2016-05-03_PERMIT FILE - C1981008 (9)

Home Browse Search

Other System Components Several existing ditches route parts of the affected area runoff to the pond. As shown on Map 1, for the areas west of 2700 Road, C 15 Ditch collects the westernmost portion of the area and C23 Ditch collects portions of the eastern parts of the drainage area. Runoff from the pasture across (east of) 2700 Road are collected in the CI Ditch, IP Tailwater Ditch 1, IP Tailwater Ditch 2, and C2 Ditch, and then report to Pond 009 via a culvert system under the road. All ditches are modeled as vegetative lined trapezoidal ditches with a bottom width, side slopes and 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. Culvert C141 is a 10" HDPE pipe located near the inlet of Pond 009 as shown on Map 2.05.3(3)-1. Culvert C142 is a 6" HDPE irrigation pipe which conveys surface runoff from the IP Tailwater Ditch 2 to Ditch Cl. Finally, culvert C143 is a 4" HDPE irrigation pipe which conveys surface runoff from the IP Tailwater Ditch 1 to Ditch Cl. The design for each of these culverts is included in the SEDCAD model for the 10 year, 24 hour precipitation event. Pond Construction Pond 009 has been designed and constructed as a largely subgrade impoundment with a low (approximately 6 foot high) embankment across a natural swale along the northern end. The details can be observed on Map 1. The spillway consists of an 18 -inch perforated riser connecting to an 18 -inch CMP under the above described low embankment. The overflow lip is at elevation 5651.5. There is a pair of 4 - inch diameter holes in the riser at elevation 5647.5 fitted with 4 -inch gate valves, normally left open. This sets the maximum normal pool at elevation 5647.5. The pond floor is at approximate elevation 5646. 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 holes on the drop inlet riser pipe (elevation 5647.5) at the start of the storm. It can be observed in the SEDCAD printout that the storm generates runoff of 0.46 ac ft of runoff and that the pool rises only 0.29 feet during the routing of this event to a peak pool elevation 5647.79. The SEDCAD results show the pond has a trap efficiency of 92%, and it Attachment 2.05.3(3)-30 Revision Date: 3/14/2016 Page 3 Revision No.: TR -70