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Exhibit 13-9B). Consequently, the watershed sediment yield is greatly <br />reduced from the worst-case conditions evaluated in the 1996 design. • <br />Embankment and Excavation <br />The embankment is a homogeneous earth fill structure. The top width of the <br />embankment is approximately 25 feet with an upstream slope of 3.75H:1V and <br />downstream slopes of approximately 3.75H:1V. The principal spillway is a 6- <br />inch diameter dewatering orifice cut into a 36-inch CMP riser, with the <br />bottom of the hole at 6, 784.0 feet, which drops into a 29-inch CMP barrel <br />pipe. The emergency spillway is a trapezoidal open channel 8.7 feet wide <br />with an invert elevation of 6,767.7 feet. Exhibit 13-9A, As-Built Pond 010, <br />shows the pond configuration and spillway specifications. A portion of the <br />pond capacity was the result of excavating topsoil and sub-grade material <br />below natural ground level. Excavated suitable material was used in <br />embankment construction and came from the Yoast Haul Road B between Stations <br />4+00 and 10+00 and Stations 13+00 to 15+00. <br />Desiqn AnalVSis <br />The TR-30 hydrology and sedimentology evaluation demonstrates that Pond 010 <br />meets or exceeds the design and performance requirements of the Regulations • <br />of the Colorado Mined Land Reclamation Board for Coal Mining. Specifically, <br />the as-built configuration adheres to the following requirements: <br />• Provide adequate stozage and tzeatment fox runoff from the 10-yeas, 24- <br />hour storm event to assure compliance with receiving water quality <br />standards in accordance with Rule 9.05.6(3)(a); <br />• Provide storage and treatment for the runoff from the 10-year, 29-hour <br />storm event without experiencing any discharge through the emergency <br />spillway (Rule 4.05.9(2)(b)); <br />• Provide an emergency spillway capable of passing runoff from the 25- <br />year, 24-hour storm event in accordance with Rule 4.05.9 (2)(c)(ii). <br />SEDCAD Demonstration. The hydrology and sedimentology o£ the watershed were <br />analyzed using the SEDCAD9 computer program. SEDCAD4 is an enhanced version <br />of the SEDCAD+ computer program that was used for the 1996 design <br />demonstration. Model methodology and assumptions are presented in Attachment <br />13-12, Yoast Mine Drainage and Sediment Control Plan. Addendums 13-3A-A, 13- <br />3A-B and 13-3A-C present the SEDCAD4 modeling demonstrations for the 10-year, <br />29-hour, 25-year, 29-hour and, 100-year, 29-hour design storms. <br />Input consists of precipitation (inches), watershed area (acres), time of <br />concentration (hours), runoff curve number (CN), representative particle-size <br />TR-35 13-3A.2 Revised 09/07 <br />