2006-02-13_REVISION - M1984044 - Laserfiche WebLink

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Knight Piesold CONSULTING ' 2.0 Design Summary 2.1 Sediment Dam P5 Design Sediment dam PS was originally intended to be constructed to the east of the East Pit Access Road of the American Gypsum Mine. It was later decided that the dam would be more effective ' if placed southeast of the East Pit Access Road. The new dam is located at the coordinates N13,875 and E16,475 at an elevation of approximately 6,666 feet. The sediment dam is designed to route the water flow associated with the 100-year/24-hour ' storm event of 2.8 inches. The dam design includes the construction of a perforated riser and emergency spillway. A Sediment, Erosion and Dischazge by Computer Aided Design (SEDCAD) analysis was used to determine the size of the dam, riser, spillway, and stilling basin. The contributing watershed in the azea is approximately 74 acres with a peak discharge of 44.6 cubic feet per second (cfs) and a runoff volume of 10.5 acre-feet (ac-ft). To sufficiently handle ' the water flow associated with the storm event, a dam with a peak capacity of 4.15 ac-fr is required. Given the topography at the chosen location of the dam, the required storage area I creates a dam height of 12 feet (an elevation of 6,678 feet) and a width of 15 feet. A spillway with a bottom width of 10 feet will be constructed at elevation 6,676 feet, 2 feet below the dam crest. ' A corrugated steel pipe (CSP) riser, perforated with small diameter holes, regulates the water ' flow to achieve the desired detention time for storm events smaller than the 100-year/24-hour storm event. A graveljacket and wire mesh screens aze used to prevent clogging. A perforated riser with a pipe diameter of 18 inches and a height of 5 feet is required to regulate the water flow. An outlet CSP with a diameter of 18 inches will connect to the riser pipe and run under the dam to transport the water to the stilling pond located to the south of sediment dam P5. A concrete pad poured level to the ground surface will be located under the elbow where the perforated riser pipe and CSP outlet join. The gravel jacket surrounding the perforated riser pipe ' will be sloped to 1.7 (H):1 (V) (horizontal vertical). A stilling basin is also included in the design of the sediment dam as the water moving through the spillway contains a large amount of energy. This energy may cause excessive erosion at the outlet which will compromise the structural stability of the embankment material of the sediment dam and undermine the spillway. The stilling basin will absorb and dissipate the energy from the spillway discharge. The plunge pool stilling basin is constructed underneath the outlet of the G \101100100.02NW eportsV(P1Design Reporl.Ox 2 January ?0. 2006