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<br />CHANNEL ROUTING - HYDROLOGIC METHODS <br /> <br />1. BACKGROUND THEORY. <br /> <br />Routing is a process used to predict the temporal and spatial variations <br />of a flood hydrograph as it moves through a riverreach or reservoir. The effects of <br />storage and flow resistance, within a river reach, are reflected by changes in <br />hydrograph shape and timing as the flood wave moves from upstream to downstream. <br /> <br />In general, routing techniques may be classified into two categories: <br />hydraulic routing and hydrologic routing. Hydraulic routing techniques are based on <br />the solution of the partial differential equations of unsteady open channel flow. These <br />equations are often referred to as the St. Venant equations or the dynamic wave <br />equations. Hydrologic routing employs the continuity equation and either an analytical <br />or an empirical relationship between storage within the reach and discharge at the <br />outlet. <br /> <br />Flood forecasting, reservoir and channel design, flood plain studies and <br />watershed simulations generally utilize some form of routing. Typically, in watershed <br />simulation studies, hydrologic routing il? utilized on a reach-by-reach basis from <br />upstream to downstream. For example, it is often necessary to obtain a discharge <br />hydrograph at a point downstream from a location where a hydrograph has been <br />observed or computed. For such purposes, the upstream hydrograph is routed <br />through the reach with a hydrologic routing techniques that predicts changes in <br />hydrograph shape and timing. Local flows are then added at the downstream location <br />to obtain the total flow hydrograph. This type of approach is adequate as long as <br />there are no significant backwater effects, or discontinuities in the water surface due <br />to jumps or bores. When there are downstream controls that will have an effect on <br />the routing process through an upstream reach, the channel configuration should be <br />treated as one continuous system. This can only be accomplished with a hydraulic <br />routing technique that can incorporate backwater effects as well as internal boundary <br />conditions, such as those associated with culverts, bridges, and weirs. <br /> <br />Hydrologic routing employs the use of the continuity equation and either <br />an analytical or an empirical relationship between storage within the reach and <br />discharge at the outlet. In its simplest form, the continuity equation can be written <br />as inflow minus outflow equals the rate of change of storage within the reach: <br /> <br />I - 0 = dS <br /> <br />(1 ) <br /> <br />dt <br /> <br />Where: <br /> <br />= The average inflow to the reach during dt <br />o = The average outflow from the reach during dt <br /> <br />7-38 <br />