2021-02-17_REVISION - C1981019 - Laserfiche WebLink

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Curve numbers are selected in accordance with Table I in Exhibit 7 in Volume 2D in the approved permit as required, and the model methodology is further descried in Volume 2D, Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Evaluations, Section 1.5. The East Taylor Pond watershed in its current condition, a large portion of which the southern portion of the watershed is not reporting to the pond due mostly to the existence of the final cut of the West Pit, and other areas that have not been backfilled and graded to date. Therefore, a large volume ofsurface water flows are being contained within the final cut of the West Pit and other mining related disturbances in the southern portion of the watershed, and are not reporting to the East Taylor Pond. West Pit reclamation areas currently reporting to the East Taylor Pond are well established and the majority are Phase 11 released further indicating surface water flows from these reclamation parcels in the West Pit reclamation are being reduced by successful revegetation. Once the southern portion of the East Taylor watershed is reclaimed, the post mine condition, a much larger area that was not previously reporting to the pond will be contributing within the watershed to the East Taylor Pond that at this time is not due to the West Pit and other disturbed areas limiting flows. The Tri-State response does not completely address the Division's comment. Please explain why the worst -case condition, soon after the West Pit has been entirely reclaimed and a large part of the watershed is bare soil, is not modeled for the East Taylor Pond analysis. The Division refers Tri-State to the following language on page Exh. 7-ET-2: "The following pages present the results of the SEDCADT"I models for the worst -case hydrologic conditions under the post mining condition. At this stage the oldest reclamation is on the northern extent of the reclaimed West Pit, and the younger (topsoil and seeded) reclamation is the southern reaches of the East Taylor Pond watershed." It is our opinion that this language suggests that the southern parcels of the watershed should by modeled with a curve number higher than 62. 6. No additional response required. 7. No additional response required. 8. No additional response required. 9. Figure Exh. 7-145-2 (Section 16 watersheds) appears to have an error with the stationing for East Section 16 Ditch. The label "13+15" should be revised to "23+15." 10. The scale on Figure Exh. 7-145-2 is incorrect. Please edit this error. This very minor scale issue has been corrected as noted. The scale on Figure Exh. 7-145-2 still appears to be incorrect. Please compare the scale bar to the stations for the ditches. For example, if the scale bar is used, the distance along the East Section 16 Ditch from 10+00 to 23+15 is over 2,000 feet. That is not correct. 11. Please explain an apparent discrepancy between the Section 16 Pond As -Built (Figure Exh. 7- 145-1) and the associated SEDCAD model. The spillway perforations are at an elevation of 7746 on the Stage Storage Curve of the drawing, but there is no discharge in the Detailed Discharge Table in the model until the water reaches 7753. Tri-State found an error in the SEDCADTM model for the perforation elevations, which has been corrected. All the models for the Section 16 Pond have been corrected and resubmitted accordingly.