PERMFILE136432 - Laserfiche WebLink

Home Browse Search

• SEDIMENTATION POND HYDRAULICS AND DESIGN GENERAL The proposed sedimentation ponds are designed to detain the inflow from runoff produced by the 10-year, 24-hour precipitation event and runoff produced by groundwater pumping to allow removal of sediment particles. Groundwater will be pumped to the ponds at the approximate rate of 0.28 cubic feet per second. The ponds are designed to contain the three-year sediment level in addition to the runoff volume. The ponds range in size from 3.03 to 11.63 acre-feet with none of the ponds exceeding 20 acre-feet. The embankments range 1n height from 6.2 to 9.7 feet measured from the upstream toe to the top of the design embankment. Pond embankments will be constructed to allow for five (5) percent settlement. Embankment crests and slopes of all proposed ponds will be protected with vegetative cover. Sideslopes are specified at 3H:1V for both upstream and downstream slopes. The Proposed Pond and Ditch Design Map. (Map 23-1), contains a pond schedule with information describing the five (5) ponds. This map also contains a cross-section illustrating the typical embankment configuration. Figures 23-14. through 23-18, Pond Plan Views. illustrate the pond configurations. All proposed ponds utilize a 24-inch riser pipe with an 18-inch conduit for an emergency spillway, and one (1) to three (3) orifices located dust above the three-year sediment level for a principle sp111May. The riser is designed to adequately pass runoff from the 25-year. 24-hour storm plus runoff from groundwater pumping walntaining one (1) foot of freeboard between the top of the 25-year pond level and the top of the embankment. The orifices limp outflow to such an extent that the entire 10-year, 24-hoc:• event is assumed to be stored, allowing the ponds to be dewatered in four (4) to six (6) days. Flood routing through the ponds assumes an Initial water surface at the required sediment storage level. This elevation corresponds to the 23-121