2017-08-15_PERMIT FILE - C1980007A (3)

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West Elk Mine - Mine Water Control Plan Mountain Coal Company 2.0 SEDCAD+ INPUT PARAMETERS APPLICABLE TO ENTIRE PROJECT 2.1 Storm Type The storm type was selected as NRCS Type II. The hydrograph convolution interval (SEDCAD+ model time increment) was selected as 0.10 hour. 2.2 Rainfall Depth The rainfall depths for the 10-, 25-, and 100 -year 24 hour design events were previously determined to be 1.9 inches, 2.3 inches, and 2.8 inches, respectively, using NOAA Atlas 2, Precipitation -Frequency Atlas of the Western United States, Volume III — Colorado (Greystone, 1993). No provision was made for snowmelt or a rain -on -snow event. 2.3 Structure Naming Convention The West Elk Mine water control system includes six separate pond outfalls (MB -3, M13-4, MB - 5E, NSSA, RPE, and SG -1), along with several cleanwater diversions, and the natural channels within Sylvester Gulch. The original Mine Water Control Plan established a structure labeling convention, wherein ditches and culverts are named to correspond to the pond or outfall to which they report. Ditch names all have the prefix "D", followed by the pond/outfall abbreviation, a hyphen, and then the ditch identification number. For example, D5-1 is the first ditch that flows into Pond M13 -5E. Culverts follow a similar pattern, with the prefix "C' instead of "D". As the last number increases, the structure is usually farther away from the pond, or a tributary to a second structure flowing into the pond. The addition of several new ditches and culverts to the system has resulted in the use of suffixes (A, B, Al, 131, AA, etc.) to maintain the original ordering system as much as possible. Structure numbers associated with systems draining to decommissioned ponds were revised to follow the naming convention. A summary of culverts and ditches is presented in Table 44E, later in this exhibit. Tetra Tech October 2010 2