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Mr. Crellin Scott <br />July 30, 2002 <br />Pale 2 of 3 <br />~. Rail Loop Watershed SEDCAD 4 Model Input. These sheets provide a summary <br />of the input data into the new SEDCAD 4 model of the Rail Loop watershed. <br />6. CN Breakdown. This color figure shows the ctiirve number breakdown for the <br />watershed. Dark green is undisturbed (CN=47), grf:en 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 ZC). <br />7. Rail Loop Sedimentation Pond Summary. This table provides a quick reference to <br />the differences between the current model of the Fail Loop 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 Rail Loop <br />Pond has 13.0 years of sediment storage. <br />8. SEDCAD 4, Rail Loop 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 Rail <br />Loop Pond is in compliance with an effluent dischar€;e of less than 0.5 ml/l settleable <br />solids and no discharge in the emergency spillway. <br />9. SEDCAD 4, Rail Loop 25-year, 24 hour Model. The spillway is adequate. <br />10. SEDCAD 4, Rail Loop 100-year, 24-hour Model. l'he spillway is adequate. <br />11. SEDCAD 4, Rail Loop TR-60 Emergency Spillway Evaluation. The Rail Loop <br />Pond was assumed to be a Class B pond for a TR-60 emergency spillway evaluation. <br />The rainfall amount used for this model was the 10(1-year. 6-hour amount plus 0.12 <br />times the difference between the probable maximum fi-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 was determined from <br />HMR 49. <br />12. SEDCAD 4, Rail Loop TR-60 Emergency Spilh~a} Freeboard Evaluation. The <br />Rail Loop Pond was assumed to be a Class B pond for a TR-60 emergenc} spilhcay <br />freeboard evaluation. The freeboard is adequate to withstand the runoff from the 100- <br />cear. 6-hour rainfall plus 0.-t times the difference behcee^ the Pi\~IP-6-hour and the <br />]00-cear. 6-hour amount. The SEDCAD 4 anahsis indicates the peak elevation will <br />approach to within 1.82 feet of the top of the embankment without ovet7opping. <br />W <br />WC Engineering <br />