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• 9. Provide a nonclogging dewatering device and principal spillway <br />that will provide adequate detention time for the 10-year, 24-hour <br />storm event and still allow a method to dewater the pond in a <br />reasonable amount of time. <br />This sedimentation pond was constructed within a disturbance area that has <br />been backfilled and graded. A conservative procedure was used to estimate <br />the runoff; the structure was analyzed with the assumption that the entire <br />drainage area that would contribute runoff to the structure was disturbed <br />and unvegetated. In addition, the structure was conservatively designed to <br />include adequate capacity for sediment storage and to treat the runoff from <br />a 10-year, 29-hour precipitation event. Finally, the design conservatively <br />assumed the principal spillway is clogged during the 25-year, 29-hour <br />precipitation event and provides for maintenance of adequate freeboard to <br />prevent overtopping of the embankment. The SEDCAD output for the pond as- <br />built conditions are presented in Addendum 13-19-A, 13-19-B and 13-19-C. <br />Summary <br />The capacity of Sedimentation Pond Olla is attributed partially to the <br />• embankment and partially to the pond excavation. During passage of the 10- <br />year, 29-hour storm the pond is designed to effectively treat the runoff <br />and will not discharge flow through the emergency spillway. In addition, <br />the emergency spillway was designed and constructed to safely discharge the <br />25-year, 24-hour storm event. <br />The embankment was constructed to be stable and provide adequate freeboard. <br />Therefore, based on the design, this structure complies with the <br />appropriate regulations for a sedimentation structure. Seneca Coal Company <br />will periodically inspect the structure far sediment storage, structural <br />integrity and hazardous conditions. Maintenance and remedial work will be <br />performed in accordance with the approved mine permit. <br />Construction Summary <br />In accordance with Rule 9, Part 9.05.6(10) of the Rules and Regulations of <br />the Colorado Mined Land Reclamation Board pursuant to the Colorado Surface <br />Coal Mining Reclamation Act 39-33-001 et seq., I, John Pellicer, state that <br />during construction of Sedimentation Pond Olla inspections were conducted <br />by MWH personnel under my supervision. Seneca Coal Company constructed <br />• Pond Olla under the supervision of Mr. Roy Karo, Reclamation Manager, and <br />TR-27 2 Revised 09/09 <br />