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2. COMMENT The downslope erosiove potential of outflow from the primary and emergency spillways has not been addressed by the operator. The outslope of Pond ;94 is steep and has been mapped by the operator's consultant as landslide debris. As such, the outslope will be subject to erosion and possible movement if suf- ficient measures are not taken by the operator to channel any outflow from the pond down the slope. No calculations are provided to show that the 6-inch pipe proposed is sufficient to carry discharge from both spillways. Therefore, the applicant must submit to the division calculations and design for the following: a. Individual outflow volume from the primary and emergency spillways during a 25-year, 24-hour storm event. b. Provide the design whereby discharge from the emergency spillway will be routed into discharge from the primary spillway with adequate energy dissipators, drop structures, or other means to prevent erosion at out- flow point. c. Provide a design to carry combined outflow of both spillways downslope in a channelized flow to Grassy Creek. 2. RESPONSE Hydrographs were developed for both the 10-year and 25-year, 24-hour storms and routed through the sediment pond. Peak outflows from both the principal spillway and dewatering orifice were determined. A pipe size was calculated to convey the peak discharge from either storm down the slope to Grassy Creek thereby reducing the erosion potential. The calculations are attached and spe- cific points addressed below. a. Total volume of runoff for 25-year, 24-hour storm event = 9.9 acre-feet. The available storage above the 1-year sediment accumulation to the crest of the emergency spillway = 12.5 acre-feet. Therefore, the total outflow volume from principal spillway = 9.9 acre-feet. Total outflow volume from emergency spillway = 0 acre-feet. b. There will be no discharge from the emergency spillway during the 25- year, 24-hour event. All discharges will be from the principal spillway. c. An 8-inch diameter corrugated plastic pipe will convey the discharge from the 25-year, 24-hour storm to Grassy Creek. Peak discharge = 4.3 cfs with maximum velocity = 12 fps. As indicated in response l.d., riprap will be placed at the discharge point to Grassy Creek to reduce erosion.