Laserfiche WebLink
rapped portions of Coal Canyon. The drop structures were revised to <br />use a D50 riprap size of 30" on a slope of 4h:ly. According to <br />Design of Sediment Control Measures for Small Areas in Surface Coal <br />Mining, critical factors should be considered in the design; <br />* a short transition channel upstream of the structure to protect <br />the channel from scour form the high approach velocities. <br />* an apron must be provided downstream of the structure to <br />prevent scour form the surging action and turbulence below the <br />drop. <br />* a general rule of thumb is the length of the upstream <br />transition channel and downstream apron should be at least 6 <br />times the flow depth of the channel. <br />The Division believes the drop structure designs should include a <br />riprap transition upstream and downstream of the structures to be at <br />least 6 times the flow depth of the channel. Please revise the <br />permit pages and maps accordingly. <br />102. In the cover letter, Powderhorn states that there is no easy way to <br />divert the runoff around the Roadside Refuse Pile. However, it is <br />known that a small Swale exists between the refuse pile and the <br />natural ground surface. It appears that this Swale has been <br />diverting the runoff around the pile for a number of years, with .no <br />indication of excessive erosion. Powderhorn may want to consider <br />utilizing this Swale as the 100-year diversion ditch, especially <br />since the estimated runoff is less than 1 cfs. The Division would <br />need designs for this diversion as well as it being located on a <br />map. Also, since the diversion is in contact with the refuse pile, <br />Powderhorn would need to ensure that the pile is protected against <br />potential scouring, by vegetation or other protective material, as <br />appropriate. <br />103. Powderhorn included roadway drainage designs in the application. We <br />have the following comments regarding the road designs; <br />a. There appear to be no designs for the roads leading to the 2W <br />Portal area and various substations within the permit boundary. <br />Please provide the Division with drainage designs for these <br />roads. <br />b. The roadway drainage acreage calculated by the Division for <br />Haul Roads 2, 4, 5 & 6 do not match the operator's estimate. <br />The following discrepancies were noted and should be addressed <br />by the operator; <br />Roadway Drainage along Haul Road 4 by CBA 2; Page 13-1-43 <br />AREA PEABODY DMG <br />111 0.40 0.58 <br />121 0.42 0.56 <br />