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Page 7 of 7 <br />• SEDIMENT POND N0. 1 (PIT) AREA SIZING <br />The total 10 yr. - 24 hr. storm runoff in the watershed at the pit is <br />24.8 ac-f t. with an annual sediment contribution of 5.3 ac.-f t. This water <br />will be held in the pit until most sediment settles and then be pumped to <br />Sediment Pond No. 1 in a totally controlled manner utilizing the West <br />Collection Channel. Sediment reduction is assumed at 75Z with this <br />treatment. The surface facilities component of runoff and sediment from <br />preceding tables is 4.1 ac.-f t. <br />Design of the pond for the pit component excluding 75S sediment in <br />addition to the surface facilities sediment component results in a pond <br />with a volume requirement of 21.7 ac.-f t. The pit component of the 25 yr. <br />- 24 hr. precipitation event will be contained in the pit. The emergency <br />spillway design is then for the surface facilities and backfilled pit <br />component only and is 50 ft. long X 1 ft. high. No water is ever expected <br />to flow through this spillway. The pond dimensions are 9.0 ft. deep from <br />the toe of the dam embankment, vertically, to the emergency spillway. Pond <br />details are shown on Pond Figures Appendix 5-2. <br />The Weat Collection Channel, including freeboard handling pit pumpage, <br />is sized to handle 72.26 CFS below the pit and 53.30 CFS within the pit. <br />• Thia design accomodates the 10 yr. - 24 hr. event for the backfilled pit <br />east of the realigned channel and the surface facilities after final <br />reclamation. After a 10 yr. - 24 hr. event when pit storage is available, <br />the dewatering procedure would be to dewater the pond in 2 phases by a <br />manual dewatering 12 1n. diameter pipe with removable cap. Phase I would <br />dewater the surface facilities component in the pond. Phase II would pump <br />the pit component of the event to the pond for the time required for <br />sediment to settle including use of flocculating gell logs, if necessary, <br />to meet the NPDES Permit suspended sediment limitation. Using this <br />procedure, repetitively, much larger events than the 10 yr. - 24 hr. event <br />could be handled. Pond bottom dimensions are 200 ft. vide x 530 ft. long x <br />9 ft. deep. Total available volume is conservative in excess of 21.7 <br />ac-f t. Sediment volume is 5.64 ac-f t. Pond cleaning will occur when <br />sediment is 2 ft. deep, before it reaches the lower edge of the dewatering <br />device. <br /> <br />