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• <br />Larry Oehler - 3 - <br />Option 1 <br />Define the 25-year floodplain for the Middle Fork of the Si <br />River. Commit to restricting all proposed mining operatioi <br />the 25-year floodplain. Since the anticipated life-of-min< <br />Placer is ten years, the Division has deemed the 25 year-2~ <br />an appropriate design criteria. (A 10 year-24 hour storm ~ <br />recurrence interval of 10 years may occur during the life c <br />OtPion2 <br />April 4 , 1990 <br />uth Platte <br />s outside of <br />for the Alma <br />hour storm as <br />nich has a <br />f mine). <br />Commit to construction of a berm along the west side of the operation <br />(approximately 4800 feet) which will be adequate to reject the flow of a <br />25 year-24 hour storm for the Middle Fork of the South Pla River. <br />Embankment design should include a minimum freeboard of 1 f of above the <br />water surface elevation of the 25 year-24 hour storm. <br />If Option 2 is chosen, the following information should be ubmitted: <br />a. An estimate of the 25-year peak flow discharge; <br />b. The elevation of the river water surface at the 25-ye r flow <br /> condition along the length of the berm, (and immediate y above and <br /> below the berm.) ? <br />c. A description of channel bank re-enforcement measures , such as <br /> riprap and riprapize to insure channel stability at 2 -year flow <br /> conditions; <br />d. A description of channel bottom reinforcement measure ,such as a <br />~~ ~ <br />~ <br /> drop structure placed upstream of the pit, armouring, e <br />xw <br />or existing <br /> channel-bottom conditions to insure stable conditions across the <br /> channel at 25-year flow conditions. <br />e. A discussion of any other structural measures such as berm width, <br /> flow deflectors, and side-channel spillways <br />contempla ed for use to <br /> ~ <br />insure stability during flood conditions; 7 <br />f. hydraulic calculations and cross-sections used to der ve these <br /> designs; <br />g. A statement that the design and construction of the d ke has been <br /> inspected and certified by a qualified registered pro essional <br /> engineer. Certification of the constructed dike shal be forwarded <br /> to the Division immediately following construction. <br />The measures outlined in Options 1 and 2 will minimize the ~otential for <br />flooding to occur in the disturbed area as well as ensure c mpliance with <br />other applicable State and Federal requirements. <br />4. Site Stability - Excavation <br />Reclamation Resources has proposed a mining method which re wires the use <br />of bulldozers to excavate, move, and stockpile ore bearing ravels. They <br />propose to remove gravels down to bedrock which varies from 10 to 70 feet <br />below the surface. Since the area to be mined consists of glacial <br />deposit within a confined sub-alpine valley, several questi ns arise <br />concerning groundwater inflows and stability within the exc vation. <br />