2016-05-03_PERMIT FILE - C1981008A (17)

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overflow lip is at elevation 5555.0. There is a pair of 6 -inch diameter valves in the riser with invert at elevation 5552.0, which sets the maximum normal pool at this level. The pond is normally operated as a "pass through" design with both valves left in the open condition. In addition, there are three parallel 18 -inch diameter 20 -foot long horizontal corrugated metal pipes with upstream invert elevation 5557.0 which serve as an emergency spillway system. Inflow Calculation Results The results of the runoff calculations and synthesized constant inflow are presented in the attached SEDCAD model printouts. The system was modeled using SEDCAD for its response to a 10 year, 24 hour storm of 2.0 inches for the sediment control analysis, and the 25 year, 24 hour storm of 2.4 inches for the spillway evaluation. In both cases, a constant base case inflow of 1,000 gpm was also included. For continuity, the results of these models 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, along with its spillway details. The model watersheds were input as a series of sub -watersheds, each with its own acreage, its own flow response parameters (slope, distance, time of concentration), and the specific runoff Curve Numbers from Table 2.05.3(3)-lA. The 10 year, 24 hour storm was then applied to the composite watershed, and routed down to the sediment pond. SEDCAD allows the user to override the customary starting pool that is set by the lowest drain hole. In the case of the 10 year storm, where the starting pool is normally set by the twin 6 -inch valves with invert at elevation 5552.0, the starting pool at the start of the design storm needs to be 1.43 feet higher, or elevation 5553.43 to account for the 1,000 gpm already flowing through the two orifices at the start of the storm. It can be observed in the SEDCAD printout that the storm and fixed inflow generate runoff of 6.12 ac ft of inflow and that the pool rises to 5554.32 feet during the routing of this event. The SEDCAD results show the pond has a trap efficiency of 66.8°/x, and it releases water with a 24-hour weighted settleable solids concentration of 0.11 ml/1, well below the 0.50 ml/1 standard. The 25 year, 24 hour storm has also been modeled as the spillway design event. For this analysis, it has been conservatively assumed that the twin 6 -inch orifice holes are closed, and normal starting pool is the principal spillway elevation of 5555.0. This must be raised 0.23 feet to account for the 1,000 gpm over the riser lip, treated as a sharp -crested weir. The starting pool for the 25 year spillway design storm was thus set to elevation 5555.23. The runoff combined with the constant inflow produce a total runoff volume of 9.14 ac -ft. In the attached SEDCAD printout for that storm it can be observed that the pool only rises to elevation. 5556.10, or 1.10 feet above the riser pipe overflow. As such, the routed 25 year storm does not raise the pool high enough to engage the three horizontal CMPs at El 5557.0, which are the emergency spillway. The following table summarizes the storm response flow. Pond 013 SEDCAD Results - With Fixed Inflow Attachment 2.05.3(3)-32 Revision Date: 3/09/2016 Page 4 Revision No.: TR -70 SEDCAD Peak Case Q;,,, cfs Pool, ft SS mUl out 10 yr, 24 hr sedimentation storm 34.19 5554.32 0.11 25 yr, 24 hr spillway design storm 48.06 5556.10 n/a Attachment 2.05.3(3)-32 Revision Date: 3/09/2016 Page 4 Revision No.: TR -70