Laserfiche WebLink
Felnrrary 7999 YoartMin< *73-11 Drainage and Sediment Control Plan f5 <br /> <br />TABLE 73-12.4 <br />PRACTICE FACTOR <br />Surface Surface 7 e Practice Practice Factor <br />Undisturbed Varies None 1.0 <br />Disturbed Topsoil Stripped/Spoil Rough and Irregular 1.0/0.8 <br />3.6 SEDIMENT PARTICLE DISTRIBUTION <br />A sediment particle size distribution was input into the SEDCAD4 computer model. The distribution was <br />obtained from previous analyses presented in the Seneca mine permit and is shown on the SEDCAD4 <br />printouts. <br />4.0 PONDS <br />All ponds are designed to effectively neat the mnoff produced from the 10-year, 24-hour storm event. The <br />ponds were designed utilizing the SEDCAD4 computer model. The input parameters and results aze shown <br />on the SEDCAD4 printouts located in Appendix 13-12.1, Sediment Ponds SEDCAD4 Results, and <br />summarized in Table 13-12.5, Yoast Mine Sedimentation Pond Summary. <br />The location of the ponds and their corresponding watersheds aze shown on Exhibit 13-12.1, Yoast Mine <br />Watershed. A simplistic approach was taken in identifying disturbed and undisturbed azeas contributing <br />mnoff to each pond. All areas which could possibly be disturbed during the life of the projec[ were <br />identified as either pit/pre-strip or spoil, with curve numbers of 91 and 86, respectively. Areas of concurrent <br />reclamation or in-pit containment of mnoff were not taken into account. This approach resulted in a <br />conservative design for each pond. Pond and embankment configurations for Ponds 012, 013 and 014 can <br />be found on Exhibitsl3-12.2, Proposed Pond 012 Design, 13-12.3, Proposed Pond 013 Design, and 13-12.4, <br />Proposed Pond 014 Design, respectively. <br />• <br />All ponds have principal and emergency spillways. The principal spillways Eor ponds 013 and 014 consists <br />of a 5 foot tall, cmp riser, with an accompanying cmp outlet and aze designed to safely pass the peak flow <br />Eiom the 10-year, 24-hour storm event, without discharging from the emergency spillway. Pond 012 also <br />has a 5 foot tall cmp riser, however, the riser is perforated. The inlet to the principal spillway will contain a <br />trash rack. The emergency spillways consist of a trapezoidal channel with 3H:1V side slopes. The outslope <br />of the spillways will be armored. The emergency spillways aze designed to safely pass the peak flow from the <br />25-year, 24-hour storm event while maintaining 1 foot of fieeboazd. The spillways are conservatively <br />designed assuming the principal spillway is clogged and the wares level is at the invert level of the spillway <br />prior to the storm even[. Refer to Exhibits 13-12.2, 13-12.3, and 13-12.4 for spillway designs. <br />All ponds will have a volume of less than 20 ac-ft as measured from [he invert of the emergency spillway to <br />the original ground surface. Pond 12 is incised, which accounts for its total contained volume exceeding 20 <br />ac-ft. (refer to Exhibit 13-12.2, Proposed Pond 012 Design} <br />The embankments for all ponds will be constructed such that the height from the upstream toe (original <br />ground) to the emergency spillway invert will be 20 feet or less. The embankments will be cons[ructed with <br />3H:IV slopes and a top width of 15 feet. All embankment crests and outslopes will be vegetated with the <br />approved temporary seed mix. A stability analysis Eor Pond 12 has been performed and is attached to this <br />submittal. Refer to Attachment 13-13, Sedimentation Pond 12 Stability Analysis. The Pond 12 <br />embankment will be cons[ructed and certified as described in the stability analysis report. <br />Montgomery [P/ahon Mining Group * 165 S Union BoulevarA, Suite 460 * Lakemoad, Colorado 80718 * (303J 763-5140 <br />