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L J <br />Mine Water Control Plmi <br />event was run to determine the emergency spillway design for ponds MB-1 and 2. These <br />values have been summarized in Table 42 and Table 45. However, these discharges are <br />approximations as the ditching system conveying storm water to the ponds was designed to <br />the 10-year 24hour event. Ditch design freeboard would tend to divert flows to the ponds <br />until cumulated ditch flows exceed ditch and/or culvert capacities, at which time some storm <br />flows would likely proceed overland, by-passing the ponds or result in a re-distribution of <br />storm flows into the ponds. The design spillways discharges are not likely to be realized. <br />Mountain Coal employs several different types of outlet works on the mine's sediment <br />ponds. <br />The West Elk Mine has gated valves on the primary spillways of MB-1, MB-2R, MB-3R, and <br />MB-5. These valves are typically closed. This allows the mine to increase their storage, if <br />required, which will in turn provide a longer time period for sediment to settle out. This <br />allows the mine to better regulate the discharge and suspended sediment load before it is <br />discharged. Ponds MB-4 and MB-5 utilize trickle tubes for the primary outlet and open <br />channel spillways for the secondary outlet works. <br />Ditches <br />• Ditches have been designed using the utility in SEDCAD+ to handle the 10-year 24-hour <br />storm event with either erodible or non-erodible channels. The ditches that were designed <br />as erodible had to maintain a discharge velocity less then 5.5 feet per second and are <br />sediment laden. <br />Channels that were designed as non-erodible were either lined with riprap, concrete, or 1/2 <br />culvert. This allowed the stream velocity to increase as the water flowed down significant <br />gradients without eroding the channel bed. Some channels have been overgrown with native <br />vegetation. This is the case for ditch DCW-1. <br />Small Area Exemptions and Other Exceptions <br />• <br />There are three areas that have been categorized as small area exemptions. One area is <br />located in the Sylvester Gulch watershed at the fan house located at the top of the road. <br />This structure was able to pass the requirements of SAE since there was no increased <br />erosion. Water that fell onto this structure flowed off of the metal roof and onto rock or <br />concrete. Once the water was on the ground if flowed directly into the roadway and into <br />9 <br />