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Carol Pahlke December 23, 1980 Page 13 (unvegetated) condition. However, the coal mining site is in the acid west where annual rainfall is on the order of 9 to 12 inches. After the snowmelt season, ditches will flow only in response to occasional rainfall, and only if the rainfall is significant enough to produce excess runoff. This condition may or may not be exceeded during the period required to establish the vegetated lining. LRCWE. recommends that ditches be constructed as early as possible after the snowmelt season. This will give the maximum amount of time for establishing the vegetated lining for contin- uous flow the following snowmelt season. During the time required to establish the vegetated lining, LRCWE suggests that any erosion that might occur be considered as a 'maintenance item' by xerr Coal Company. Kerr Coal Company could correct any erosion prob- lem, filling and reseeding, if necessary, as part of their maintenance program. As pointed out on Plate 1 of Exhibit 29, the diversion ditches were designed for the 10-year, 24-hour event or the 100-year, 24- hour event. According to the OSM regulations, a minimum design for temporary diversion is the 2-year, 24-hour event. LRCWE would further suggest that the minimum level of design is the AMC-II condition; LRCWE in its design used the AMC-III condition, which is more conservative. LRCWE believes that enough conservatism has been incorporated into the design of the ditches, that the risk of erosion during the period required for establishment of the vege- tative lining is acceptable. Kerr Coal Company should argue against further conservatism requested by MLR. The conservative design requested by MLR would require a larger ditch cross section and a flatter channel slope. This would result in greater excavation costs and greater costs for drop structures. LRCWE believes that the incremental cost is not justified when the risks are considered."