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<br />2013 <br /> <br />17. A backwater model (HEC-2 or HEC-RAS) can be developed to determine the <br />effect of cross-section variability on hydraulic parameters. Note that HEC-RAS output files <br />are not compatible with SAM. <br /> <br />18. Suspended-sediment data from gages upstream from John Martin Reservoir on the <br />Arkansas River and the Purgartoire River can be used to help determine which sediment <br />transport equation is appropriate for use in determining the sediment-discharge rating curves. <br />It is important that bed-material gradation at the gages be determined so that the wash load <br />fraction can be established and wash load "can be subtracted from the measured suspended <br />sediment data. <br /> <br />19. Using data collected from the new field surveys, the width vs. channel-forming <br />discharge relationship can be used to assess the effect of the dam on stable channel width. <br />Additional analyses, similar to the preliminary analysis conducted for this study, should be <br />conducted for several of the historical ranges to determine possible variability in channel- <br />forming discharge due to cross-section geometry. Discharge variability in the 58-mile reach <br />downstream from John Martin Dam can be determined by developing flow-duration curves <br />from other gages. The coefficient for the hydraulic geometry relationship will vary depending <br />on the bank characteristics. Using field observations, different values for the coefficient can <br />be established for different conditions, e.g. percent silt and clay and/or percent vegetative <br />cover. <br /> <br />20. The channel-forming discharge and the hydraulic geometry relationships can be <br />used to design a stable channel geometry and an appropriate management strategy. Although <br />not part of a geomorphological assessment, a hydraulic analysis should be conducted to insure <br />appropriate conveyance capacity for irrigation delivery and floods. <br /> <br />Conclusions <br /> <br />21. The principle geomorphological processes that seem to be affecting channel <br />development of the Arkansas River downstream from John Martin Dam are increased growth <br />of vegetation along the banks and on channel berms or bars; and a decrease in the channel- <br />forming discharge. The increase in vegetation is supported by several decades of increasing <br />availability of water in the channel due to the increased irrigation supply, of which John <br />Martin Dam is a part. The dam also reduces the occurrence of major flood peaks, flood peaks <br />that would have provided erosive forces sufficient to wash out significant reaches of vegetative <br />encroachment. <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />15 <br /> <br />August 8, 1997 <br />