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• The emergency spillway flowing 10-feet wide and 6-inches deep has a capacity of 16.76 cfs as shown page 5. The emergency spillway ie therefore sized to pass the 8.62 cfs run-off generated by the 25-year 24- hour precipitation event. The sediment pond will collect and contain the run-off from drainage areas 1 through 7. Two new 12" diameter culverts will be installed ae shown on Map 14 to direct the run-off from the upper areas to the sediment pond. Dugout Pond The dugout pond as is implied by its name, was constructed by excavating in the natural ground surface, therefore the dugout pond does not have designed embankments. The dugout pond must be capable of containing the run-off generated by the 10 year 24 hour precipitation event from area 8. The run-off calculations presented on page 9 show that the dugout pond must contain 0.01 acre-feet. The sketch on page 11 shows that the dugout pond does have adequate capacity to contain the run-off from area 6. The dugout pond has a combination, open channel, primary and emergency spillway. The spillway is sized to handle the 0.16 CFS flow generated by the 25-year 29-hour event as presented on page 10a. The spillway is sized on page 6a to handle the design flow and a section view of the spillway is presented on the sketch on page 11. • Ditches Two collection ditches are sized on pages 5a and 6. Ditch C collects flow on the east side of the haul road and the north side of the gravel road that leads to the bathhouse shop area. Ditch C is a triangular channel, a minimum of 1 foot deep with 1.5h:1v side slopes. New twelve inch (12") diameter culverts (CMP #1 b #2) will be installed where this ditch crosses the roads. Ditch B is the collection ditch located on the north edge of the coal pad. This ditch collects drainage from nearly the entire site. Ditch B will be a trapezoidal channel, a minimum of 1 foot deep, 1.6 feet wide at the bottom with 1.Sh:ly side slopes. Eighteen inch (18") diameter culverts (CMP #3 b #4) are installed where ditch B crosses the roads. Diversion ditch A will be left in place. The culvert that previously conveyed the flow from diversion ditch A under the haul road to the west undisturbed area will be plugged. According to the Handbook of Steel Drainage b Highway Construction Products the maximum flow that 12" and 18" diameter culverts can handle without headwater is 2 cfs and 5.5 cfs respectively. With only 1 inch of headwater depth, an 18" diameter culvert can handle 6 cfs. • -9- 6/03/93