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Table 1. Drainage Culverts <br />~ Culvert * Design Size (in.) Installed Size(in.) <br />C1 existing county road culvert <br />C2 24 24 <br />C3 30 30 <br />C4 24 not yet constructed + <br />C5 24 24 <br />C7 42 42 <br />C8 15 15 <br />C9 15 24 <br />* see Mine Grading Plan for locations <br />+ located in area of site not yet constructed <br />POND A <br />Pond A, the storm water detention and sedimentation pond, is <br />located downgradient of the Coal Ridge #1 Mine surface <br />facilities. The drainage ditches and culverts, as reviewed <br />above, direct runoff from storms and snowmelt upto and including <br />the 10-year, 24-hour design storm and sediment to the pond. The <br />sediment capacity is for 2 years of erosion from the fully <br />• developed site. <br />The pond was permitted with a required capacity of 7.5 acre feet <br />(AF) of storage. Sediment storage capacity amounts to 4.1 AF of <br />two years of erosion from the fully exposed site and 3.1 AF of <br />storm water from the design storm. The total permitted volume <br />also includes 0.3 AF of "gray" water storage. Calculations of <br />the constructed capacity of the pond, based upon field <br />measurements of length, width and depth, indicate the actual <br />capacity to be 9.9 AF. Therefore, there is an additional 2.4 AF <br />of extra capacity for detention and sediment storage. The actual <br />capacity is 1308 of the required and permitted volume. This <br />additional 308 capacity provides greater detention and sediment <br />accumulation capacity and hence provides extra protection. <br />The detention pond spillway consists of a 15 feet wide concrete <br />channel with a sharp crest provided by a metal plate bolted to <br />a sill block extending across the spillway. The maximum possible <br />head of water above the sharp crested plate is 1.25 feet. The <br />spillway is designed to pass the 25-year peak runoff rate of 77 <br />cubic feet per second (CFS). The constructed configuration of <br />the spillway would allow a maximum discharge rate of 85 CFS <br />(Reference 1). The additional 308 of detention volume and 108 <br />additional spillway capacity combine to provide an increased <br />level of protection to the storm drainage control system. <br />Pond A was designed and constructed as having two cells for the <br />sediment storage. This concept was chosen as a practical <br />solution to facilitate sediment cleanout rather than a <br />2 <br />