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<br />CHAPTER 7 <br />CHANNEL ROUTING - HYDROLOGIC METHODS <br /> <br />1. BACKGROUND THEORY. <br /> <br />Routing is a process used to predict the temporal and spatial variations of a flood <br />hydrograph as it moves through a river reach or reservoir. The effects of storage and flow <br />resistance, within a river reach, are reflected by changes in hydrograph shape and timing <br />as the flood wave moves from upstream to downstream. <br /> <br />In general. routing techniques may be classified into two categories: hydraulic <br />routing and hydrologic routing. Hydraulic routing techniques are based on the solution of <br />the partial differential equations of unsteady open channel flow. These equations are often <br />referred to as the 5t. Venant equations or the dynamic wave equations. Hydrologic routing <br />employs the continuity equation and either an analytical or an empirical relationship <br />between storage within the reach and discharge at the outlet. <br /> <br />Flood forecasting, reservoir and channel design, flood plain studies and watershed <br />simulations generally utilize some form of routing. Typically. in watershed simulation <br />studies. hydrologic routing Is utilized on a reach-by-reach basis from upstream to <br />downstream. For example, it is often necessary to obtain a discharge hydrograph at a <br />point downstream from a location where a hydrograph has been observed or computed. <br />For such purposes, the upstream hydrograph Is routed through the reach with a hydrologic <br />routing techniques that predicls changes in hydrograph shape and timing. Local flows are <br />then added at the downstream location to obtain the total flow hydrograph. This type of <br />approach is adequate as long as there are no significant backwater effects, or <br />discontinuities in the water surface due to jumps or bores. When there are downstream <br />controls that will have an effect on the routing process through an upstream reach, the <br />channel configuration should be treated as one continuous system. This can only be <br />accomplished with a hydraulic routing technique that can incorporate backwater effects as <br />well as internal boundary conditions. such as those associated with culverts, bridges, and <br />weirs. <br /> <br />Hydrologic routing employs the use of the continuity equation and either an <br />analytical or an empirical relationship between storage within the reach and discharge at <br />the outlet. In its simplest form, the continuity equation can be written as inflow minus <br />outflow equals the rate of change of storage within the reach: <br /> <br />1- 0 = d5 <br />dt <br /> <br />(1) <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />78.1 <br /> <br />fRIJFf <br />