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<br /> <br />Mr. Luke Russell -4- September 21 1993 <br />riprap on the channel bottom and sideslopes up to the computed water depth (Table 2), plus <br />two feet of freeboard. <br />Lower Tailings Pond. The existing diversion channels should safely convey the 100-year, <br />24-hour runoff event, however some erosion of the channel bottom and sides may occur <br />due to the computed channel velocities. For these channels to pass the PMP event without <br />overtopping or eroding significantly, placement of 14 inch mean diameter (based on <br />Ishbash Curve, SCS 1984) angulaz rock riprap on the channel bottom and sideslopes would <br />be required. <br />Upper Tailings Pond. The slope of the existing diversion channel should be graded to a <br />consistent one percent grade to safely pass the 100-year, 24 hour runoff event. At this <br />flow, some erosion of the channel bottom and sides may occur due to the computed <br />channel velocities. For this channel to pass the PMP event without overtopping or eroding <br />significantly, placement of 14 inch mean diameter (based on Ishbash Curve, SCS 1984) <br />angular rock riprap on the channel bottom and sideslopes would be required. <br />If you have any questions regarding the hydrologic evaluation or design criteria described <br />in this letter, please call me at (208) 345-5865. <br />Sinceerely, ,~ 1 <br />J ~(.~ <br />Scott Benowitz, P.E. <br />