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<br />relation become very serious as the longer durations (with lower rain- <br />fall rates and greater time and area variations) occur. <br />Where it is important to consider longer sequences of rainflood <br />runoff, as in studies to assure sufficient time between storms to empty <br />flood control space, flood periods can be added before and after the <br />standard, project computed runoff. These can be sequences that occurred <br />in relation to large recorded rainfloods or can be derived from flood <br />volume frequency curves. In the latter case, incremental runoff vol- <br />umes for incremental durations of about 3 days can be computed for an <br />exceedence interval of about 200 years. These volumes can be used with <br />a typical time pattern to develop hydrograph components, the larger of <br />which should ordinarily precede the main period of computed runoff. <br /> <br />Section 3.09. Summary of procedure--rainfloods <br /> <br />Standard project rainfloods are derived in the following steps: <br /> <br />a. Develop a map of the entire hydrologic region showing relative <br />values of storm precipitation potential. This map would nDt be necessary <br />if the storm precipitation potential is relatively uniform throughout <br />the region. as in some plains areas far removed from ocean sources of <br />moisture. This could be a map of normal annual precipitation or an <br />iSDhyetal map of 3-day precipitation having an exceedence frequency of <br />10 years per hundred years (3-day la-year preCipitation). <br />b. For each large recorded storm in the region, develop the depth- <br />area-duration relationship of maximum total precipitation expressed as a <br />ratio to the index of storm precipitation potential from step a. <br />c. Construct a standard project depth-area-duratiDn relationship <br />that enveloPS all but any radically unusual values in the various storms. <br />d. For the drainage area for which the standard project flood is <br />to be derived, select a representative time distribution of precipita- <br />tion from recorded storms and adjust the quantities to correspond to <br /> <br />3-12 <br />