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<br />Mr. Crellin Scott <br />July 3Q 2002 <br />Page 2 of 3 <br />5. Work Area Watershed SEDCAD 4 Model Input. These sheets provide a summary <br />of the input data into the new SEDCAD 4 model of the Work Area watershed. <br />6. CN Breakdown. This color figure shows the 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 watershed is reclaimed and is considered the worst-case scenario. Curve <br />numbers were obtained from '`Methodologies and Assumptions for Sedimentation <br />Pond Evaluations" of the approved permit (Volume 2C). <br />7. Work Area Sedimentation Pond Summary. This [able provides a quick reference <br />to the differences between the current model of the V/ork Area watershed and earlier <br />versions. The summary sheet indicates that the drainage area decreased by 14.5 acres <br />between 1998 and 2002 and the dischazge parameters for the 10-year and 25-year <br />storms generally did not change. The pond summary sheet also predicts the Work <br />Area Pond has 37 years of sediment storage. <br />• 8. SEDCAD 4, Work Area 10-year, 24-hour Model. This is the printout of the 10- <br />year. 24-hour design storm. This model demonstrates that using the design storm, the <br />Work Area Pond is in compliance with an effluent discharge of less than 0.5 ml/1 <br />settleable solids and no discharge in the emergency spillway. <br />9. SEDCAD 4, Work Area 25-year, 24 hour Model. The spillway is adequate. <br />10. SEDCAD 4, Work Area 100-year, 24-hour Model. The spillway is adequate. <br />] 1. SEDCAD 4, Work Area TR-60 Emergency Spilhvay Evaluation. The Work Area <br />Pond was assumed to be a Class B pond for a TR-60 Emergency Spilhvay Evaluation. <br />The rainfall amount used for this model was the 100-year. 6-hour amount plus 0.12 <br />times the difference between the probable maximum 6-hour amount and the 100-year. <br />6-hour amount. The model shows the spillway is adequate to meet the TR-60 criteria <br />for a Class B structure. The probable maximum precipitation ~cas determined from <br />HMR 49. <br />12. SEDCAD 4, Work Area TR-60 Emergency Spilhvay Freeboard Evaluation. The <br />V<'ork Area Pond was assumed to be a Class B pond for a TR-60 Emergence Spilhvay <br />Freeboard Evaluation. The h-eeboard is adequate to ~cithstand the runoff from the <br />100-Fear. 6-hour rainfall plus 0.4 times die difference between the PMP-6-hour and <br />the 100-year. 6-hour amount. The SEDC:\D ~4 anal}sis indicates the peak elevation <br />will approach to within ] .» feet of the top of the embankment ~~ ithout overtopping. <br />W <br />WC Engineering <br />