2011-09-21_REVISION - C1982056 (2)

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TWENTYMILE COAL, LLC — FOIDEL CREEK MINE TR11 -78, 16 -LEFT UTILITY BOREHOLE TECHNICAL ADEQUACY RESPONSES 1) The proposed drawing and design calculations for the 16LT proposed access road culvert, to be inserted into Exhibit 49Z, includes design assumptions that are not generally used for this area. Please address and validate the following discrepancies in the design inputs: a. Although the NRCS Storm Type Distributions are considered very conservative, the common Storm Type for the area of the Foidel Creek Mine would be a NRCS Type II. The NRCS Type I storm used in the submitted model changes the cfs design discharge used. b. The TR78 Application proposes 6 culverts to be installed along the 1 6LT access road. Designs and drawings for only two culverts were included with the application package. Please provide design details for all structures associated with drainage and sediment control. c. Structure detail was included for a trapezoidal erodible channel, however, there was no network structure included that involved the erodible channel. Please describe the use of the channel and at what point the channel is utilized in the modeling network d. It is noted that diversion ditches are to be utilized for sediment and drainage control at the 16LT pad. Please provide designs or drawings to confirm that these diversions are adequate per Rule 4.05.3(7). Response: The identified changes, responses, and additional documentation requested have been incorporated into the Exhibit 49Z materials, which are provided with this response submittal for replacement in the PAP. b. Of the 6 culverts identified, only three are designed culverts; the two culverts south of the existing 17 -Left Pad, and the culvert just north of the 16 -Left Pad. One of the provided culvert designs is for the two twinned culverts near the 17 -Left Pad, while the other is for the culvert near the 16 -Left Pad. The remaining culverts will provide for drainage passage along the light - use road, do not have any significant contributing drainage area (near the top of the sub - watershed), and will be 30 -inch diameter culverts due to standardization and clean -out considerations. Relevant permit text has been revised to document these considerations, and accompanies these responses for replacement in the PAP. c & d. The channel design provided is for the short diversion channel that runs along the west and north sides of the 16 -Left Pad. A collection ditch runs along the south and east sides of the pad and discharges through a rock -lined silt trap to the adjacent natural drainage. Although it will not drain as large a contributing drainage area, this ditch will be sized and constructed to the same specifications as the designed diversion ditch, to facilitate construction. Relevant permit text has been revised to document these considerations, and accompanies these responses for replacement in the PAP.