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pool elevation of 5606.11. The SEDCAD results show the pond has a trap efficiency of 95%, <br />and it releases water with a 24-hour weighted settleable solids concentration of 0.00 ml/1, well <br />below the 0.5 ml/1 standard. <br />The 25 year, 24 hour storm has also been modeled as the spillway design event. It is <br />conservatively assumed that both valves are closed and that the starting pool is the overflow <br />elevation 5611.0. It produces a total runoff volume of 0.38 ac -ft. In the attached SEDCAD <br />printout for that storm it can be observed that the pool only rises to elevation 5611.08, or 0.08 <br />feet above the overflow lip on the riser. This minimal storm response is due to the now <br />relatively low runoff curve numbers in combination with a relatively large sediment pond for the <br />remaining tributary watershed in its current condition. <br />SEDCAD model allows for an estimate of the annual sediment collected in the pond. The <br />methodology is described on page 64 of the SEDCAD User's Manual, and requires an Annual R <br />factor (rainfall -erosivity factor). The value of R at 27 can be interpolated from Figure 5.3 <br />presented in Barfield, Warner and Haan (1981). In addition the total tonnage of sediment from <br />the modeled storm, in this case 10 year, 24 hour storm, is required. This is obtained from the <br />SEDCAD output by subtracting the sediment tonnage into the null below the pond (0.0 tons) <br />from the tons flowing into the pond (0.8) tons. The net 0.8 tons from a single 10 year , 24 hour <br />storm is converted to annual tons via the method described in the SEDCAD Users Manual to 1.1 <br />tons. This weight is then converted to volume, using a density of 78 lb/cu ft, to arrive at the <br />projected annual volume collected in the pond of 0.0007 ac -ft. Thus, over a period of 10 years <br />approximately 0.007 ac -ft of sediment would be accumulated in the pond bottom with the <br />watershed in its current revegetated condition. This is small enough to not significantly affect <br />the trap efficiency of this pond. <br />The SEDCAD printout shows the 10 year, 24 hour storm flows in Ditches C7 and C8 to be <br />respectively 0.20 and 0.30 cfs. Both ditches flow at a velocity less than 1.0 fps, well below the <br />allowable velocity of 7 fps for the grass lined channels at these conditions. <br />Attachment 2.05.3(3)-31 Revision Date: 1/08/2016 <br />Page 4 Revision No.: TR -70 <br />