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Mr. Rex Burns <br />Page Three <br />May 1, 1998 <br />Table 1. Right Overbank Rating Curve. <br />Total Discharge <br />(cfs) Right Overbank <br />Discharge (cfs) <br />4,000 0 <br />5,000 65 <br />6,000 300 <br />7,000 482 <br />8,000 1,277 <br />9,000 1,780 <br />10,000 2,336 <br />11, 000 2, 944 <br />12, 000 3, 633 <br />13,000 4,378 <br />14,000 5,319 <br />15,000 6,288 <br />16,000 7,262 <br />17,000 8,223 <br />The average right overbank velocity, even at the highest flows, is approximately 1.5 feet per <br />second. This is well below an erosive velocity for the overbank spills, so significant erosion <br />under existing conditions would not be expected to occur there. This is significant in that we can <br />focus on the erosion that would occur as a result of the gravel pit itself -there will be minimal <br />additional erosion to be of concern. <br />Hydrology <br />The 100-year hydrograph for the Poudre River downstream of Boxelder Creek was discretized <br />according to the discharge ranges shown in fhe table above. The hydrograph is shown in <br />Figure 2. Giscretization of the hydrograph permitted determination of the duraticn of each <br />range of flow. The durations were used to determine the quantity of sediment transported <br />during the hydrograph, as described later in this report. <br />Sediment Transport <br />We assumed that erosion of a potential cut-off channel will take place on the steep ground <br />surface where the undisturbed grade transitions into the gravel pit on the upstream side of the <br />pit. As the overland flow cascades down this steep face, it will reach critical flow conditions and <br />erode the ground surface. It is unlikely that supercritical flow would develop because erosion of <br />the bed occurs, disrupting the flow. Generally, in mobile channels the transport of sediment and <br />bed changes keep the flow at critical conditions. As soil is removed, this erosion zone will <br />advance in the upstream direction. For a large channel having a unit discharge of 4 cfs/ft (as <br />our modeling indicates for the right overbank area), the energy slope in this critical region is <br />0.019 and the velocity is 5.16 fps. <br />BURNS-4A.LTR <br />34-0648.00 <br />