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<br />Step 4. Compute the time of concentration Te' <br /> Description <br />Reach of flow Length Velocity. Travel ti;:;:e <br />- <br /> (ft. ) (ft./sec.) (sec.) <br />A to B Over land 500 0.7 714 <br />B to C Overland 900 2..3 321 <br />C to D Storn drain 2,000 10.0 200 <br />D to E Open channel 3,000 8.2 366 <br /> Total 1,601 <br /> <br />Thus: <br /> <br />Tc = 1,60l sec = (I 44 h <br />(3,600 sec/hr)' r. <br /> <br />4.4.2 Assumptions and limitations <br /> <br />This SCS method uses a dimensionlees unit hydrograph technique and <br />a storm pattern based on ratios to thE', 24-hour precipitation event. It <br />assumes that the watersheds are totally contributing, and various por- <br />tions of the watershed will contribute in accordance with their curve <br />numbers. It also assumes that rainfall intensity and rainfall patterns <br />do not affect the results. This lattE:r assumptio:l has been challenged <br />recently by Hawkins (1973, 1978). <br /> <br />This procedure is applicable for estimating the peak rate of surface <br />runoff from sunnner storms .,,;th recurrE:nce intervals from 2 years to 100 <br />years. The method is limited to small watersheds with times of concen- <br />tration less than 10 hours or areas lE:S$ than 10 square miles. The method <br />should be used for determining peak flows only. Despite its limitations <br />this method can produce us,.ful results: if properly used. Results, how- <br />ever, should be compared with actual gaged runoff events whenever possible. <br />Refer to Section 9.3 of this Manual for desi.n of detention basins using <br />this method. <br /> <br />The SCS developed this basic metr,od for nation-"ide use. It is <br />updated periodically; the most recent revisi,on should be used. <br /> <br />4.4.3 Procedure for determinin~ runoff volume <br /> <br />Fig. 4.4.3-1 shows graphically thE! relations~ip among precipitation <br />(P), curve numbers (eN), and direct tunoff (Q). :~athematically it can <br />be expressed as <br /> <br />Q " <br /> <br />(P - 0.2S2-=-_ <br />(P + 0.8S) <br /> <br />~ <br /> <br />(Eq. 4.4.3-1) <br /> <br />where: <br /> <br />S " lOOO - 10 <br />CN <br /> <br />LCS-l,'M Manual <br /> <br />4./,----17 <br /> <br />April 1979 <br />