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_. -., <br />Mr. William T. Davis - 4 - June 7, 1989 <br />If the operator chooses to substantiate a smaller sediment yield value, then a <br />revised USLE application must be used. The R value should be about 80; the K <br />factor .37 to .43; the CP factor 1.0. The LS factor is a key variable. <br />Division measurements of Exhibit KK indicate average slopes which are about <br />25% and slope lengths which vary from about 150 to 400 feet. These numbers <br />indicate a much higher LS value than has been used. It is possible the <br />applicant is projecting disturbed area configurations to achieve lower LS <br />factors, but this remains to be substantiated. <br />The pond designs are not adequately detailed to conduct proper evaluation. <br />The cross section is not specifically correlated to plan view diagram, and <br />contour information is not provided with plan view. The designs must be of <br />sufficient accuracy and detail to allow volume calculations. The surface area <br />shown is 0.61 acres with a depth shown to be two feet from primary dewatering <br />device to base of pond. These factors indicates a pond volume of 1.22 acre <br />feet at the level of the primary dewatering device and 2.75 acre feet at the <br />100 year, 24 hour storm stage level. These values are far less than the <br />required volumes and do not corroborate the calculated figures. <br />As part of the additional detail requested are elevations of the upstream and <br />downstream dam toes, maximum sediment level, 10 year-24 hour storage level, 25 <br />year-24 hour or 100 year-24 hour storage level (depending on pond size as <br />specified in Rule 4.05.6), primary spillway inlet, emergency spillway crest, <br />and dam top. <br />The primary dewatering device specifications must include the diameter, pipe <br />length, pipe slope, and mannings n for the pipe. The emergency spillway <br />specifications must include spillway channel bottom width, side slope <br />gradients, crest and outs lope gradients, length of crest and out slopes, <br />anticipated peak flows and velocities, riprap calculations if erosive <br />velocities are expected, and spillway riprap safety factor. Calculations <br />should be provided to include anticipated detention time, (must be greater <br />than 24 hours) and dewatering time (should be less than 10 days). <br />As drawn, the embankment dimensions do not comply with tfie requirements of <br />Rule 4.05.6(8)(h). A scale diagram should be provided with specific <br />elevations as noted above. <br />The method should be described by which sediment accumulation will be <br />monitored and the level at which sediment removal will occur. The level <br />should never exceed 60% of the total three year accumulation. <br />