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<br />Modified Puis procedure. In river reaches, storage-outflow relationships can be determined <br />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 hydrographs. <br /> <br />Steady-flow water surface profiles. computed over a range of discharges, can be <br />used to determine storage outflow relationships in a river reach. The storage-outflow <br />relationship required for routing is determined by computing a series of water surface <br />profiles. corresponding to a range of discharges. The range of discharges should <br />encompass the range of flows that will be routed through the river reach. The storage <br />volumes are computed by multiplying the cross-sectional area, under a specific flow profile, <br />by the channel reach lengths. Volumes are calculated for each flow profile and then <br />plotted against the corresponding discharge at the outlet. If channel or levee modifications <br />will have an effect on the routing through he reach, modifications can be made to the cross <br />sections, water surface profiles recalculated, and a revised storage-outflow relationship can <br />be developed. The impacts of the channel or levee modification can be approximated by <br />routing floods with both pre and post project storage-outflow relationships. <br /> <br />Observed water surface profiles, obtained from high water marks, can be used to <br />compute storage-outflow relationships. Sufficient stage data over a range of floods Is <br />required for this type of calculation; however, it is not likely that enough data would be <br />available over the range of discharges needed to compute an adequate storage discharge <br />relationship. If a few observed profiles are available. they can be used to calibrate a <br />steady-flow water surface profile model for the channel reach of interest. Then the water <br />surface profile model could be used to calculate the appropriate range of values to <br />calculate the storage-outflow relationship. <br /> <br />Normal depth associated with uniform flow does not exist in natural streams; <br />however, the concept can be used to estimate water depth and storage in natural rivers <br />if uniform-flow conditions can reasonably be assumed. With a typical cross section, <br />Manning's equation is solved for a range of discharges, given appropriate 'n' values and <br />an estimated slope of the energy grade line. Under the assumption of uniform-flow <br />conditions, the energy slope is considered equal to the average channel bed slope; <br />therefore, this approach should not be applied in backwater areas. <br /> <br />Observed inflow and outflow hydrographs can be used to compute channel storage <br />by an inverse process of flood routing. When both inflow and outflow are known, the <br />change in storage can be computed, and from that a storage vs. outflow function can be <br />developed. Tributary inflow, if any, must also be accounted for in this calculation. The <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />78.3 <br /> <br />fRAFT <br />