2011-06-21_REVISION - C1982057 - Laserfiche WebLink

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Pond 016 Slide Remediation Plan TR-73 Response to Adequacy Comments-2"`i Round Page 4 22. Response previously accepted 23. Response previously accepted 24. Response previously accepted 25. The Division asked SCC to clarify whether the two stock tanks were primary sediment control structures and, if they were, to provide design information. SCC responded that the structures are not primacy sediment control structures, and directed the Division to closely review the SEDCAD design. SCC maintains that these structures are an integral part of the drainage system in that they provide some runoff control by slowing the velocity of runoff at that point. SCC is correct; these do provide velocity control as they are currently constructed. Since SCC is not committing to maintaining these structures in the current configuration or providing design information, however, SCC must demonstrate that Pond 016 is adequately sized to treat the runoff from a 10 year124-hour event if the stock tanks are silted in (null structures). Please revise the SEDCAD design to designate the stock tanks as null structures. In addition, SCC should submit an additional SEDCAD run for the 25 year/24-hour event to verify the adequacy of the emergency spillway channel sizing, and updated design (sizing) information for all of the postmining drainage channels and culverts in the Pond 016 watershed that are affected by the TR-73 revised drainage plan. The design submitted for Pond 016 includes the two stock tanks (T-26 and T-27) as sediment control features. The previously approved language in the permit application package, on page 11 of Tab 13, indicates that the stock tanks are not primary sediment control structures. If SCC intends to rely on these structures to control runoff and sedimentation, the language of Tab 13 will need to be revised accordingly. Also, SCC will need to submit detailed design information, as required by Rule 4.05.6(5), indicating that Stock Tanks T-26 and T-27 will comply with Rules 4.05.6 and 4.05.9. Response: A revised SEDCAD analysis is attached designating the two stock ponds as null structures. The original emergency spillway was constructed to handle 134.2 cfs and the new 25-year/24- hour SEDCAD analysis (attached) indicates that the emergency spillway only needs to handle 59.5 cfs. The difference in the spillway size requirement is due to the conditions that the spillway was designed for. The original design and as-built condition were based on worst-case totally disturbed conditions. The remediation plan conditions are based on the majority of the area being revegetated; therefore the existing emergency spillway as constructed is adequate. The Stock Tanks, as designed and built, have been approved in the permit and do not appear to require further detailed design information until they are requested as permanent structures as Seneca Coal Company • P.O. Box 670 • Hayden, Colorado 81639 Telephone (970) 276-5219 • FAX (970) 276-5222