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<br />2-3 <br /> <br />4f <br /> <br />respect to time, that is developed during the analysis of a watershed. <br />For example, if a ten day outflow hydro graph is needed, a main time in- <br />crement of 0.8 hour would provide the necessary 240 hours (300 x 0.8 =240). <br />However, for a small watershed with a time of concentration of 0.25 hour, <br /> <br />, ' " /, ,'. . /, I; <br />a main time increment of la.t..ieas.t 0'.1 hour is needed to define the composite <br />hydrograph. The user must also consider, when selecting the main time <br />increment how much of the hydrograph volume at the end of the watershed is <br />needed to develop the peak outflow. The maximum of 300 ordinates applies <br />to all hydrographs that are generated by the operations in the program. <br /> <br />~. <br /> <br />Reservoir Routing <br />The composite flood hydrograph is routed through a reservoir using the <br />storage-indication method as described in Chapter 17, NEH-4. The working <br />curve is not used; instead the working equation is solved during a <br />process in which interpolations are made in the elevation-discharge-storage <br />data for the structure. The starting elevation for routing or the pool <br />elevation when runoff begins must be specified by the user. The outflow <br />hydrograph in cfs and/or elevation can be printed at multiples of the <br />main time increment (subject to the 300 point limit). The peak discharges <br />and associated elevations are determined using the peak flow routine <br />described in the previous Hydrograph Development section. The following <br />criteria applies to the main time increment, At, for reservoir routing <br />(pg. 17-22, NEH-4): <br /> <br />S2 > O2 <br />At 2 <br /> <br />where At=the main time increment in hours <br /> <br />~ : / <br /> <br /> <br />S2=storage volume at end of time period <br />02=outflow rate at end of time period Reach Routing <br /> <br />The composite flood hydrograph is routed through a valley reach using a <br />Modified Attenuation-Kinematic (Att-Kin) method as described in Appendix <br />G. The routing time interval is the main time increment. The routing <br />reach length (L) is specified by the user. <br /> <br />.. <br /> <br />The routing coefficient (C) is developed from the valley storage-discharge <br />curve (Q = kSm, where S = valley storage) that represents the reach and <br />