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<br />Section 2.10. Routing and combining operations <br /> <br />Computation of flood hydrographs from specified storm rainfall and <br />snowmelt for each sub-basin is accomplished first for the most upstream <br />areas in each tributary, and each succeeding sub-area runoff computation <br />is Made as the routing and combining operation proceeds downstream. <br />This is a straightforward computation for cases where the storm rainfall <br />quantities do not vary with size of drainage area. In cases where lower <br />precipitation amounts are used for larger areas. some means of recompu- <br />ting upstream contributions for each successive downstream location must <br />be used. One technique for doing this is described in Chapter 7. <br />Routing computations in each stream reach between points where <br />determinations are required or at reservoirs must use a computation <br />interval equal to that used in subsequent sub-basin runoff computations <br />so that hydrographs can be combined in a digital computation (or sub- <br />sequent computation intervals can be an exact multiple of earlier com- <br />putation intervals). <br />Routing and COmbining operations for a stream system can be accom- <br />plished using the computer program described in Appendix 2. <br /> <br />Section 2.11. River routing of floods <br /> <br />Routing of floods through river reaches to account for the time of <br />travel and storage effects can be accomplished by a number of methods. <br />A few that are widely used in project studies are described in the <br />following paragraphs. <br />The Muskingum or coefficient .ethod of flood routing uses the <br />following function of the rate of change of inflow during the computa- <br />tion interval and difference between inflow and outflow at the start of <br />the co.putation interval to determine the rate of change of outflow <br /> <br />2-16 <br />