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Table 3: Runoff Coefficient "C" <br /> FLAT ROLLING HILLY <br /> 0%to2% 2%to10% Over10% <br /> Developed Surface Types: <br /> Impervious Areas .9 .9 .9 <br /> Gravel Pavement .5 .55 .6 <br /> Landscape Areas(Except Lawns) .35 .4 <br /> Lawns .17 .22 .35 <br /> Undeveloped Surface Types: <br /> Meadow, Pasture,or Farm .25 .3 .35 <br /> Mixed .15 .2 .25 <br /> Woodland and Forest .1 .15 .2 <br /> Development Types: <br /> Commercial Development .8 .85 .9 <br /> industrial Development,Heavy .7 .8 .9 <br /> Dense Residential(over 6 units/acre) .7 .75 .8 <br /> Industrial Development, Light .6 .7 .8 <br /> Normal Residential (3 to 6 units/acre) .5 .55 .6 <br /> Light Residential (1 to 3 units/acre) .35 .4 .45 <br /> Parks .15 .2 .25 <br /> b. Rainfall Intensity"I" <br /> The cumulative rainfall intensity shall he derived from Figure 1. The design <br /> storm interval is typically based on the longest time of concentration for the <br /> drainage area. <br /> c. Time of Concentration "T,' <br /> Calculations for time of concentration should be divided into two segments: <br /> sheet flow and controlled flow.For overland flow distances up to 300 feet,the <br /> sheet flow time of concentration can be calculated with the kinematic wave <br /> equation: <br /> T,=0.93 La6no611"So.3 <br /> T,=flow time(minutes) <br /> L=overland flow length (feet) <br /> n=Manning's Roughness Coefficient(see Table 4) <br /> I=rainfall intensity(inches/hour) (see Figure I) <br /> S=average slope of overland area(foot/foot) <br /> Stormwater Management Stormwater Quantity Page III-2 <br /> IEIGJaPWE9vm3raunvARKsttxux,TEn Mwwe.ENT wvo oan 22002 <br />