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hyetographs are provided in Appendix G, The hyetographs are shown in Appendix G; their derivation is described below. area, a weighted impervious percentage for each subwatershed was calculated, For exist- ing conditions, imperviousness was typically I percent. For future conditions, the imper- viousness ranged from 2 to 85 percent. For area adjustment I, 2-hour duration hyetographs were created by distributing the I-hour rainfall depths according to Table 3-1 of the Rainfall chapter of the USDCM (UDFCD, 1969). No area adjustments were applied, 3.6.3 Slope For area adjustment 2, 3-hour duration hyetographs were created by adding rainfall exceeding the 2-hour storm depth unifonnly over the extended duration, The 3-hour hyetographs were then adjusted according to the 10 to 20 square mile area adjustments shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969). 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 and corrected slopes were entered into Equation 4-5 of the Runoff chapter of the USDCM (UDFCD, 1969) to determine the weighted subwatershed slope, Weighted subwatershed slopes varied from 0.2 percent to 4,9 percent in the study area. For area adjustment 3, 6-hour duration hyetographs were created by adding rainfall exceeding the 2-hour storm depth unifonnly 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), 3.6.4 Time of Concentration 3.6 CUHP Modeling In accordance with the USDCM, the modified CUHP procedure was used for subwater- sheds less than 90 acres (UDFCD, 1969). The modified CUHP procedure produces peak flows similar to the rational method and requires a time of concentration value for each subwatershed modelled (for example, subwatersheds less than 90 acres), For existing development conditions, Equations 3-2 and 3-3 from the Runoff chapter of the USDCM were used to determine the time of concentration (UDFCD, 1969), The velocity parameter used to determine the travel time variable in Equation 3-2 was estimated from Figure 3-1 of the Runoff chapter of the USDCM using the curve for short grass pasture and lawns (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 100-year event. CUHP models for existing and future development conditions were developed for each watershed (total of twelve base models), The 24 storms 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 H, For future development conditions, Equation 3-4 from the Runoff chapter of the USDCM was used to determine the time of concentration, since it yielded shorter concentration times than Equation 3-2 and 3-3 and Figure 3-1 of the USDCM (UDFCD, 1969), Time of concentration values generally ranged from 30 to 120 minutes for existing development conditions. For future development conditions, the time of concentration values ranged from 17 to 48 minutes, 3.6.5 Retention Storage Losses 3.6.1 Delineation of Subwatersheds Retention storage losses were determined according to Table 2-1 of the Runoff chapter of the USDCM (UDFCD, 1969), For existing conditions, the retention storage losses were assumed to be 0,05 inch for impervious areas (representing sloped roof areas) and 0,40 inch for pervious areas (representing wooded areas and open fields). For future development conditions, the retention storage losses were assumed to be 0,05 inch for impervious areas (representing sloped roof areas) and 0,35 inch for pervious area (representing lawn grass), Subwatersheds were delineated on I :24,000 scale digitized USGS quadrangle maps, The boundaries were determined 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 3.6.6 Inf'Iltration Losses The imperviousness of each subwatershed was determined based on superimposing the maps of existing and future development condition imperviousness (Appendices B and C) onto the subwatershed maps, Based on the assumed impervious values for each land use The infiltration parameters (initial infiltration, fmal infiltration, and decay coefficient) were computed from Table 2-2 of the Runoff chapter of the USDCM based on a weighted DENIOO1768l.WP5 3-3