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<br />total storage is computed from some base level storage at the beginning or end of the <br />routing sequence. <br /> <br />Inflow and outflow hydrographs can also be used to compute routing criteria <br />through a process of iteration in which an initial set of routing criteria is assumed, the <br />inflow hydrograph is routed, and the results are evaluated. The process is repeated if <br />necessary until a suitable fit of the routed and observed hydrograph is obtained. <br /> <br />2.2 DETERMINING THE NUMBER OF ROUTING STEPS. In reservoir routing, <br />the Modified Puis method is applied with one routing step. This is under the assumption <br />that the travel time through the reservoir is smaller than the computation interval At. In <br />channel routing, thet travel time through the river reach is often greater than the <br />computation interval. When this occurs the channel must be broken down into smaller <br />routing steps in ordetr to simulate the flood-wave movement and changes in hydrograph <br />shape. The number of steps (or reach lengths) affects the attenuation of the hydrograph <br />and should be obtained by calibration. The maximum amount of attenuation will occur <br />when the channel routing computation is done in one step. As the number of routing steps <br />increases, the amount of attenuation decreases. An initial estimate of the number of <br />routing steps (NSTPS) can be obtained by dividing the total travel time (K) for the reach <br />by the computation interval At. <br /> <br />L <br />K;- <br />Vw <br /> <br />(3) <br /> <br />NSTPS= ~ <br /> <br />(4) <br /> <br />Where: <br /> <br />K = total travel time through the reach <br />L = channel reach length <br />V w = velocity of the ftood wave <br />NSTPS= number of routing steps <br /> <br />The time interval At is usually determined by ensuring that there is a sufficient <br />number of points on the rising side of the inflow hydrograph. A general nule of thumb is <br />that the computation ,interval shouid be less then 1/5 of the time of rise (t ,) of the inftow <br />hydrograph. <br /> <br />_ t r <br />at -5 <br /> <br />(5) <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />78.4 <br /> <br />a=w=r <br />