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<br />If one knows the precipitation (P) and the cJrve number (eN), it is a <br />simple matter to compute the direct runoff i1 inches (Q). To convert <br />this to acre-feet, one must multiply (Q) by the area of the ,,'atershed in <br />acres and divide by 12. Thus a (Q) of 1.2 i1ches over a 100-acre ,,'ater- <br />shed will be equivalent to 10.0 acre-feet. <br /> <br />4.4.4 Procedure for determining peak i'lows <br /> <br />The basic chart used for determining peak flQ\,s (Fig. 4.4.4-1) <br />presents relationships developed for use in Colorado. In applying this <br />to Larimer County, the TYI.e II storm curve i" reco!1l1:\ended for areas whose <br />average watershed elevations are above 8,000 feet. The Type IIA sto~ <br />curve is recommended for watersheds ",hose average elevations lie between <br />6,000 and 8,000 feet. <br /> <br />One "nters this chart with a time of concentration (see Section <br />4.4.1.4), goes to the appropriate storm type curve, and then reads the <br />"peak discharge csm/inch." This value is the peak flow expressed in <br />cubic feet per second per square mUe pe,r inch of runoff. Thus to com- <br />pute the peak flow, one takes this val~e'and multiplies it by the water- <br />shed area, and then multiplies this product by the inches of runoff (see <br />Section 4.4.3). <br /> <br />4.4.4,.1 Example of techriique" TCle land use (present and future) <br />and hydrologic soil groups are evenly distributed, i.e., runoff charac- <br />teristics are uniform throughout the watershed. Hhat will be the effect <br />of the planned development on runoff and peak discharge at the lOa-year <br />frequency? <br /> <br />~l. <br /> <br />Present condition: <br />Q = 1.88 inches for eN = <br />From Fig. 4.1.,,4 -1 for T <br />c <br />inch of runoff. <br /> <br />q = qp AQ = 410(1.5) (1.8t:) = 1156 cis <br /> <br />80 and P24 = 3.8 <br />= 0.9 hour, qp = <br /> <br />inches. <br />410 csm per <br /> <br />(Eq. 4.4.4.1-1) <br /> <br />~~. Future condition: <br />Q = 2.28 inches for eN = 85 anc, P24 = 3.8 inches. <br />From Fig. 4./,.4-1 for Tc = 0.6 hour, qp = 460 csm per <br />inch of runoff. <br /> <br />q = qp AQ = 525 (1.5) (2.28)= li96 cis <br /> <br />(Eq. !,.4.4.l-2) <br /> <br />~;!.. The proposed project will increase the total volu",e of <br />runoff by 21 percent and decrease the time of concen- <br />tration by 33 percent; result will be an increa'se in <br />peak discharge of 55 percent (from 1,156 cis to 1,796 cfs). <br /> <br />LCS-WM Manual <br /> <br />4. (,---18 <br /> <br />April 1979 <br />