2011-04-08_PERMIT FILE - C1981012A (7)

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Ditch 27 will direct runoff from Warehouse 1 to Containment Area 2. The total drainage area is 2.9 acres. A total of 0.28 acre -feet of storage is required to handle the 10 -year 24 -hour event and 0.38 • acre -feet of storage is required to handle a 25 -year 24 -hour event. The three -year sediment volume is 0.02 acre -feet. Ditch 28 will direct disturbed runoff from a 15.45 -acre disturbed area to Containment Area 1. A total of 1.51 acre -feet of storage is required to handle a 10 -year 24 -hour event and 2.01 acre - feet for a 25 -year 24 -hour event. The three -year sediment storage volume is 0.07 acre -feet. Containment Area 1 also controls drainage from the non -coal waste disposal area. Ditch 29 collects runoff from 67.2 acres of undisturbed runoff and directs this runoff to the laydown area without impacting any disturbed area. Culvert C -27 has been installed at the east end of the laydown area to handle peak flows from Drainage Basins CC and DD as shown on Map 13 -A Sediment and Surface Water Control Silo Area. 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. 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. The catchment area is 4.09 acres and is isolated from other catchments by existing diversion ditches and grade breaks. The 100 year, 24 -hour storm precipitation is 4.2 inches. The model predicts the peak runoff to be 12.76 inches with 4.2 inches of 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. • TR 55 Page 2.05 -10 03/11