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4.3.2.4 Ponds, Impoundments and Diversions <br />All overland flow in the loadout area is conveyed through temporary <br />diversion structures. All structures in the temporary diversion <br />system are designed to safely pass the peak runoff from a 10 -year, <br />24-hour precipitation event. Design criteria for both disturbed and <br />undisturbed area diversions are included in the Engineering <br />Calculations, Appendix 4-B. <br />Flow from disturbed areas is conveyed to sediment control ponds. <br />Three sedimentation ponds are used at the loadout site. Pond Number 2 <br />is located to control disturbed area runoff from the railroad spur. <br />Sediment storage volume is based on 0.1 acre-foot per acre of <br />disturbed area. Runoff storage capacity accommodates discharge from a <br />100 -year, 24-hour precipitation event. Calculations and design cri- <br />teria are shown in the design section of Appendix 4-B. <br />Pond Number 2 controls sediment from the washdown process plus adja- <br />cent disturbed area runoff. Capacities and operation details are pre- <br />sented in the final design section of Appendix 4-B. Storage capacity <br />accommodates the discharge from a 100 -year, 24-hour precipitation <br />event. The State Engineer confirms that dewatering is not required. <br />Each sediment pond will be designed and inspected during construction, <br />under the supervision of, and certified after construction by, a <br />qualified registered professional engineer. <br />Sedimentation Pond Number 3 controls disturbed area runoff at the <br />truck dump. The area, 200 feet square, is paved and bermed to contain <br />rainfall or snowmelt. The pavement is sloped gently toward a sedimen- <br />tation pond in the western most corner of the site. Sediment storage <br />volume and temporary diversion capacity are based on the peak <br />discharge from a 10 -year, 24-hour precipitation event. Design calcu- <br />lations and details for this pond are presented in the final design <br />section of Appendix 4-B. <br />4-38 <br />