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<br />It is often desirable to develop a compc)site runoff coefficient <br />based on the percentage of differe~t types of surface in the drainage <br />area. This procedure is often applied to typical "sample" blocks as a <br />guide to selection of reasonable values: of the coeffLci,ent for an entire <br />area. <br /> <br />The range and recommended (Table e. 2.6-1) coefficients are appli- <br />cable for storms of 5-year to la-year frequencies" Less frequent higher- <br />intensity storms will require modification of the co,~fficient because <br />infiltration and other losses have a proport:.onally smaller effect on <br />runoff (see Section 4.3.8.3). <br /> <br />The adjustment of the Rational Method for use with major storms <br />can be made by multiplying the right side of the Rational Formula by a <br />frequency factor Cf (,,'right-McLaughlin EnginE,ers, 1969), which is used <br />to account for antecedent precipitatior, cond!.tions. The Ratio:1al Formula <br />now becomes: <br /> <br />Q = CCfIA <br /> <br />(Eq. 4.2.6-1) <br /> <br />where: <br /> <br />Recurrence <br />interval <br />(years-) <br /> <br />2 to lO <br /> <br />25 <br /> <br />50 <br /> <br />100 <br /> <br />Cf <br /> <br />1.0 <br />1.1 <br /> <br />1.2 <br />1.25 <br /> <br />The product of C times Cf sho\Jld not exceed 1.0. <br /> <br />4.2.7 Application of the Rational Mett'o(~ <br /> <br />The first step in applying tho, Rational Method is to obtain a good <br />topographic map and define the boundaries of all of the relevant drain- <br />age basins. Basins to be defined include all. bas:;,ns tributary to the <br />area of study and subbasins in the study area. A field check and possi- <br />bly field surveys should be made for each basin. <br /> <br />The major storm drainage basin does not always c.oi,ncide with the <br />minor storm drainage basin. This is often the case In urban areas where <br />a low flow will stay next to a curb and foll"w the lowest grade, but <br />when a large flow occurs the water will, be dE:ep enough so that part of <br />the y,~ater will overflow street crm.;rns and flow int.o a new subbasin. <br /> <br />LCS-WM Manual <br /> <br />e.2-'--6 <br /> <br />April 1979 <br />