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<br />• resistant bedrock in many areas and is armored with large sandstone rocks and boulders <br />from mass-wasting of adjacent steep slopes), and well established, heavy shrub cover. <br />34. The two particle size distributions used by OCM in determining the required sediment capacity <br />of the Pond C include particles larger than those that would be entrained in sheet flow. <br />Typically, the largest particles entrained in rill or sheet flow are sand sized, up to 2. S mm. The <br />handbook for the SEDCAD+T"' mode[, which was used for OCM's submittal, indicates that the <br />input for particle size distribution is the erodible particle size. The model does not account for <br />erosion caused by channelized flows, which would be expected to entrain particles larger than <br />sand. The particle size distribution used for Pond B is appropriate. Please provide a similar <br />particle size distribution for the Pond C watershed. <br />Responses: The particle size distribution originally utilized by WESTEC in the <br />sedimentology calculations for Pond C has been replaced with the particle size distribution <br />utilized for the Pond B calculations, and the sedimentology calculations for Pond C have <br />been re-run. Please see accompanying revisions. <br />35. Map OXWVF-03 shows Ditch D•]0 at a 3% slope, and references noss-section B/03. Cross- <br />section B on OXWVF-03 show two ditches which do not resemble the description of D-10 in the <br />design (triangular with 1:1 side slopes). Also, the design for D-10 indicates the slope for the ditch <br />is 0.01 %. Please make the necessary corrections so that the design and illustrations are consistent. <br />Responses: Ditch D-10 is unnecessary and has been eliminated as indicated by the revised <br />• drainage calculations provided for insertion in Appendix B and Map 3211, Drainage and <br />Sediment Control Plan. Please see accompanying revisions. <br />36. The designs of the refuse pile collection ditches specify rip rap lining with a 23" DsP OCM <br />proposes to reduce the Dso to 19" and augment with grout. Grout can degrade causing loss of <br />integrity of the rip rap installation. Please provide additional information which further <br />explains the 4"reduction being offset by grout. A similar proposal has been made for the upland <br />diversion at the refuse pile. This also should be further explained <br />Response: Ditch designs have been reviewed and revised to minimize riprap requirements <br />where possible. In certain steep-slope segments, lazge diameter riprap is indicated by the <br />design calculations and will be utilized. In other areas where flow velocities are not <br />excessive, smaller diameter riprap and regulaz channel maintenance will be utilized to assure <br />that ditches continue to perform as designed without excessive erosion. Oxbow has <br />utilized this approach for existing drainage channels with good success. Please see <br />accompanying revisions. <br />37. 7Tie permit application indicates discharge from Pond C will be routed to an existing 18"culvert. <br />Should the 100-year, 24-hour precipitation event occur, would be culvert safely convey al[ water <br />discharged from the refuse area? If not, it would appear that overflow from the pond might <br />damage the railroad track adjacent to the spillway. Whife it is not a regulatory requirement that <br />OCM size the culvert for the 100-year, 24-hour event, it might be additional insurance for OCM <br />in preventing damage to the track. <br />• Response: Under a 100-year storm scenario, elevated runoff flows would occur from all <br />hillslopes and from the normally dry ephemeral drainages to the area, many of which drain <br />to or across the existing railroad grade. Given the topographic and drainage configuration <br />of the azea, it is probably impractical and unrealistic to try to accommodate major storm <br />flows except where specifically required by applicable regulatory requirements. The <br />