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West Elk Mine - Mine Water Control Plan <br />Mountain Coal <br />• 2.0 SEDCAD INPUT PARAMETERS APPLICABLE TO ENTIRE <br />PROJECT <br />2.1 Storm Type <br />The storm type was selected as NRCS Type ll. The hydrograph convolution interval (SEDCAD <br />model time increment) was selected as 0.10 hour. <br />2.2 Rainfall Depth <br />The rainfall depths for the 10-, 25-, and 100-year 24 hour design events were previously <br />determined to be 1.9 inches, 2.3 inches, and 2.8 inches, respectively, using NOAA Atlas 2, <br />Precipitation-Frequency Atlas of the Western United States, Volume III - Colorado (Greystone, <br />1993). No provision was made for snowmelt or a rain-on-snow event. <br />2.3 Structure Naming Convention <br />The West Elk Mine water control system includes eight separate pond outfalls (MB-1, M13-21R, <br />M13-3, M13-4, M13-5, NSSA, RPE, and SG-1), along with several cleanwater diversions, and the <br />natural channels within Sylvester Gulch. The original Mine Water Control Plan established a <br />structure labeling convention, wherein ditches and culverts are named to correspond to the <br />pond or outfall to which they report. Ditch names all have the prefix "D", followed by the <br />pond/outfall abbreviation, a hyphen, and then the ditch identification number. For example, D2- <br />1 is the first ditch that flows into Pond MB-2R. Culverts follow a similar pattern, with the prefix <br />"C" instead of "D". As the last number increases, the structure is usually farther away from the <br />pond, or a tributary to a second structure flowing into the pond. The addition of several new <br />ditches and culverts to the system has resulted in the use of suffixes (A, B, Al, 131, AA, etc.) to <br />maintain the original ordering system as much as possible. A summary of culverts and ditches <br />is presented in Table 44E, later in this exhibit. <br />• <br />Tetra Tech October 2008