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For a pit like the Douglas Mountain Ranch Mine (berm length>1300 ft), according to the Urban <br /> Drainage Flood Control District, riverside berms shall have two spillways, or inflow/outflow <br /> structures. However, riverside berms of the type discussed in Urban Drainage guidelines are <br /> typically 2.5H:1 V or steeper sloped on the pit side. Since the DMR Mine stormwater berm is <br /> significantly gentler sloped (16.7H:IV) and will be vegetated,the need for such spillways is low. <br /> However, out of an abundance of caution,the DMR Mine will install two spillways along the <br /> riverside berm. Spillway locations can be seen on Map C-2B. These spillways will be created by <br /> cutting the riverside berm down four feet at each location (two feet tall instead of six feet tall) for <br /> a distance of 200'. The sideslopes of these spillways will be 4H:1 V for a total width of 232' at <br /> the top of the spillway. <br /> Flood waters will return to the West Fork of Clear Creek passively after an event. The <br /> stormwater berm will accomplish its job of retaining sediment from disturbed areas within the <br /> pit. <br /> 3. Flood Emergency Response Plan <br /> During the first ten years of the mine's life, mining will be significantly above the floodplain <br /> elevation. Over 90% of the mining area will be outside of the floodplain.Nonetheless, a Flood <br /> Emergency Response Plan will be in place throughout the operation's life. Empire Aggregates <br /> personnel will follow the following response plan, at a minimum: <br /> 1. Monitor available weather data for signs of area storms that could lead to flooding. <br /> 2. Monitor the water level of the West Fork of Clear Creek. <br /> a. When the water level is within six feet of the bottom of the stormwater berm, <br /> move all portable and mobile equipment to higher ground. <br /> 3. In the event of site flooding, contact DRMS within 24-hours. <br /> 4. Following a flood, repair any damaged stormwater control structures. <br /> 4. Conclusion <br /> The Douglas Mountain Ranch Mine mining and reclamation plans will create a better flood <br /> condition along the West Fork of Clear Creek than currently exists by providing a large <br /> hydrologic sink along the creek course that flood waters can safely enter and exit. Stormwater <br /> control structures will trap sediment from flood waters on site and be constructed in a manner <br /> that will resist damage during flood events. <br /> Please contact me if you have any further questions or concerns. <br /> Sincerely, <br /> Ben Langenfeld, P.E. <br /> Greg Lewicki and Associates <br /> DMR Mine Flood Analysis 4 i.n•l 1-11 and\stion.1---t' <br />