2016-02-29_REVISION - C1981019 (4)

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0 Mr. Zach Trujillo February 24, 2016 Page 4 solids performance standards in both cases, although again, the sediment inflow and outflow results are different, as would be expected. In the Exh. 7-23, Part C scenario, there will be a large pile of spoil located immediately upstream of the pond in each drainage. However, a mitigating factor in this time frame is the fact that that the majority of the watershed runoff does not report directly to the ponds because much of the runoff will be intercepted by the pit. So the sediment concentration would be expected to be higher, but the total volume of inflow would be lower. Both factors affect the trap efficiency of the ponds. In the Exh. 7-23, Part I scenario, the lower portions of the watershed are largely revegetated except for the bottommost 1i of the former spoil pile's footprint. This location happens to be immediately above the ponds. The remainder of the removed spoil pile footprint will be in various states of revegetation growth by the time of this scenario. Much of the regraded pit area will also be reclaimed in this time frame. In this scenario, the pit will no longer intercept the runoff, and it all reports to the pond. For that reason, the series of upstream stock ponds was included, to accomplish the flow attenuation that was previously accomplished by the pit in the Exh. 7-23, Part C scenario. " Response: The requested text change to the introductory text has been made as noted by the Division. Furthermore, Exh. 7-23, Part I has also been revised with this submittal due to backfill progression timing changes on Map 29B, and associated hydrology timing as shown on Map 41 C. Regarding Part I and the related Map 41C, the Division has the following additional comments/questions: a. The model appears to include an error related to Structure 14. The drainage area above this channel includes an SWS of 10.67 acres, but it appears, looking at Map 41C, that it should also include the larger area of 93.58 acres. Response: The error in the West Side Model for Structure 14 (10 year and 100 year SEDCAD models) has been corrected as noted. Please note that due to the backfill plan changes a slight shift in the sub - watershed acreage reporting to Structure 13 (a "null" in the model) which reports to the West Collector Ditch (Structure 14) has occurred. The watershed was reduced from 93.58 acres to 92.33 acres. This slight reduction in the watershed is reflected in the SEDCAD model. b. Map 41C depicts the conditions in year 2041. Should the ramps be shown on the map for this time? Or should they be included but a note added to the map stating that they are left as landmarks but will not exist in 2041? Response: As discussed above in response 45a, backfill timing changes have resulted in revisions to Map 29B which also affected the timing presented Map 41C. As such, the map title and associated text changes have been made to delete all references to Year 2041. Regardless, the haul road ramps have been deleted from Map 41 C. AN EOUAL OPPORTUNITY/AFFIRMATIVE ACTION EMPLOYER A Touchstone Energy' Cooperative.