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POST REGRADING POND DEMONSTRATION <br />In response to the Division additional issues regarding the Raton Creek pond, MWH has completed <br />the following pond calculations to demonstrate that the pond under existing conditions will meet <br />effluent standards fora 10-year, 24-hour event. The mine site was regraded and seeded in 1998 and <br />moderate vegetation has been reestablished. The clean water diversions are still in place and all runoff <br />from the reclaimed azeas are still diverted to the pond. To demonstrate that the pond discharge <br />during a 10-year, 24-hour event will meet effluent levels, MWH utilized the SEDCAD 4 computed <br />model developed by Civ$ Software Design. MWI-I used the same hydrology inputs outlined in the <br />existing permit with the following exceptions: <br />CN Reclaimed <br />Soil Type: B <br />Ground Covu: Poor <br />Cover Type: Herbaceous <br />CN: 80 <br />C Reclaimed <br />Type and Height of Canopy: <br />Percent Cover: <br />C Factor: <br />P Reclaimed <br />Moderately Rough: <br />No Appreciable Canopy <br />20% <br />0.20 <br />0.9 <br />The following is a summary of the pond information taken from the As-built Survey drawing: <br />Total Pond Volume: 4.91 ac-ft <br />Principal Spillway <br />Type: <br />Invert Elevation: <br />Emergency Spillway <br />Type: <br />Invert Spillway: <br />4" riser with a 4" barrel <br />6342.1 ft <br />Trapezoidal Channel <br />6350.5 ft <br />All input infotmation and results can be found on the SEDCAD 4 computer printout sheets. The <br />following is a summary of the results: <br />Peak Discharge <br />In: 43.2 cfs <br />Out: 0.5 cfs <br />Peak Stage: 6349.7 <br />Total Runoff: 3.80 ac-ft <br />Peak Settleable Concentration <br />In: 106.6 ml/1 <br />Out: 0.0 ml/I <br />Sediment Storage Level: 6341.8 ft <br />~,~~~ <br />