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I I I I I I I I I I I I I I I I I I I For area adjustment 3, 6-hour duration hyetographs were created by adding rainfall exceeding the 2-hour stonn depth uniformly over the extended duration. The 6-hour hyetographs were then adjusted according to the 20 to 30 square mile area adjustments shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969). For area adjustment 4, 6-hour duration hyetographs were created as described for area adjustment 3 and adjusted according to the 50 to 75 square mile area adjustments shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969). Hyetographs for the 500-year event were based on the same temporal distribution as the lOO-year event. 3.6 CUHP Modelling CUHP models for existing and future development conditions were developed for each watershed (total of twelve base models). The 24 stonns described in the previous section were modelled in each of the twelve base CUHP models. The subwatershed characteris- tics required for the CUHP models included the identification number, area, length, centroid length, slope, impervious percentage, retention storage losses, and infiltration losses. The methods for estimating the subwatershed characteristics are described below and a summary table of the characteristics is shown in Appendix J. 3.6.1 Delineation of Subwatersheds Subwatersheds were delineated on 1:24,000 scale digitized USGS quadrangle maps. The boundaries were detennined based on the topography of the maps, existing roadways! bridges, and existing culverts. The subwatershed areas, lengths, and centroid lengths were estimated from the digitized 1:24,000 scale USGS quadrangle maps. 3.6.2 Imperviousness The imperviousness of each subwatershed was determined based on superimposing the maps of existing and future development condition imperviousness (Appendices C and D) onto the subwatershed maps. Based on the assumed impervious values for each land use area, a weighted impervious percentage for each subwatershed was calculated. For exist- ing conditions, imperviousness was typically 1 percent. For future conditions, the imper- viousness ranged from 2 to 85 percent. 3.6.3 Slope The slopes for each subwatershed were estimated according to the weighted slope method documented in the USDCM. The length of each subwatershed was broken up into small segments. The slope of each of the segments was computed and corrected according to Figure 4-1 of the Runoff chapter of the USDCM (UDFCD, 1969). The segment lengths DENlOOISI86.WP5 3-5