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<br />-. <br /> <br />.- <br /> <br />Mr. Bill Crepeau <br />September 7, 1984 <br />Page two <br /> <br />The 1984 discharges were: <br /> <br />Date <br />May 18th <br />May 24th <br />May 26th <br />June 3rd <br /> <br />Discharge in cfs <br />20,200 <br />28,000 <br />31,200 (1984 peak) <br />25,100 <br /> <br />- <br /> <br />The existing approved Colorado Water Conservation Board <br />hydrologic valves for the same location are: <br /> <br />Frequency <br />10-year <br />50-year <br />100-year <br />500-year <br /> <br />Discharge in cfs <br />22,000 <br />33,000 <br />40,000 <br />57,000 <br /> <br />Using these hydrologic data, we estimate the 1984 flood <br />frequency to be approximately a 40-year flood event. Since there <br />are not any major tributaries between Glenwood Springs and Silt, <br />these values are representative for the Silt reach during a <br />snowmelt flood event. It is our understanding that the 1984 <br />event nearly overtopped the sandbag dike system; therefore, a <br />100-year or larger event will completely inundate the treatment <br />plant site. <br /> <br />Groundwater Infiltration <br /> <br />The island's soil composition is one of gravel and cobble <br />overlaid with a shallow layer (less than 6 inches) of organic <br />matter. With this type of sub-surface stratum, as the river <br />stages rise during a flood, muddy, silty floodwaters will <br />percolate through the stratum and will surface within the diked <br />area. Since the island is a relatively small area, the <br />floodwater levels will seek equilibrium during prolonged flood <br />stages, which do occur during a snowmelt flood on the Colorado <br />River. Therefore, for any level or dike system to completely <br />protect the island it must include a cutoff system which will <br />provide protection from infiltration of floodwaters. However, <br />caution must be exercised so Silt's water supply is not also cut <br />off. <br /> <br />, <br />