2016-11-02_PERMIT FILE - C1982057A

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All topsoil at the stock tank sites will be salvaged prior to construction; the topsail will be used to reclaim the embankment and disturbed areas above the peak storage elevation. Spoil material will be used for construction of the embankments. The spoil material will be placed in controlled lifts and compacted. Attachment 13-11 presents the stability analysis SCC has used for the stock tank designs at the Seneca II-W mine, The SLOPEW computer model will be used to complete a force limit stability analysis. A Morgenstern Price analysis will be used to estimate the lowest safety factor for the embankment. A worst case evaluation will be examined using the same embankment geometry characteristics and material strength properties used in generic stock tank stability analysis conducted for the existing stock tanks. The worst case scenario, using the generic assumptions presented above, estimate a safety factor of 1.56 using the limit equilibrium analysis. This safety factor value is greater than the 1.3 safety factor value required by Rule 4.05.9(8-(b). This indicates that a stock tank with an embankment design limited in size to the generic geometry characteristics above will present minimal or no risk to human health and safety. The hydrologic and hydraulic designs were calculated using SEDCAD4 computer model. The SEDCAD4 modeling methodology is summarized in Attachment 13-58, Drainage and Sediment Control Plan II-W South Extension. Results of the SEDCAD4 analyses are presented in Attachment 13-11 A, Stock Tank T-25, T-26 and T-27 Design. Design drawings for the three stock tanks are included in the permit as: • Exhibit 13-27.T25 • Exhibit 13-27.T26 • Exhibit 13-27.T27 Discharae Structures Discharge from sedimentation ponds, diversions and pit dewatering will be controlled by energy dissipaters, riprap channels, and other means or devices, where necessary, to reduce erosion, to prevent deepening or enlargement of stream cfianneis, and to minimize disturbance of the hydrologic balance. Discharge structures will be designed according to standard engineering design procedures. Drainaae Channels As part of the mining activity in the North Area, SCC mined through two unnamed watersheds in the headwaters of Dry Creek. The northem watershed, named the 005 Gulch, drains the northern most coal recovery area and is an ephemeral tributary to Dry Creek. The southern watershed, named the . 006 Gulch, drains the southertt part of the North Area, and is an ephemeral tributary to Hubberson Gulch. Exhibit 13-2 shows the two North Area watersheds and the existing drainage network. PR-05 12 Revised 01/06