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Tables C-1 and C-la present recom- <br />mended ranges for C values. It is <br />readily apparent that there is a large <br />opportunity for interpretation; ideally, <br />a calibration procedure would be used <br />to refine the values, but this is <br />seldom practical. This represents a <br />further limitation on the use of the <br />Rational Method. For purposes of <br />planning, the values in Table C-1 are <br />recommended due to better definition of <br />specific coefficients. <br />It is often desirable to develop a <br />composite runoff coefficient based on <br />the percentage of different types of <br />surface in the drainage area. This <br />procedure is often applied to typical <br />"sample" blocks as a guide to selection. <br />of reasonab.e values of the coefficient <br />for an entire a::ea. The procedure is <br />illustrated at the bottom of Table C-1. <br />Development of a hydrograph <br />This "hydregraph" synthesis method <br />assumes that once the peak rate of <br />runoff for a given storm has been <br />reached (after the time of concentra- <br />tion; the runoff rate remains constant <br />until the design storm duration is <br />reached. This is consistent with the <br />assumptions of linear runoff response <br />and uniform rainfall intensity described <br />Topography and I Open Sandy Clay and <br />Vegetation Loan Silt Loam Tight Clay <br />_ _ <br />Woodland <br />Flat 0-5t slope <br />0.10 <br />0.10 <br />0.40 <br />Hollrng 5-10% slope <br />0.25 <br />0.15 <br />0.50 <br />Hilly 10-1t slope <br />0.10 <br />0.50 <br />0.60 <br />Pasture, Lawn _-- <br />Flat <br />0.10 <br />0.10 <br />0.40 <br />Polling <br />0.16 <br />0.16 <br />0.55 <br />Hilly <br />0.22 <br />0.42 <br />0.60 <br />cultivated <br />Flat 0.10 0.50 0.60 <br />Falling 0.40 0.60 0.70 <br />Hilly _ _ 0.52 0.72 _ O.62_ <br />10% of area 50% of area 70% of area <br />impervious impervious impervious <br />U[ban Areas, General <br />Flat 0.40 0.55 0.65 <br />Pollin _ - 0.50 0_65 <br />Minimum Optimum Maximum <br />Foots 0.90 0.95 1.00 <br />concrete or asphalt <br />).as <br />0yy95 <br />1.00 <br />roads and pavements <br />_ <br />Bituminous macadar <br />0.60 <br />0.70 <br />0.80 <br />toads and pavements <br />Gravel areas and walks <br />Packed <br />0.70 <br />Loose <br />0.10 <br />-- - <br />Vacant lots, unpaved street <br />--- <br />Light plant growth <br />0.60 <br />No plant growth <br />0.75 <br />parks, lawns and meadows 0.10 0.40 0.60 <br />Farming country 0.100.10 0.60 <br />woodland I 0.20 <br />To calcu:ate average coefficient for contributary areas <br />Acres 60 P coeff 0.5 0.5 x 60 • 10 30 + 10 • 40 : 100 (acres) <br />Acres 40 F coetf 0.25 0.25 x 40 • 10 0.6 <br />Combined coeff • (el x A11 • IC2 x A21 <br />�oF> • �t�E4'laU�•Ni"S <br />(Yallr. cC C In Q-CIA) <br />•Yli'2N �VOtM1I•• <br />aV:A;l:l_1t"a'ti.`L C.FS F�'C1FZ,.nCN <br />Inl 1-;41(J i;iwlt li 14Etc <br />LN(VGR411y GF GEI-h.XA1'E <br />AWIL, 197A <br />