2009-04-29_PERMIT FILE - C1982056 (3)

Home Browse Search

r] *Total contributing watershed at Pond 2 25-year, 24-hour storm event (in) 2.1 CNdisturbed 85 CNundisturbed 81 K (Soil erodibility factor) 0.1 Cdisturbed 0.45 Cundisturbed 0.003 Ch dromulched 0.05 Scenario 2: After Construction After construction is complete, the disturbed area within the watershed will decrease from 4.0 ac to 2.4 ac. The reduction in disturbed area occurs when the cuttings pits are constructed because they will collect runoff without discharging. In addition, the completed Shaft Pad will be surfaced with a coarse gravel and the Shaft Pad outslopes will be hydromulched. The gravel surfacing is expected to produce a greater volume of runoff than would the disturbed condition, but the relatively coarse particle size distribution of the gravel will produce runoff with a significantly lower sediment concentration. After construction of the Shaft Pad installation is complete, silt fence will no longer be necessary to pre-treat runoff before it enters Pond 1. The following values were applied to the Scenario 2 modeling parameters (Table 2). Table 2. Scenario 2 SEDCAD Inputs • Parameter Value Watershed Area (ac)* 5.9 total, 2.4 disturbed 10-year, 24-hour storm event (in) 1.8 25-year, 24-hour storm event (in) 2.1 CNShaft Pad 88 CNundisturbed/Shaft Pad outslo a 81 K (Soil erodibili factor) 0.1 CShaft Pad 0.05 Cundisturbed 0.003 Ch dromulched 0.05 *Total contributing watershed at Pond 1 The following pond elevation-area-capacity data were input into Pond 1 in SEDCAD (Table 3): Table 3. Pond 1 Elevation-Area-Capacity Elevation (ft amsl) Area (ac) Capacity (ac-ft) References 6862 .000 .000 Bottom of and 6863 .088 .029 6864 .106 .126 6865 .126 .242 6866 .147 .378 6867 .170 .536 6868 .194 .718 Emergency spillway 6869 .220 .924 6870 .247 1.158 6871 .276 1.419 Twentytmile Coal Company 18L Shaft Pad 4 Water & Earth Technologies, Inc. Drainage and Sediment Control System Design March 2, 2009