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~• Mc Crellin Scott <br />July 30, 2002 <br />Page 2 of 3 <br />5. West Pit Watershed SEDCAD 4 Model Input. These sheets provide a summary of <br />the input data into the new SEDCAD 4 model of the West Pit watershed. <br />6. CN Breakdown. This color figure shows [he curve number breakdown for the <br />watershed. Dark green is undisturbed (CN=47), green is reclaimed 3+ year growth <br />(CN=56), blue is reclaimed 1 to 2 year growth (CN=61), and red is reclaimed <br />topsoiled and seeded (CN=86). This figure represents the watershed immediately <br />after the pit is backfilled and is considered the worst-case scenario. Prior to that time. <br />the pit intercepts a large portion of the watershed. Curve numbers were obtained <br />from "Methodologies and Assumptions for Sedimentation Pond Evaluations" of the <br />approved permit (Volume 2C). <br />7. West Pit Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the `JVest Pit watershed and earlier <br />versions. The summary sheet indicates that the drainage area decreased 321.5 acres <br />between 1998 and 2002 and the discharge parameters for the 10-year and 25-year <br />storms generally decreased. The pond summary sheet also predicts the West Pit Pond <br />. has 5.2 years of sediment storage. <br />8. SEDCAD 4, West Pit 10-year, 24-hour Model This is the printout of the 10-year, <br />24-hour design storm. This model demonstrates that using the design storm the West <br />Pit Pond is in compliance with an effluent discharge of less than 0.5 ml/1 settleable <br />solids and no discharge in the emergency spillway. <br />9. SEDCAD 4, West Pit 25-year, 24 hour Model. The spillway is adequate. <br />10. SEDCAD 4, West Pit 100-year, 24-hour Model. The spilhva}~ is adequate. <br />11. SEDCAD 4, West Pit TR-60 Emergency Spillway Evaluation. The West Pit Pond <br />was assumed to be a Class B pond for a TR-60 emergency spillway evaluation. The <br />rainfall amount used for this model was the 100-year. 6-hour amount plus 0.12 times <br />the difference behv'een the probable maximum 6-hour amount and the 100-year. 6- <br />hour amount. The model shows the spilhvay is adequate to meet the TR-60 criteria <br />for a Class B strucnu'e. The probable maximum precipitation was determined from <br />HMR 49. <br />12. SEDCAD 4, West Pit TR-60 Emergency spilhvay Freeboard Evaluation. The <br />~l%est Pit Pond was assumed to be a Class B pond for a TR-60 emer~~ency spilhvay <br />freeboard evahiation. "fhe freeboard is adequate to withstand the runoff from the 100- <br />cear. 6-hour rainfall plus 0.4 tithes the difference between the PIP-6-hour and the <br />W <br />WC Engineering <br />