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Mr. Crellin Scott <br />July 30, 2002 <br />Page 2 of 3 <br />4. Streeter Pond, OOlA As-Built. This drawing, provided by Colowyo Coal Company, <br />was used to obtain the dimensional information necessary to model the pond in <br />SEDCAD 4. <br />5. Streeter Watershed SEDCAD 4 Model Input. These sheets provide a summary of <br />the input data into the new SEDCAD 4 model of the Streeter 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 pit is backfilled and is considered the worst-ease 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. Streeter Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the Streeter watershed and earlier <br />versions. The summary sheet indicates that the drainage area increased by 129.4 <br />acres between 1998 and 2002 and the discharge parameters for the ]0-year and 25- <br />year storms generally decreased. The pond summary sheet also predicts the Streeter <br />Pond has 2.5 years of sediment storage. <br />8. SEDCAD 4, Streeter 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 />Streeter Pond is in compliance with an effluent discharge of less than 0.5 m1/1 <br />settleable solids and no discharge in the emergency spilhvay. <br />9. SEDCAD 4, Streeter 25-year, 24 hour Model. The spilhvay is adequate. <br />10. SEDCAD 4, Streeter 100-year, 24-hour Model The spillway is adequate. <br />11. SEDCAD 4, Streeter TR-60 Emergency Spilh~ay Evaluation. The Streeter Pond <br />~~as assumed to be a Class B pond for a TR-60 emergence spilhca} evah~ation. The <br />rainfall amount used for this model .vas the ] 00-eear. 6-hour amount plus 0.13 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 suucture. The probable maximum precipitation was determined from <br />Hi\-IR 49. <br />W <br />WC Engineering <br />