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• Mc Crellin Scott <br />July 3Q 2002 <br />Page 2 of 3 <br />~. Gossard Watershed SEDCAD 4 Model Input. These sheets provide a summary of <br />the input data into the new SEDCAD 4 model of the Gossard 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. Gossard Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the Gossard watershed and earlier <br />versions. The summary sheet indicates that the drainage area does not change <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 Gossard Pond <br />has 7.2 years of sediment storage. <br />• 8. SEDCAD 4, Gossard 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 <br />Gossard Pond is in compliance with an effluent discharge of less than 0.5 tnl/I <br />settleable solids and no discharge in the emergency spillway. <br />9. SEDCAD 4, Gossard 25-year, 24 hour Model. The spillway is adequate. <br />] 0. SEDCAD 4, Gossard 100-year, 24-hour Model. The spillway is adequate. <br />1 I. SEDCAD 4, Gossard TR-60 Emergency Spilhvay Evaluation. The Gossard 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 between the probable maximum 6-hour amount and the 100-year. 6- <br />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 was determined from <br />HMR 49. <br />12. SEDCAD 4, Gossard TR-60 Emergency Spillway Freeboard Evaluation. The <br />Gossard Pond was assumed to be a Class B pond for a TR-60 emergency spilAvac <br />freeboard evaluation. The freeboard is adequate to withstand the runoff from the 100- <br />}ear, 6-horn- rainfall plus 0.-! times the difference betwee^ the Pi\dP-6-horn- and the <br />100-}'ear. 6-hour amount. The SEDCAD ~ anal}sis indicates the peak elevation will <br />approach to within 6.~6 feet of the top of the embanl:n~ent without overtopping. <br />W <br />WC Engineering <br />