2016-05-03_PERMIT FILE - C1981008 (12)

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sections will be at least as hydraulically capable and erosion resistant as the modeled template described above for the modest runoff flows described below. Pond Construction Pond 015 a subgrade impoundment. The details can be observed on Map 1. The pond floor is at approximate elevation 5559.0. Normal pool for evaluating containment of the 100 year storm has been assumed to be 3 feet above this, at elevation 5562.0 to allow for accumulation of sediment over the pond's expected service life. Runoff Calculation Results The results of the runoff calculations are presented in the attached SEDCAD model printout. For sedimentation accumulation estimates, the system was modeled using SEDCAD for its response to a 10 year, 24 hour storm of 2.0 inches. For 100 year containment evaluation, the 100 year, 24 hour storm of 3.0 inches was used. For continuity, the results of the modeling are presented below along with the discussion of the physical dimensions and properties of the pond. The volume of the below grade pond was input into the SEDCAD model. The model watershed was input with its acreage, flow response parameters (slope, distance, time of concentration) and the specific runoff curve number from the above referenced table. The 10 year, 24 hour storm was then applied to the composite watershed, and routed down to the sediment pond. The pond was assumed to have 3 feet of water or sediment in the bottom to an elevation of 5562.0 at the start of the 10 year, 24 hour event. It can be observed in the SEDCAD printout that the storm generates runoff of 0.129 ac ft, and does not discharge in the 10 year event. The SEDCAD model does not calculate a trap efficiency in cases where there is 100% containment, but trap efficiency is obviously 100% in this case. It should be noted that the 10 year event was modeled not as a performance consideration, but only to provide an estimate of annual sediment generation, as discussed below The 100 year, 24 hour storm of 3.0 inches was modeled to evaluate Pond 015's ability to completely contain the 100 year event. Again, it was assumed the starting pool would be at elevation 5562, three feet above the pond floor, which could be either impounded water or retained sediment, or both. The 100 year storm generates 0.268 acre feet of runoff, raises the pool to elevation 5565.81 by interpolation on the stage -storage curve in the as -built drawing (SEDCAD does not provide a peak pool elevation when the inflow is entirely contained). Thus, inflow from the 100 year event peaks about 4.69 feet below the top of the embankment. 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 (10.0) tons. The net 10.0 tons from a single 10 year , 24 hour storm is converted to annual tons via the method described in the SEDCAD Users Manual to 1.1 tons. This weight is then converted to volume, using a density of 78 lb/cu ft, to arrive at Attachment 2.05.3(3)-33 Revision Date: 1/08/2016 Page 3 Revision No.: TR -70