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3.4.3 Hyetographs Table 3-3 Composite Future Imperviousness Values Watershed Composite Future Imperviousness (percent) Oak Gulch 33 Unnamed Tributary 42 Lemon Gulch 24 A total of 8 hyetographs were developed for the four recurrence intervals (10" 50" I 00" and SOO,year), and two storm types (2,hour with no area adjustment and 2,hour with an area adjustment). The derivation of the hyetographs is described below. The 2,hour duration hyetographs with no area adjustments were developed by inputting the one,hour depths and letting the CUHP program automatically develop the 2-hour distribution. The 2-hour duration hyetographs with the area adjustment were taken directly from the Douglas County Drainage Criteria Manual (1986). The hyetographs used were adjusted according to the 5 to 10 square mile area adjustments shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969). 3.5.3 Slope Hyetographs for the SOO,year event were based on the same temporal distribution as the 100, year event The slopes for each sub watershed 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 1.2 percent to 5.6 percent in the study area. 3.5 CUHP Modeling 3.5.4 Time of Concentration CUHP models were developed for the 8 storms described in the previous section. The subwatershed characteristics 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 E. In accordance with the USDCM, the modified CUHP procedure was used for subwatersheds 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 sub watershed modeled. 3.5.1 Delineation of Subwatersheds For rural land use conditions (future imperviousness less than 20%), 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). Time of concentration values generally ranged from 36 to 124 minutes. Subwatersheds were delineated on 1 :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. 3.5.2 Imperviousness Under urban conditions (future imperviousness greater than 20%), 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 from the USDCM (UDFCD, 1969), For urban development conditions, the time of concentration values for individual subwatersheds ranged from 19 to 40 minutes, The imperviousness of each subwatershed was determined by superimposing the map of future development condition imperviousness onto the subwatershed maps. Based on the assumed imperviousness values for each land use area, a weighted impervious percentage for each sub watershed was calculated. The composite future imperviousness values for the Oak Gulch watershed, Unnamed Tributary watershed, and the Lemon Gulch watershed are shown in Table 3,3. 4 Oak Gulch and Lemon Gulch FHAD.DOC