Laserfiche WebLink
,, <br />David Berry -14- June 20, 1996 <br />analysis was conducted again using a curve number of 83, as <br />suggested by DMG, for all natural watershed sub-areas <br />including A3b, A3a, A4a, and A4b. The original modeling <br />scenario was a fully constructed coal mine waste bank without <br />reclamation. This scenario was used to provide a conservative <br />result for hydrology and sedimentology analysis. However, <br />this scenario is not representative of activities that would <br />be performed at this site. Another approach was taken to more <br />accurately model an actual condition at the waste bank and to <br />include runoff curve numbers and degrees of reclamation that <br />are more appropriate. <br />The waste bank was modeled in the conservative state of <br />complete construction, with reclamation up to the second bench <br />level below the crest. The runoff curve number for the <br />disposal area was kept at 90, and the reclaimed portion of the <br />waste pile was 80. The results of this recalculation show <br />that the existing sediment pond is adequate under this <br />approach. Calculated runoff and sediment volumes were <br />slightly lower in this waste bank configuration, as compared <br />to the original, and therefore, the existing sediment pond and <br />outlet works were adjusted accordingly. These results are <br />provided in updated Appendix B. <br />59. The construction of the underdrain for the coal waste pile <br />does not mention the use of a filter blanket over the rock <br />underdrain, to add further protection from clogging. Please <br />comment on this. <br />As indicated on page 7 of Volume IV, the drain material <br />gradation was designed to naturally filter fine material from <br />the waste pile to prevent clogging without the use of a <br />filter. Filter fabric is not required unless the gradation <br />requirements for the drain material cannot be met during <br />construction. <br />60. As requested by the DMG, the coal waste pile topsoil pile and <br />coverfill stockpile were added to revised Map 20, <br />Sedimentation Control Plan, and to revised Map 21-1, Drainage <br />Plans. Enclosed are revised Maps 20 and 21-1. <br />61. On page 2.04-36, it is stated that mining will not impact a <br />surface source of water. The DMG requests a comment on the <br />possibility of subsidence impacting water flow in Terror Creek <br />and Hubbard Creek. Please refer to Map No. 27 Subsidence. <br />The projected extent of subsidence does not come close to <br />either Terror Creek or Hubbard Creek. Therefore, these creeks <br />were not specifically mentioned on page 2.04-36. Section <br />2.05.6(6)(a) states in part "Hubbard Creek is located <br />immediately east and northeast of the Permit Area. Terror <br />Creek is located west of the proposed permit area. Neither of <br />these perennial streams are considered to be adjacent to the <br />