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Mr. Crellin Scott <br />July 30, 2002 <br />Page 2 of 3 <br />5. Gulch A Watershed SEDCAD 4 Model Input. These sheets provide a summary of <br />the input data into the new SEDCAD 4 model of the Gulch A 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 ,+ year growth <br />(CN=56), blue is reclaimed 1 to 2 year growth fCN=61), and red is reclaimed <br />topsoiled and seeded (CN=86). This figure represents the watershed immediately <br />afrer 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. Gulch A Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the Gulch A watershed and earlier <br />versions. The summary sheet indicates that although the drainage area increased 76.4 <br />acres between 1998 and 2002, discharge parameters for the 10-year and 25-year <br />• storms generally decreased. The pond summary sheet also predicts the Gulch A Pond <br />has 4.9 years of sediment storage. <br />8. SEDCAD 4, Gulch A 10-year, 24-hour Model. This is the printout of the ]0-year. <br />24-hour design storm. This model demonstrates that using the design storm the <br />Gulch A Pond is in compliance with an effluent discharge of less than 0.5 ml/I <br />settleable solids and no discharge in the emergency spillway. <br />9. SEDCAD 4, Gulch A 25-year, 24 hour Model. The spillway is adequate. <br />] 0. SEDCAD 4, Gulch A 100-year, 24-hour Model. The spilhvay is adequate. <br />1 I. SEDCAD ~, Gulch A TR-60 Emergency Spilhvay Evaluation. The Gulch A Pond <br />vvas assumed to be a Class B pond for a TR-60 emergency spilhvay 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 />13. SEDCAD 4, Gulch A TR-60 Emergency Spillway Freeboard Evaluation. The <br />Gulch A Pond was assumed to be a Class B pond for a TR-60 emergency spillwaJ <br />freeboard eeah~ation. The freeboard is inadequate to withstvid the runoff from the <br />100-year. 6-hour rainfall plus 0.4 times the difference '~etween the PMP-6-hour and <br />W <br />WC Engineering <br />