2016-08-08_PERMIT FILE - C1981019A (115)

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sediment. They will be constructed as each stock pond site reaches final PMT, and will be constructed according to the with design requirements shown on Figure 2.05-6 in Volume 1. Their nominal locations are shown on Map 41 C (Volume 22). As an aid in understanding the function of the six stock ponds, Exh 7-23 Figure I2 "East Side SEDCAD Networking Schematic Diagram" has been provided. This shows in schematic form the network of the entire East Side System in a format more user friendly than that provided in the SEDCAD computer output. Also added are the runoff volumes in and out of the several sediment ponds, including the six stock tanks. It can be observed that each stock tank captures and retains about 2.8 ac ft of runoff, thereby reducing the direct load to the Section 25 Sediment Pond during the passage of the 10 year 24 hour storm. The entire length of D-5 Ditch will be removed when the top of the remaining spoil pile has dropped below the ditch invert, which is the condition addressed in this time frame. Drainage will at this point either flow via re-established tributaries into re-established Little Collom Gulch, or will flow directly into the Section 36 Pond and Middle Pond in Badger Draw. The two ditches D-3 and D-4 that convey runoff from the north face of the temporary spoil pile will remain in service through this period. These two ditches are relatively steep trapezoidal rip -rap lined ditches, with bottom width 12 feet, sideslopes 2H:1 V, and gradients of 12.7% and 10.4% for D-3 and D-4, respectively. To evaluate this condition in this East Sediment Control System, two extremely complex SEDCAD models have been required, which are presented here. The following SEDCAD models accompany this section: Eastside End of Mine Life Model, 10 Year 24 Hour Event, and Eastside End of Mine Life Model, 100 Year 24 Hour Event Both are discussed in more detail below. This modeling is included to demonstrate that upon removal of the final lift, i.e. the "valley plug" discussed above, the sediment control system consisting of the three existing sediment ponds augmented by the six "string-of-pearls" small stock ponds will continue to meet the effluent standards previously demonstrated for the active mining condition in Part C. Ultimately all of the runoff in the eastern system passes through Section 25 Pond. The first SEDCAD analysis shows that with this upstream system of stock ponds, Section 36 Pond and Middle Pond in place, and the entire eastern watershed reporting, the 10 year 24 hour storm surge pool peaks below the invert of the emergency spillway (Elevation 6873.13 as compared to the emergency spillway invert Elevation 6875.00), and the 24-hour weighted settleable solids will be 0.00 ml/1 (versus allowable 0.5 ml/1). When delivering the 100 year spillway design storm, the pool peaks at Elevation 6876.92, or about 1.92 feet deep in the emergency spillway. The following Items accompany this submittal: • Exh. 7-23I-1 through 154. Exh. 7-231-4 Revision Date: 2/15/16 Revision No.: PR -04