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2.0 Design Analysis <br />• Portal Sump #1 (PS -1): PS -1 will be an excavated pit approximately 15 feet deep with a top surface <br />area of approximately 0.05 acres. The top elevation will be at 6,675 feet. The total capacity at the <br />top elevation is 0.4 ac -ft. A minimum of two (2) 18 -inch corrugated metal pipe (CMP) straight <br />barrels will be installed to convey water from PS -1 to Portal Sump #2 (PS -2). A MSHA safety berm <br />will be installed along the sump margins that abut the pit access road and pit floor. The berm will <br />provide additional freeboard; however, berm elevations and areas were not included in the design <br />calculations. <br />The PS -1 sump design assumed a permanent pool elevation of 6,673 feet, which equates to a <br />permanent pool volume of 0.3 acre -feet. The SEDCAD modeling calculated a 10 -year, 24 -hour <br />event runoff volume into PS -1 of 0.5 ac -ft, with a peak water elevation of 6,674.0 feet. <br />Portal Sump #2 (PS -2): PS -2 will be an excavated pit approximately 15 feet deep with a top surface <br />area of approximately 0.23 acres. The top elevation will be at 6,675 feet. The total capacity at the <br />top elevation is 2.2 ac -ft. The sump design assumed a permanent pool elevation of 6,669.2 feet, <br />which equates to a permanent pool volume of 1.0 ac -ft — this means there would be approximately <br />1.2 ac -ft of storage available. The SEDCAD modeling calculated a 10 -year, 24 -hour event runoff <br />volume into PS -2 of 0.8 ac -ft, with a peak water elevation of 6,673.1 feet. A sensitivity analysis <br />looking at the 25 -year, 24 -hour event runoff indicates a peak water elevation of 6,674.6 feet. This <br />shows that if the permanent pool elevation is maintained at 6,669.2 feet, there is sufficient capacity to <br />• store the calculated runoff from a storm less than or equal to the 25 -year, 24 -hour event. A MSHA <br />safety berm will be constructed along the top of the excavation where is abuts the pit access road and <br />pit floor. The berm will provide additional freeboard; however, berm elevations and areas were not <br />included in the design calculations. <br />Groundwater calculations indicate that the mine working will generate a maximum inflow of <br />approximately 75 gpm. Assuming a conservative mine water discharge rate of 80 gpm, <br />approximately 0.35 ac -ft of water could be discharged from the mine workings into PS -2 daily. With <br />a total capacity of 2.2 ac -ft, assuming no stormwater runoff inputs, there approximately 6.2 of mine <br />water storage in the PS -2 — or 3.4 days of storage with a permanent pool elevation of 6,669.2 feet. <br />SEDCAD4 requires that a pond design include a spillway structure so it can complete the calculation <br />iterations. Consequently, a principal spillway was input as a place holder into the PS -2 design. The <br />invert elevation for the "ghost" spillway was set at 6,669.0 feet (the top of the exaction). Artificially <br />small dimensions for the pipe diameter, barrel slope and an extremely long barrel length create a zero <br />(0) discharge scenario but assure that the calculations can be completed. <br />Surface runoff from the pit floor and ramp road will be routed to PS -2 through a series of short <br />culverts (or cuts) through the MSHA berm. The culverts (cuts) will safely convey runoff from a 10- <br />• year, 24 -hour event. The calculated peak flow is 3.2 cubic feet per second. Assuming a maximum <br />headwater depth of 0.5 feet, the minimum pipe diameter for a single pipe is 48- inches. To assure <br />there is sufficient carrying capacity to convey runoff through the MSHA berm, either seven (7) 18- <br />2.05.3 -E2.F2 12/09 <br />