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<br />2.1 DETERMINATION OF THE STORAGE-OUTFLOW RELATIONSHIP. <br />Determining the storage-outflow relationship for a river reach is a critical part of the <br />Modified Puis procedure. In river reaches, storage-outflow relationships can be <br />determined from one of the following: <br /> <br />- steady-flow profile computations; <br /> <br />- observed water surface profiles; <br /> <br />- normal-depth calculations; <br /> <br />- observed inflow and outflow hydrographs; and <br /> <br />- optimization techniques applied to observed inflow and outflow <br />hydrographs. <br /> <br />Steady-flow water surface profiles, computed over a range of discharges, <br />can be used to determine storage outflow relationships in a river reach. The storage- <br />outflow relationship required for routing is determined by computing a series of water <br />surface profiles, corresponding to a range of discharges. The range of discharges <br />should encompass the range of flows that will be routed through the river reach. The <br />storage volumes are computed by multiplying the cross-sectional area, under a specific <br />flow profile, by the channel reach lengths. Volumes are calculated for each flow <br />profile and then plotted against the corresponding discharge at the outlet. If channel <br />or levee modifications will have an effect on the routing through he reach, <br />modifications can be made to the cross sections, water surface profiles recalculated, <br />and a revised storage-outflow relationship can be developed. The impacts of the <br />channel or levee modification can be approximated by routing floods with both pre and <br />post project storage-outflow relationships. <br /> <br />Observed water surface profiles, obtained from high water marks, can be <br />used to compute storage-outflow relationships. Sufficient stage data over a range of <br />floods is required for this type of calculation; however, it is not likely that enough data <br />would be available over. the range of discharges needed to compute an adequate <br />storage discharge relationship. If a few observed profiles are available, they can be <br />used to calibrate a steady-flow water surface profile model for the channel reach of <br />interest. Then the water surface profile model could be used to calculate the <br />appropriate range of values to calculate the storage-outflow relationship. <br /> <br />Normal depth associated with uniform flow does not exist in natural <br />streams; however, the concept can be used to estimate water depth and storage in <br />natural rivers if uniform-flow conditions can reasonably be assumed. With a typical <br />cross section, Manning's equation is solved for a range of discharges, given <br />appropriate "n" values and an estimated slope of the energy grade line. Under the <br /> <br />7-40 <br />