Laserfiche WebLink
• 3. Provide adequate storage for sediment generated as determined from the <br />SEDCAD 4 computer program. <br />4. Provide a nonclogging dewatering device and principal spillway that will provide <br />adequate detention time for the 10-year, 24-hour storm event and still allow a <br />method to dewater the pond in a reasonable amount of time. <br />In some instances, mining will cause the drainage area to change during the life of the <br />sedimentation structure. \X~hen the pit moves into the a~atershed, runoff is intercepted by <br />the pit. In these cases, a conservative procedure was used; the structure was analyzed for <br />the largest anticipated disturbed drainage area that will contribute runoff to the structure. <br />It was estimated that this condition would occur if all of the proposed mining area is <br />disturbed and the area is in different phases of mining and reclamation. 1n addition, the <br />structure was conservatively designed to include adequate capacity for sediment storage <br />and to treat the runoff from a 10-year, 24-hour precipitation event. Finally, the design <br />conservatively assumed the principal spillway is clogged during the 25-year, 24-hour <br />precipitation event and provides for maintenance of adequate freeboard to prevent <br />• overtopping of the embankment. The SEDCAD output for the pond as-built conditions <br />are presented in Addendum 13-17-A, 13-]7-B, and 13-77-C. <br />Summary <br />The capacity of Sedimentation Pond 014 is attributed partially to the embankment and <br />partially to the pond excavation. During passage of the ]0-year, 24-hour storm the pond <br />is designed to effectively treat the runoff and will not discharge flow through the <br />emergency spillway. ]n addition, the emergency spillway was designed and constructed to <br />safely discharge the 25-year, 24-hour storm even[. <br />The embankment was constructed to be stable and provide adequate freeboard. <br />Therefore, based on the design, this structure complies with the appropriate regulations <br />for a sedimentation structure. Seneca Coal Company will periodically inspect the <br />structure for sediment storage, structural integrity, and hazardous conditions. <br />Maintenance and remedial work will be performed in accordance with the approved mine <br />. permit. <br />t3-t7-2 <br />