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Due to the steepness of drop structure FD5-1 with resulting high flow <br />• velocities, a splash apron at the outlet of FDS-1 will be constructed. The <br />splash apron will be built by placement of riprap at the outlet of FDS-1 to <br />dissipate energy and reduce the water velocity as the flow enters the sediment <br />pond. <br />The facilities sediment pond consists of a pre-existing pit which will <br />be modified to incorporate a single open channel primary/emergency spillway. <br />The spillway will be riprapped to minimize potential erosion. The <br />configuration of the pit is such that the lowest point along the perimeter of <br />the pond is at an elevation of 7812 feet. This low spot occurs in the <br />northwest corner of the impoundment. However, the location designated for the <br />spillway is at the south end of the pit where the top of pond elevation is <br />7817 feet. Although the top of pond elevation at the location of the emergency <br />spillway is not the topographic low point along the pond perimeter, placing <br />the spillway at this location requires less material to be excavated during <br />spillway construction. Additionally, this location is the closest point to a <br />natural drainage channel for releasing discharge, should it occur, and <br />minimizing the potential for erosion. <br />The total pond capacity below the 7812 elevation is 41.99 ac-ft. Since all <br />• of the pond capacity is incised below grade the pond does not meet the size <br />criteria of 30 CFR 77 .216ia). Allocation of the capacity of the pond is <br />distributed as follows: sediment storage 16.50 ac-ft, permanent pool = 21.59 <br />ac-ft, and storm runoff routing 3.9 ac-ft. This allocation of storage <br />capacity ensures that the pond will have sufficient sediment storage capacity <br />for the life of the mine. <br />The SEDCAD+ computer simulation indicates that the total runoff expected <br />from the 10-year and 25-year precipitation events are 1.36 and 1.95 ac-f t, <br />respectively. The crest of the single open channel spillway is set at the <br />elevation of 7810.5 feet. The peak stage of the pond during the 10-year, 29- <br />hour storm event is 7810.77 while the peak stage of the pond during the 25- <br />year, 24-hour storm event is 7810.89. Due to the configuration of the pond, a <br />minimum distance 1.11 feet between the peak stage of the pond and the top of <br />the pond is measured using the lowest top of pond elevation of 7812 feet <br />(northwest corner of pond). <br />• 4.6-34c May 20, 1993 <br />