of 011 the bosin characteristics is presented in Table 111-1 for the existing and future development conditions. 2. Design Rainfall A depth-duration-frequency graph for Grange Hall Creek Major Drainageway Planning Basin was obtained from the storm isohyetals presented in the Criteria Manual. A review of the isohyets showed that a difference of 0.04 inches to 0.32 inches of total rainfall existed between the upper and lower end of the basin, with the higher point rainfall occurring over the upper end of the basin. This difference was not considered sufficient to warrant different rainfall volues over the upper and lower reaches of the basin. A weighted average was used for the total basin. The resulting point precipitation values ore listed in the table below and plotted in Figure 111-2. 2- Yeor 10-Year 100- Year Storm* Recurrence Recurrence Recurrence Duration Interval Interval Interval ( Hours) ( Inches) (Inches) (Inches) I 0.99 1.47 1.88 6 1.52 2.22 2.99 24 1.86 2.76 3.60 3. Storm Runoff and Hydrogrophs The storm runoff peak volues and total volume are dependent upon various factors, which include total precipitation, incremental precipitation pattern, infiltration rates, antecedent soil moisture conditions, and basin characteristics. The total precipitation and incremental pattern chosen for this analysis is based upon a thunderstorm-type occurrence, since a thunderstorm precipitation arrangement yields higher peak flows. The peak runoff values are generally the critical design element for urban drainage faci Iities. One exception is the design of lorge storm detention reservoirs where maximum inflow volume is important. The infi Itration rates used for this analysis were based on recommendations contained in the Criteria Manual and the U.S.D.A., S.C.S. publications, plus field appraisal. Based upon the soils in the study area, the probable soil moisture conditions, and soil cover complexes, a maximum value of one-half inch per hour infiltration rate was used for this analysis. The rainfall-runoff model used for this analysis is the Colorado Urban Hydrograph Procedure (CUHP) with May 15, 1975 revisions to the basic data and procedure. A computer program was used to generate the unit hydrograph, the excess precipitation, and resulting storm runoff hydrograph for each of the 44 sub-basins, four design storms, and two development conditions. A sample output from the program is shown in Table 111-3. D. Flood Routing The next step in the hydrologic analysis consisted of routing and combining the individual sub-basin storm hydrographs to determine the peak flows and runoff volumes at the hydrologic points. The process of routing consisted of channel routing, storoge or detention routing, and hydrograph separation procedures for special applications of flow splitting (separation of chonneO. The results of the flood routing for the existing and future development conditions are presented in Table 111-4. Storm hydrogrophs for severo I *Two hour duration was used in study. The data of Figure 111-2 was tobulated for five minute intervals for the various recurrence interval storms and the five minute rainfoll increments were rearranged in a pattern which places the maximum increment at 25 minutes. The resulting storm pattern is consistent with the general arrangement of incremental precipitotion of the thunderstorms in the Denver Metro area. The design storms used in this analysis are presented in Table 111-2. 10