2016-05-03_PERMIT FILE - C1981008 (8)

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SEDCAD model allows for an estimate of the annual sediment collected in the pond. The methodology is described on page 64 of the SEDCAD User's Manual, and requires an Annual R factor (rainfall -erosivity factor). The value of R at 27 can be interpolated from Figure 5.3 presented in Barfield, Warner and Haan (1981). In addition the total tonnage of sediment from the modeled storm, in this case 10 year, 24 hour storm, is required. This is obtained from the SEDCAD output by subtracting the sediment tonnage into the null below the pond (0.0 tons) from the tons flowing into the pond (37.7) tons. The net 37.7 tons from a single 10 year , 24 hour storm is converted to annual tons via the method described in the SEDCAD Users Manual to 54.0 tons. This weight is then converted to volume, using a density of 781b/cu ft, to arrive at the projected annual volume collected in the pond of 0.0318 ac -ft. This indicates that the above described assumption of a starting pool at elevation 5614.0 for the 100 year containment evaluation is reasonable. A spillway evaluation is not necessary because the pond completely contains the runoff from the 100 year event. The SEDCAD printout shows the 10 year, 24 hour storm flows in the steepest reach (4.6%) of the Tailwater Ditch along the north boundary to be 0.32 cfs, which flows at a velocity of 0.83 fps, well within the allowable limit for a grass lined ditch. In the postmine channel below the Tailwater Ditch, which is modeled as a "cobbles and shingles" erodible ditch, the flow is 9.0 cfs , at a velocity of 3.4 fps, which is below the allowable 5 fps for this channel lining. The construction will be under the direction of a professional engineer and upon completion of construction, an as -built drawing and construction report will be completed by a professional engineer. Attachment 2.05.3(3)-29 Revision Date: 1/08/2016 Page 4 Revision No.: TR -70