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• Mc Crellin Scott <br />July 3Q 2002 <br />Page 2 of 3 <br />4. Prospect Pond, 007A As-Built. This drawing, provided by Colowyo Coal <br />Company, was used to obtain the dimensional information necessary to model the <br />pond in SEDCAD 4. <br />~. Prospect Watershed SEDCAD 4 Model Input. These sheets provide a summary of <br />the input data into the new SEDCAD 4 model of the Prospect 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 I to 2 year growth (CN=61), and red is reclaimed <br />topsoiled and seeded (CN=86). This figure represf:nts the watershed immediately <br />afrer the pit is backfilled and is considered the worst-c;ase 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 ZC). <br />7. Prospect Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the Prospect watershed and earlier <br />versions. The summary sheet indicates that the drainage area decreased 38.2 acres <br />between ] 998 and 2002 and the discharge parameters for the 10-year and ZS-year <br />storms generally decreased. The pond summary sheet also predicts the Prospect Pond <br />has 1.2 years of sediment storage. <br />8. SEDCAD a, Prospect 10-year, 24-hour Model. Thais is the printout of the ]0-year, <br />24-hour design storm. This model demonstrates that using the design storm the <br />Prospect Pond is in compliance with an effluent discharge of less than 0.~ mI!I <br />settleable solids and no discharge in the emergency spilhvay. <br />9. SEDCAD a, Prospect 25-year, 24 hour Model. The spillway is adequate. <br />]0. SEDCAD ~, Prospect 100-year, 2=1-hour Model. The spilhvay is adequate. <br />11. SEDCAD ~, Prospect TR-60 Emergency Spilhva}• Evaluation. The Prospect Pond <br />~~as assumed to be a Class B pond for a TR-60 emergency spilhca} evaluation. The <br />rainfall amount used for this model was the 100-year, 6-hour amount plus 0.12 times <br />the difterence behveen the probable ma~inuim 6-hour amount and the 100-vear. 6- <br />hour amount. The model shows the spilhca} is adegi_iate to meet the TR-60 criteria <br />for a Class B structure. The probable mazinunn precipitation was determined from <br />HMR -19. <br />W <br />WC Engineering <br />