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February 1999 Yoast Mine " 13-11 Drainage and Sediment Control Plan ~ 5 TABlE 13-72.4 PRACTICE FACTOR SurFace Surface Type Practice Prectice Factor Undisturbed Varies None t.0 Disturbed Topsoil Stripped/Spoil Rough and Irregular T.0/0,8 3.6 SEDIMENT PARTICLE DISTRIBUTIQN A sediment particle size distribution was input into the SEDCAD4 computer model. The distribution was obtained from previous analyses presented in the Seneca mine permit and is shown on the SEDCAD4 printouu. 4.0 PONDS All ponds are designed to effectively treat the runoff produced from the 10-year, 24-hour storm event. The ponds were designed utilizing the SEDCAD4 computer model. The input parameters and resuIu aze shown on the SEDCAD4 printouts located in Appendix 13-12.1, Sediment Ponds SEDCAD4 Results, and summarized in Table 13-12.5, Yoast Mine Sedimentation Pond Summary. . The location of the ponds and their corresponding wazersheds are shown on Exhibit 13-12.1, Yoast Mine Watershed A simplistic approach was taken. in identifying distwbed and ttadisnrrbed areas contributing runoff to each pond -All azeas which could possibly be disturbed during the life of the project were identified as either pit/pre-strip or spoil, with curve numbers of 41 attd.66, respectively. , Areas of concurrent reclamation or in-pit containment of runoff were not taken into account. This approach resulted in a conservative design for each pond: Pond and embankment configurations for Ponds 012, -023 aad 014 can be found on Exhibits1312:2,. Proposed Pond 012 Design, 13-12.3, Proposed Pond 013 Design, and 1112.4, Proposed P,oad 014Design; respectively. - ,;.r All ponds Gave principal and emergency spillways. The principal spillways for ponds 013 and 014 consists of a S foot tall, cmp riser, with an accompanying cmp outlet and are designed to safely pass the peak flow from the IO-yeaz, 24-hour storm event, without discharging from the emergency spillway. Pond 022 also has a 5 foot tall cmp riser, however, the riser is perforated. The inlet to the printpal spillway will contain a trash rack- The emergency spillways consists of a trapezoidal channel with 3H:1V side slopes. The outslope of the spillways will be armored. The emergency spillways are designed to safely pass the peak flow from the 2Syear, 24-hour storm event while maintaining 1 foot of freeboard. The spillways aze conservatively designed assuming the principal spillway is clogged and the water level is at the invert Level of the spillway prior to the stone event. Refer to Exhibits 13-12.2, 13-12.3, aad 13-12.4 for spillway designs. All ponds will have a volume of less than 20 ac-ft as measured from the invert of the emergency spillway co the original ground surface. Pond 12 is incised, which accounu for iu total contained volume exceeding 20 ac-h. (refer to Exhibit 13-12.2, Proposed Pond 012 Design) The embankments for all ponds will be constructed such that the height from the upstream toe (original ground) to the emergency spillway invert will be 20 feet or -ess. The embankments will be constructed with 3H:IV slopes and a top width oI ]5 feet- All embankment crests and outslopes wi[l be vegetated with the approved temporary seed mix- A stability analysis Eor Pond 12 has been performed and is ateached ro this submittal- Refer to rSrtachment i3-13, Segmentation Pond 12 Stability Analysis- The Pond 12 embankment will be constructed and certified as described in the stability analysis report. ,Wontgo.+tery Wauan Af~nrng Group " 165 $. Unton $wdrvrnJ, Satre 460 ' I okrwood, Colo+ndo 802..>B "/_30 vJ 16 ~ t 1 n0