2016-09-16_PERMIT FILE - M2016010

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%P Transit Mix Concrete Co. b. Sediment Detention Basins Down drain ditches discharge to sediment detention basins, and these basins are designed to contain runoff from the 100-yr, 24-hr storm event. Where possible, sediment detention basins are located within the pit and other disturbance boundaries to minimize disturbances. Norwest determined runoff volumes using the same design criteria (curve numbers and routing) as the ditches. A key element of the stormwater management plan is maintaining a high available sediment capacity in the sediment detention basins, and Norwest designed the basins to have minimum capacities of the runoff volume plus 20% for sediment detention. The basins will require periodic maintenance when sediment reaches this sediment level. The calculated runoff volumes and recommended capacities are included in Table G-5. The basins will typically be 10 feet deep, but the depths will vary depending on the necessary capacities and area available for the pond. It is not anticipated that the height of the embankments will require permits for the Colorado Office of the State Engineer. If conditions will require a higher height of embankment, the proper permits from the State Engineer will be obtained. The sediment detention basins are planned to be excavated into the pit floor at a location that is away from the pit walls and mine infrastructure. Inside the pit areas, where the detention basins will be constructed into bedrock, the sediment detention basins walls will be sloped at 0.5H:1 V. The design includes an access ramp sloped at 10% to provide access for maintenance equipment. Sediment detention basins located in unconsolidated materials will be excavated with walls sloped at 3H:1 V. El Paso County requires installing a forebay to collect large sediment, and the forebays will have capacities of approximately 10% of the storm runoff volume. Outlet works that drain the sediment detention basins but retain solids are required by El Paso County and will be installed at each sediment detention basin. The outlet works will cause basin water levels will be drawn down following storm events and will discharge to nearby drainages. The outlet pipe will be controlled by an orifice plate at the pipe inlet and will be sized to drain the 100-year storage volume in less than 72 hours. Sediment detention basin NP-SP3, that captures water from the Deadman Creek drainage, will be pumped down within 72 hours using a floating barge. The pump equipment will be configured with a float switch that will start the pump when the level in the detention basin reaches a given level and turn it off when it drops to the bottom of the water storage level. The pump will be sized to allow a maximum rate of flow of 450 gpm which is sufficient to evacuate the anticipated storage volume within 72 hours. During the operational period, the water will be pumped back to Deadman Creek; for reclamation, the areas within the pit will be backfilled and the pit walls be breached or notched to establish positive drainage into Deadman Creek. A typical design for the sediment detention basins is shown in Figure G-14, and the SEDCAD design reports are included in Attachment G-1. Detention Basin P-DET-1 is shown as the worst-case with the greatest drainage area. This basin shifts from its initial location about 800 feet south at the beginning of Phase 2. Thus, the drainage area decreases and that change is reflected in the Figure G-6a and G-7 through G-10. Hitch Rack Ranch Quarry Permit Application M-2016-010 September 15, 2016 G-17