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Table 3: Runoff Coefficient "C" <br /> FLAT ROLLING HILLY <br /> 0%to 2% 2%to 10% Over 10% <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 /> 41.1 <br /> b. Rainfall Intensity"1" <br /> The cumulative rainfall intensity shall be 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 "TC' <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,=0.93Loan 1f4S°3 <br /> Te=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 1) <br /> S =average slope of overland area(foot/foot) <br /> (mil/ Stormwater Management Stormwater Quantity Page <br /> lfxd40:P:w[s96AS,dumAttMSToRmwAT[n1MxWE,.arvr.wro 09/12fl02 <br />