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West Elk Mine - Mine Water Control Plan Mountain Coal Company <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 It. The hydrograph convolution interval <br />(SEDCAD+ 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 six separate pond outfalls (MB-3, M13-4, MB- <br />5E, NSSA, RPE, and SG-1), along with several cleanwater diversions, and the natural channels <br />within Sylvester Gulch. The original Mine Water Control Plan established a structure labeling <br />convention, wherein ditches and culverts are named to correspond to the pond or outfall to <br />which they report. Ditch names all have the prefix "D", followed by the pond/outfall abbreviation, <br />a hyphen, and then the ditch identification number. For example, D5-1 is the first ditch that <br />• flows into Pond MB-5E. Culverts follow a similar pattern, with the prefix "C instead of "D". As <br />the last number increases, the structure is usually farther away from the pond, or a tributary to a <br />second structure flowing into the pond. The addition of several new ditches and culverts to the <br />system has resulted in the use of suffixes (A, B, Al, B1, AA, etc.) to maintain the original <br />ordering system as much as possible. Structure numbers associated with systems draining to <br />decommissioned ponds were unchanged. A summary of culverts and ditches is presented in <br />Table 44E, later in this exhibit. <br />• <br />Tetra Tech October 2008 2