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Pond Construction <br /> Pond 013 was designed and constructed as a largely subgrade impoundment with a low <br /> (approximately 5 feet high) embankment along the northwestern end across a natural Swale. The <br /> details can be observed on Map 2.05.3(3)-32. The primary, or "service" spillway consists of an <br /> 24-inch perforated riser connecting to an 24-inch CMP under the above described low <br /> embankment. The overflow lip is at elevation 5555.0. There is a pair of 6-inch diameter valves in <br /> the riser with invert at elevation 5552.0, which sets the maximum normal pool at this level. The <br /> pond is normally operated as a "pass through" design with both valves left in the open condition. <br /> In addition, there are three parallel 18-inch diameter 20-foot long horizontal corrugated metal <br /> pipes with upstream invert elevation 5557.0 which serve as an emergency spillway system. <br /> Pond 013 embankment compaction results and safety calculations are also presented in <br /> Appendix 1 and 2 of this attachment. <br /> Inflow Calculation Results <br /> The results of the runoff calculations and synthesized constant inflow are presented in the <br /> attached SEDCAD model outputs. The system was modeled using SEDCAD for its response to a <br /> 10 year, 24 hour storm of 2.0 inches for the sediment control analysis, and the 25 year, 24 hour <br /> storm of 2.4 inches for the spillway evaluation. In both cases, a constant base case inflow of <br /> 1,000 gpm was also included. For continuity, the results of these models are presented below <br /> along with the discussion of the physical dimensions and properties of the pond. <br /> The volume of the pond was inputted into the SEDCAD model, along with its spillway details. <br /> The model watersheds were inputted as a series of sub-watersheds, each with its own acreage, its <br /> own flow response parameters (slope, distance, time of concentration), and the specific runoff <br /> Curve Numbers from Table 2.05.3(3)-l. The 10 year, 24 hour storm was then applied to the <br /> composite watershed, and routed down to the sediment pond. SEDCAD allows the user to <br /> override the customary starting pool that is set by the lowest drain hole. In the case of the 10 year <br /> storm, where the starting pool is normally set by the twin 6-inch valves with invert at elevation <br /> 5552.0, the starting pool at the start of the design storm needs to be 1.43 feet higher, or elevation <br /> 5553.43 to account for the 1,000 gpm flowing through the two orifices at the start of the storm. It <br /> can be observed in the SEDCAD outputs that the storm and fixed inflow generate runoff of 6.12 <br /> ac ft of inflow and that the pool rises to 5554.32 feet during the routing of this event. The <br /> SEDCAD results show the pond has a trap efficiency of 65.7%, and it releases water with a 24- <br /> hour weighted settleable solids concentration of 0.14 ml/l, well below the 0.50 ml/l standard. <br /> The 25 year, 24 hour storm has also been modeled as the spillway design event. For this analysis, <br /> it has been conservatively assumed that the twin 6-inch orifice holes are closed, and normal <br /> starting pool is the principal spillway elevation of 5555.0. This must be raised 0.23 feet to <br /> account for the 1,000 gpm over the riser lip. The starting pool for the 25 year spillway design <br /> storm was thus set to elevation 5555.23. The runoff combined with the constant inflow produce a <br /> total runoff volume of 9.14 ac-ft. In the attached SEDCAD printout for that storm it can be <br /> observed that the pool only rises to elevation. 5556.36, or 1.36 feet above the riser pipe overflow. <br /> As such, the routed 25 year storm does not raise the pool high enough to engage the three <br /> horizontal CMPs at El 5557.0, which are the emergency spillway. <br /> Attachment 2.05.3(3)-32 Revision Date: July 2020 <br /> Page 3 Revision No.: TR-99 <br />