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Mineral Joe Mine — Drainage Design Plan 43 <br />5.5 Culvert Capacity <br />Culverts were evaluated to determine the size required to convey the 100 -year 24 -hr peak flow rate (gloo) <br />based on the parameters shown in Table 25. As shown in Figure 2, the existing culverts are located on <br />County Road EE 19 and are owned and maintained by Montrose County. Existing culverts are constructed <br />using 18 -inch diameter corrugated steel pipe. The capacity was evaluated using HY -8 (FHWA, 2012). <br />On the EE -19 Road, Montrose County installs culverts to fit their requirements. <br />Any new culverts installed by Cotter within the Mineral Joe Mine area will be installed with adequate <br />capacity to convey the 100 -year 24 -hour storm, and will be a minimum of 18 -inch diameter. <br />Table 25. Culvert Capacity for 100 -year 24 -hour Storm Event <br />Parameter <br />Culvert C -1 <br />Culvert C -2 <br />Contributing Sub -Basin <br />N -I <br />N -H <br />10 Design Flow cfs <br />0.31 <br />0.20 <br />100 Max Flow cfs <br />0.9 <br />0.53 <br />Channel Depth / Culvert Diameter ft <br />1.5 <br />1.5 <br />Culvert Flow Depth ft <br />0.34 <br />0.25 <br />Tailwater sloe <br />0.191 <br />0.075 <br />Velocity ft/s <br />3.09 <br />2.64 <br />Slope ft/ft <br />0.02 <br />0.02 <br />6. STRUCTURAL CONTROLS AND ADDITIONAL STORMWATER BMPS <br />6.1 Summary of Existing Structural Controls for Stormwater <br />The best management practices (BMPs) for stormwater evaluated above include diversion ditches to <br />convey upland runoff around the Mineral Joe Mine site, as well as drainage ditches, berms, and the <br />stormwater retention pond for the capture, control and treatment of runoff. In summary, existing structural <br />controls on stormwater flow are shown in Figure 4 and include the following: <br />• Upper Diversion Structure <br />• Middle Diversion Structure <br />• Lower Diversion Structure <br />• Stormwater and Sediment Retention Pond <br />• Crest Berm surrounding the waste rock pile <br />• Perimeter Ditch surrounding the mine facilities area. <br />• Silt Fence located at the toe of the waste rock pile <br />6.2 Sediment and Erosion Prevention <br />Several practices are employed at the Mineral Joe Mine to reduce erosion and prevent sediment delivery to <br />State waters. Although no perennial streams or springs have been mapped in the Mineral Joe Mine area, <br />these sediment and erosion prevention measures are intended to prevent sediment delivery to ephemeral <br />channels (incised channels that are mostly dry, but may flow temporarily in response to rainfall or <br />snowmelt). <br />4148B.131029 Whetstone Associates <br />