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SLOPE RESISTANCE TO 100 YEAR EVENT <br />In order to insure that the dike repair is of sufficient quality to withstand the stress of the <br />Gunnison River in this area, and thus prevent another breach into the Tri County Gravel pit, an <br />analysis of the dike repair under the conditions of a 100 year flood event has been performed. <br />According to the Delta County Flood Insurance Study, the 100 year event along the Gunnison <br />River at the Tri County dike produces a flow of 12,200 cfs. Based on these values, and a <br />AutoCAD generated cross section of the dike repair area, a determination of the dike repair's <br />integrity during a 100 year event was calculated. Appendix C contains several cross sections and <br />details related to the following calculations. <br />1. Slope Conditions <br />The dike slope is armored by a D50 of approximately 5.5 inches and a vegetative cover of <br />approximately 40%. The vegetation in place appears to be smooth brome and similiar grasses. <br />According to Design Hydrology and Sedimentology of Small Catchments (Barfield, et. al.) <br />smooth brome and other similar grasses can withstand a maximum of 7 feet per second in non- <br />erosive soil. For these grasses in an erosive soil, the maximum water velocity is 5 feet per <br />second. These can be seen in the Table 4.5 from Barfield, et. al. on the following page. The slope <br />of the embankment is approximately 3H:1 V and the height is 6 feet above the river at the crest of <br />the embankment. <br />1. Flow Cross Section Breakdown <br />The integrity of the dike slope during a 100 year event can be analyzed by looking at the velocity <br />along the slope and comparing to accepted velocities for that type of material and vegetation. In <br />order to calculate velocity along the bank, we must first determine the depth at which the 100 <br />year event is achieved. Accurate water flow conditions along the dike can be calculated by <br />breaking the river cross section down into components. The river flow ill vary based on each <br />areas surface conditions (e.g. Manning's number). The river was divided into five subsections <br />based on geometry (Figure 3). Manning's equation was applied to each subsection separately to <br />calculate an individual flow (Q;). <br />Tri County Dike Slope Stability Jan 08 8