2012-05-30_PERMIT FILE - C1981012A (2)

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Ditch 29 collects runoff from 67.2 acres of undisturbed runoff and directs this runoff to the lay down area without impacting any disturbed area. Culvert C -27 has been installed at the east end of the lay down area to handle peak flows from Drainage Basins CC and DD as shown on Map 13A— Drainage Basins. Ditches 39a and 39aa are proposed to route stormwater around the cut slope of the substation and rock dust system to maintain the integrity of the slope. At the time of construction, the slope material characteristics will be evaluated to determine if a drop structure is necessary to carry the cut slope runoff. The details of the cut slope are included as Figure 2d. Sedimentation control at the Bates Portal is shown on Map 13 -A Sediment and Surface Water Control Silo Area. Six new ditches and four new culverts and the design size and routing of each structure is provided in Exhibit 19 (16). Runoff from the 10 -year, 24 -hour event at the Portal will be routed to Pond 007. The C & W train shop, 10 stall garage and other associated structures are located on the west side of the east mine sedimentation control system. The land slopes approximate 2% from the C & W train shop to the east towards pond 7. Flow from this general area will be collected in either ditch D -5 or D -6 and conveyed to sediment pond 7 and can be found on Map -13C, West Side of the East Mine - Sedimentation Control Plan. A small area exemption has been established for subsoil storage pile #1 due to its proposed location outside of areas where disturbed runoff will be routed to a sediment pond. Alternative sedimentation control for this area is provided by berms and silt fences. The design of the silt fences are included Exhibit 19(16). This SAE area is shown on Maps 11 and 12. The proposed DWDA stormwater management above, around and on the waste pile was evaluated for a 100 year, 24 -hour storm event. The results indicate that the management system already in place for the site will safely pass the storm with the recommendation that the berms and ditches are field verified to be constructed as designed. The stormwater routing near the DWDA is shown on Figure 2e. The runoff above (North) is diverted through Ditch D -11 through culvert C -30, ditch D -1, culvert C -26, and ditch D -32 to the river. This is shown in Figure 2e. Exhibit 39 Sediment Design Specification – Development Waste Disposal Area demonstrates that the existing ditches are appropriately sized to carry the stormwater above the DWDA. The DWDA runoff is routed through a small catchment into culvert C -13 and to pond 007. The engineering evaluation of the DWDA is included as Exhibit 39 Sediment Design Specification – Development Waste Disposal Area. As part of TR -60 the DWDA was expanded to the west in order to gain more storage area for development waste. The drainage area evaluated for TR -60 is 5.1 acres and includes the DWDA, and a small portion of the haul north of the pile that will flow into Ditch D -12. The 100 year, 24 -hour storm precipitation is 4.2 inches. SedCad calculates the peak flow to be 15.58 cfs with 1.47 acre -feet of total runoff. The required ditch size to convey that flow is a trapezoidal channel of bottom width 3.0 ft, height of 1.5 ft, side slopes of 1:1 and a minimum slope of 1 %. Existing ditches are of this recommended size and shape. The minimum pipe diameter needed to convey the 100 year, 24 -hour peak flow to pond 007 is 24 inches. The existing culvert is 24 inches. The pipe installation should have a minimum of 1.2 feet of material from the top of the pipe to the top of the bank (i.e. the lowest point on any top of berm/top of bank around the DWDA shall be no less than 1.2 feet above the top of the 24 inch outlet pipe. A small area exemption has been established for a material storage area and associated activities located south of sediment pond 7 due to its location outside of an area where runoff will be routed to a sediment pond. Alternative sedimentation control for this area is provided by berms and a silt fence. The design of the silt fence is included Exhibit 19(17). This SAE area is shown on Map 11 Sheets 1, 2 and 3. The cut slopes above the upper diversion ditch at the Bates Portal fall under Rule 4.05.2(4) which states in part; for the purposes of 4.05.2, "disturbed area" shall not include those areas in which only diversion ditches sedimentation ponds, other treatment facilities, or roads are installed in accordance with the performance standard requirements of this Rule. The cut slopes will be covered with a straw blanket and seeded since there is concern the slopes may contribute suspended solids to stream flow or runoff outside the permit area. The straw blanket will be maintained until vegetation is established. TR -60 Page 2.05 -10 Revised 01/09/12