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<br />of interest in the probable Maxinum flood. Nevertheless, it is prudent <br />to adopt a base flow value that is IIlOre severe than that which would be <br />used for standard project flood derivation. There is no general gui~e <br />for this, but an additional 10 to 25 percent should suffice. <br /> <br />Section 4.08. Probable Maximum flood cOMPutation <br /> <br />Many of the guides contained in Section 3.07 are relevant to the <br />computation of the probable Maximum flood. Rainfall-runoff factors <br />should be selected in this case as the most severe that are reasonably <br />consistent with the storm and flood conditions, and should be consider- <br />ably more severe than those selected for the standard project flood. In <br />all cases where the unit hydrograph technique is used, the unit-hydro- <br />graph peak should be increased substantially to account for the more <br />rapid concentration of flood flows in the stream system. Channel routing <br />coefficients should likewise be modified toward greater translation speed <br />and less storage effects because of the more efficient hydraulic flow <br />conditions during larger floods. <br />In application of the probable maximum flood for spillway design, <br />allowance should be Made for the accelerating effect of a reservoir in <br />relation to the strealll reaches that are inundated, and the reservoir <br />level at the start of t~e flood should be the highest level reasonably <br />consistent with probable maximum flood conditions. <br /> <br />Section 4.09. Antecedent conditions <br /> <br />In many spillway design applications, flood conditions that precede <br />the probable maxi.um flood MaY have substantial influence on the regu- <br />latory effect that the reservoir has on the probable IIlaximum flood. In <br />such cases, it is appropriate to precede the probable maximum flood with a <br />flood of major magnitude at a time interval that is minimum consistent <br /> <br />4-10 <br />