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3. The reclaimed drainage calculations are based on vegetated channel conditions. When the drainages aze initially constructed, vegetation will not be. established. The Division noted this concern in previous modeling performed for channels at the Seneca Ii-W and Yoast Mines. Seneca should design the drainage channels based on the erodible channel design methods used in SEDCAD+, and provide a plan for temporary stabilization of the drainages until vegetation has established. 4. Seneca used a curve number (CN) of 71 for all of the reconstructed drainage channels. According to Table 13-1 in the Seneca II-W permit application, a CN of 71 is for a revegetated area with 30-70% cover. All of the drainages revised by Seneca will be constructed when the area is reclaimed and topsoiled. Table 13-1 lists a CN for topsoil replaced, with 0-30% cover, between the range of 76 to 84 (depending on the applicable hydrologic soil group). The Division believes that the CN range of 76 to 84 is more applicable for the conditions at the Seneca II-W mine. Please revise the channel designs accordingly. 5. Seneca used a time of concentration (TC) land flow condition of 3 (short grass pasture) and 6 (grassed waterway) in the SEDCAD+ channel designs. These numbers seem to represent a flow path along a reclaimed slope and channel. The designs of the channels should be based on a newly reclaimed surface and a drainage channel. Please revise the TC factors accordingly. 6. Seneca did not provide a peak runoff calculation for the 006 Gulch watershed. 006 Gulch will route runoff from channel segments 006-E1, 006-E2 and the undisturbed drainage. Seneca states that the channel will be designed for the 100-year, 24-hour storm event. The Division agrees that 006 Gulch should be designed for the 100-year, 24-hour event due to the large watershed size (715.8 acres) that flows across backfilled material in Gulch 006. Please provide the Division with peak runoff flow calculations for 006 Gulch based on the 100-year, 24-hour storm event. Please also revise the design of the channel based on the peak flow. For each drainage channel, Seneca designed a vegetated and arip-rap lining design to be used depending on the slope of the ditch. Please inform the Division as to how the rip- rap sections will be keyed in place between the transitions of vegetation to rip-rap. It would also be helpful if these transition azeas were identified on the map. 8. Channel 006-E2 is a new segment proposed by Seneca. The upper end of the channel is placed on a relatively flat slope with little drainage definition. It is not apparent how runoff will flow into the channel based on the uniform topography. Seneca should revise the postmining contours in this azea to create a more defined channel in the upper segment of channel 006-E2. Seneca revised the channel gradient of 006-E1 to be steeper in a number of sections than