2016-05-03_PERMIT FILE - C1981008 (10)

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Other System Components Two existing ditches route parts of the affected area runoff to the pond. As shown on Map 1, Pond 012 Area, the C7 Ditch collects the westernmost portion of the area and the C8 Ditch collects portions of the eastern parts of the drainage area. Both ditches are modeled as vegetative lined trapezoidal ditches with a 4 -foot bottom and 2H:1 V sideslopes, and with gradients determined from field surveys, as depicted on Map 1. Several cross sections are also shown on Map 1. The actual side slopes and bottom width may vary in the field, and the ditches may have a flatter side slope or a wider bottom in some locations. Some reaches may be cut in rock. As such, the actual ditch sections will be at least as hydraulically capable and erosion resistant as the modeled template described above for the modest runoff flows described below. Also presented on Map 1, in the easternmost sub -watershed that flows into Ditch C-8, are a series of rip -rap lined erosion control measures. These are provided to prevent localized erosion on the now completed reclaimed north facing slopes. The rip -rap sizing and channel depth requirements shown on Map 1 are based on the watershed area's response to the 10 year, 24 hour storm with Curve Number conservatively set at 74, notwithstanding the vegetation is now mature enough to justify a Curve Number of 62. The four SEDCAD models associated with these channels are presented on pages 25 though 56 in this section. Pond Construction Pond 012 has been designed and constructed as a largely subgrade impoundment with a low (approximately 9 foot high) embankment about 40 feet long across a natural swale on the north side. The details can be observed on Map 1. The primary, or "service" spillway consists of an 18 -inch perforated riser connecting to an 18 - inch CMP conduit under the above described low embankment. The overflow lip is at elevation 5611.0. There are a pair of 4 -inch diameter holes fitted with gate valves in the riser at elevation 5606.0. The valves are normally open, which sets the maximum normal pool at this level. The pond floor is at approximate elevation 5605.1. Runoff Calculation Results The results of the runoff calculations are presented in the attached SEDCAD model printout. For sedimentation evaluation, the system was modeled using SEDCAD for its response to a 10 year, 24 hour storm of 2.0 inches, and the 25 year, 24 hour storm of 2.4 inches for the spillway evaluation. 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, 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 number from the above 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 be full to the elevation of the open gate valves on the drop inlet riser pipe (elevation 5606.0) at the start of the storm. It can be observed in the SEDCAD printout that the storm generates runoff of 0.175 ac ft, and the pool rises only 0.11 feet during the routing of this event, to a peak Attachment 2.05.3(3)-31 Revision Date: 1/08/2016 Page 3 Revision No.: TR -70