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<br />Unit Hydrograph Calculations <br /> <br />Area 1: <br /> <br />Includes entire Big Dry Creek Basin <br /> <br />Area ~ 12,500 acres or 19.5 square miles <br /> <br />L (length along channel): <br /> <br />14.2 miles <br /> <br />Lca (length along channel from study point to centroid of Basin): <br />7.5 miles <br /> <br />Propo~~d Land Use: <br /> <br />From Table 2-] <br /> <br />(Criteria Manua]) <br /> <br />% Pervious <br /> <br />% Impervious <br /> <br />Residential <br /> <br />60 <br /> <br />40 <br /> <br />Residential (large lots) <br /> <br />70 <br /> <br />30 <br /> <br />Commercial-Industrial <br /> <br />5 <br /> <br />95 <br /> <br />Open Space <br /> <br />95 <br /> <br />5 <br /> <br />Average <br /> <br />60 <br /> <br />40 <br /> <br />From Table 4-1 (Criteria Manual) for 40% impervious <br /> <br />Ct (Coefficient reflecting time to peale for unit hydrograph) ~ 0.30 <br />C (Coefficient related to peak rate aT runoff-unit hydrograph) ~ 0.50 <br />p <br />t ~ C (L x L )0.3 ~ 0.30(14.15 x 7.48)0,3 = 1.21 hours <br />p t ca <br /> <br />or <br /> <br />73 minutes <br /> <br />where tp = time to peak of unit hydrograph from midpoint of unit rainf~Jl <br /> <br />qp = .c:~_'!..Q. = .0.50 x 640 = 264 cfs/square mi Ie <br />tp 1 . 21 <br /> <br />Where qp = peak rate of runoff In cfs per squ~re mile <br /> <br />- 30- <br />