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<br />only locally significant, mainly where the channel is narrow, <br />ei ther naturally or artificially. Wi.dening of the high banks leads <br />to development of emergent sandbars, which, with continued <br />widening, leads to vegetated islands. Occasional flood events <br />cause some channel avulsions that consist, mainly of anabranching <br />and point bar cutoffs which lead to the formation of very large, <br />relatively high-elevation islands. Once an island becomes even <br />moderately vegetated, it is less likely to erode than the adjacent <br />high bank. This leads to increased erosion of the high banks and <br />eventually, braiding and severe anabra,nch,:Lng. <br /> <br />The analysis showed that the sandbar / island located <br />immediately upstream from the bridge was formed mostly by sediment <br />deposition along the right high bank. High flows in the past <br />several years have reopened the channel along the right high bank, <br />which has increased the sedimentation rate along the left edge and <br />the downstream end of the island.. This was most likely due to <br />changes in flow distribution. Som"~ erosion of the right side of <br />the island has occurr,ed over the I cl.st few years; however, most of <br />the erosion in the reach immediately upstream from the bridge is on <br />the right and left banks. <br /> <br />Examination of the topographic maps and the aerial photography <br />indicates a naturally stable O]OEm channel width would be <br />approximately 600 feeL Although several areas have maintained an <br />open channel of widths up to l, 500 fE'et:, there are even more areas <br />wi.th emergent sandbars and islands in channels as narrow as 900 <br />feet. Some channel areas have remained stable at widths as narrow <br />as 400 feet; however, erosion upstream from these areas indicates <br />a high likelihood of channel cutoff in the near future which will <br />effectively increase 1:he top width of the river. <br /> <br />AI,TERNATIVES <br /> <br />The alternatives discussed below are intended to resolve the <br />sedimentation problem at the bri.dge. 'They are based on the <br />information presented above. Before any alternative is <br />constructed, additional analysis and data collection should be <br />completed. A summary of the suggested additional data collection <br />and analysis is presented at the end of t,his report. <br /> <br />Section 22 - South Platte RiVer, Merino, Colorado <br /> <br />Page :: <br />