2013-09-26_REVISION - C1996083 - Laserfiche WebLink

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Page 2 of 3 DRMS: BRL has revised the paragraph to indicate that the pond would be cleaned out when sediment was at or below the maximum level. BRL has, however, retained the previously proposed statement that the ability to remove sediment is contingent upon weather conditions that will allow the pond to be dewatered and sediment to be effectively dried before transporting it back to the gob pile. The contingency language suggests that there may be accumulation of sediment above the maximum storage level during inclement weather periods. As stated in the Division's August 27, 2013 concern letter, we recognize that weather conditions will determine when sediment is cleaned out, and that BRL will determine how best to schedule sediment removal from the pond as long as the requirements of Rule 4.05.6(3)(b) are met. The Division cannot, however, approve a maintenance plan that would allow sediment or water to accumulate beyond the maximum allowable level and affect the ability of the pond to treat runoff from the design event. The contingency language in the third paragraph of page 10 must be omitted or revised to make clear that regardless of weather conditions, the maximum sediment and water levels will not be exceeded at any time. 11. DRMS: The Subwatershed Hydrology Detail table is provided on page 17 of Appendix B. There are three subwatersheds that should be assigned a curve number of 90 given the current and anticipated pile expansion configuration. Please revise SWS2 for Structure #36 (Ditch F17), SWS1 for Structure #8 (Ditch F5), and SWS1 for Structure #18 (Ditch F14) to a curve number of 90. BRL: The SWS for ditch F -17 is correct. There is a small drainage are that is no coal mine waste flowing in to ditch F17 from the area in front (north) of the topsoil stockpile. The large drainage area that is currently a CN +90 is from the Gob pile. The CN for Ditch 5 has been changed to 90. Please seethe revised pages App. B -17 and 36. The curve number for Ditch 14 is correct. It will receive flow from the gob pile, but its drainage area is from undisturbed areas. DRMS: The copy of page App. B -17 that BRL submitted does not reflect a curve number of 90 for Ditch 5 (Structure 8) as indicated by BRL, and is inconsistent with the runoff volume and peak discharge shown on page App. B -36 and other pages of the design information. Please provide a revised Page App. 817 using a curve number of 90 for the Ditch 5 subwatershed. When the curve number is revised, other portions of the design will be affected (this change appears to have been carried through on the 100 yr run only). Please also revise page App. a -6 through B -9 (Pond sizing, sediment storage, and discharge structure sizing), a -13 (Sedcad structure summary for the 10 yr — 24 hr event), a -18 - B -19 (complete subwatershed hydrology detail for the 10 yr — 24 hr event), 8 -23 (structure summary for the 25 yr - 24 hr event), and 8 -26 through 8 -29 (Sedcad outputs for the 100 yr — 24 hr event). 13. DRMS: The Structure Summary table from the Sedcad run for the 100 -yr event was not included in the submittal. Please provide the Structure Summary table so that we can verify the discharge values used to size the ditches and culverts. BRL: Please see new page 26. DRMS: After review of the structure summary, it is apparent that the following structures will need to be redesigned to reflect the peak discharge values on revised page App. a -26: Ditch F4 (peak flow from page App. 8 -26 is 38.17 cfs, design is based on 37.75 cfs), Ditch F -7 (peak is 37.95 cfs, design is 37.43 cfs), Ditch F -7a (peak is 37.95 cfs, design is 37.43 cfs), Ditch F -14 (peak is 11.73 cfs, design is 11.22 cfs), and Culvert F2 (peak is 34.01 cfs, design is 33.50 cfs).