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<br />, <br />4.2.8 Major storm analysIs <br /> <br />~1'hen analyzing the major runoff occurring on an area that has a <br />storm sewer-system sized for the minor storm, care r:lus1: be used ",,'hen <br />applyIng the RatIonal Method. Normal applIcatIon of the RatIonal Method <br />assumes that all of the runoff Is coll',cted by tc,e storm sewer. In <br />thIs inItial design the tIme of concentration is, in part, dependent <br />upon the flow time in the sewer. Durjng the major runoff, however, <br />the se"'ers should be fully taxed and cannot accept all the water flow- <br />ing to the inlets. This additIonal w<lter then flows by the inlets and <br />continues overland, generally at a siVlificantly lower velocity than <br />the water in the storm se"'ers. This ""quires an analysis of different <br />concentration times between underground flow and ove:rland flow. This <br />difference in travel times provides opportunities for the storm sewer <br />to continue flowing full for a longer period and, in effect, to carry <br />significant portions of the major runoff. The basis for this Increased <br />benefIt is that the excess water from one inlet will flow to the next <br />inlet downhill, using the overland route. If that Inlet too is fully <br />taxed, the water will often continue on until capacity is available <br />in the storm sewer. The analysis of this aspect of the interaction <br />between the storm-sewer system and the, major storm runoff is complex. <br />The most useful procedure for this anelysis is the routing of hydro- <br />graphs through the t,,'o routes concurrently. <br /> <br />~ <br /> <br />LCS.-j,'M Manual <br /> <br />4.2---7 <br /> <br />April 1979 <br />