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I I I I I I I I I I I I I I I I I I I TABLE 1 DESIGN 100-YEAR RAINFALL PATTLRN the relations shown in Figure 1 and the equation: Ct = Kt (L L )0.14 ca SO.22 -' Time % of Depth Time % of Depth J~lin. ) I-Hr. Depth (In. ) (Min~ I-Hr. Depth (In. ) 5 1 .03 65 4 .10 10 2 .05 70 2 .05 15 4 .10 75 2 .05 20 8_ .20 80 2 .05 25 15 .38 85 2 .05 30 25 .64 gO 2 .05 35 14 .36 95 1 .03 40 8 .20 100 1 .03 45 5 .13 105 1 .03 50 5 .13 110 1 .03 55 4 .10 115 1 .03 60 4 .10 120 1 .03 In this equation L is the basin length (mi), L is the distance from ca the outlet to the centroid of the basin (mi) and S is the basin slope (ft/ft). The values of L, Lea and S were determined from a topographic map for each of the subareas used in the study. As mentioned earlier, the basin slope values used in this analysis ignore the existing check dams which have been placed in the flood plain by previous land owners since these check dams are not considered permanent structures. The percent imperviousness needed for finding Kt and Cp was estimated using a composite building density versus percent imperviousness for Arapahoe County shown in Figure 2. The housing density was determined from the proposed development plan for Highlands Ranch which includes the Dad Clark Gulch drainage area. For development such as commercial business, in- dustrial or school sites, or for areas of housing densities greater than 5 units per acre, percent imperviousness was estimated by analyzing indi- vidual site plans for such areas and/or similar existing developments in Arapahoe County. The imperviousness ranged from 5 percent for open space to 75 percent for community centers and industrial parks. I-Hr. Depth = 2.56 in. 2-Hr. Depth = 2.95 in. The entire basin was divided into 13 sub-basins or subareas so that Os could be determined at a number of locations. These subareas are identi- fied on the Basin Boundary Map found in the Appendix of this report (sheet 1 of 23 of the drawings). The physical characteristics and hydrograph parameters for each of these subareas as well as the entire basin are tabulated in Table 2. The CUHP computer program was used to find the peak 0 '(lOO-year, future development) at the downstream end of each of the 13 subareas. The 0 at a point was computed by considering all the subareas that contribute rather than lumping all the contributing area into one subarea. This resulted in larger, and thus more conservative, estimates of O. For example, the 0100 for the entire watershed was 4,446 cfs using 5