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Nevada and Cascade Mountains. <br />• The SCS Type II distribution was used for the Hayden Gulch Coal Mine evaluation. <br />Storm Duration <br />Per Rule 4.05 of the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining <br />(Division of Minerals and Geology, 1995), diversion channels and sedimentation structures are <br />designed to pass safely and detain the peak runoff from a precipitation event of 24 hour duration. <br />• A storm duration of 24 hours was used for the Hayden Gulch Coal Mine runoff <br />evaluation. <br />Return Period/Precipitation <br />Diversion channels and sedimentation structures must also be designed for the precipitation amount <br />associated with an appropriate recurrence interval. Per Rule 4.05.6(3)(a), a sedimentation structure <br />must contain adequate capacity to detain or treat runoff entering the pond as a result of the 10-year, <br />24-hour storm event. Per Rule 4.05.6(5)(6), the applicable design storm event for sizing a pond <br />emergency spillway, for structures which do not exceed MSHA safety criteria, is the 25-year, 24-hour <br />storm event. The following precipitation amounts were used for the mine pit reclaimed area runoff <br />evaluation: <br />• IO-year, 24-hour event 1.8 inches <br />• 25-year, 24-hour event 2.3 inches <br />The precipitation amounts were obtained from the Precipitation-Freq:terrey Alas ojthe Western <br />United Stales, NOAA Atlas 2, Volume III-Colorado (Miller, et al., ]973). <br />Hydrograph Response Shape <br />A unit hydrograph was chosen for each drainage area or sub-area model to predict the runoff <br />response. The hydrograph responses available in the SEDCAD model are slow, medium, and fast. <br />A slow response corresponds to a forested area or an area with a number of obstructions. A fast <br />response corresponds to an unvegetated or poorly protected area (Warner and Schwab, 1992). <br />Field monitoring for revegetative success indicates excellent revegetative success with a vegetative <br />cover condition rating greater than 95, with a weighted mean litter cover of approximately 28 percent <br />(I-IGCC, 1993). This cover condition, as well as field observations, suggests that a number of <br />obstructions impede surface runoff. The medium hydrograph response was used for this evaluation. <br />The internal convolution increment is 0.05 hours and values are saved at the user specified interval <br />of 0.1 hours or greater. A convolution increment of 0. I was specified for the H-G Coal Mine. It <br />should be noted that a time of concentration less than 0.125 hours bypasses the unit hydrograph <br />technique and instantaneous run-off is assumed. As explained in the SEDCAD User's Manual <br />(Schwab and Warner, 1987, page 113), the time of concentration restriction of 0.125 hours is <br />c:~87Thg8~primary4nellwdhow 7/16N6 <br />