2009-10-16_PERMIT FILE - M2009082 (27)

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Inflow and Outflow Structures In order to mine within 400 feet of the Cache la Poudre River, the Division of Reclamation Mining and Safety (DRMS) requires the construction of engineered inflow and outflow structures. A single inflow/outflow structure will be required for each of the following mining phases: 1C, 3, 4B, 4C, 5A, 5B, and 5C. Each inflow/outflow structure will be a native grass-lined trapezoidal channel excavated to a depth of two feet with 4:1 (horizontal: vertical) side slopes. Because flow can be bi-directional, these channels are to be constructed with no longitudinal slope. Each channel is approximately 200 feet in length with the exception of the channel for Phase 4B which is approximately 300 feet in length. The bottom width of the trapezoidal channel will vary from 20 to 100 feet depending on the size of the phase. Phases 1C, 3, 5A, and 5C require a bottom width of 20 feet while Phases 4B and 5B require a bottom width of 50 feet and Phase 4C requires a bottom width of 100 feet. The rate of inflow from the river will vary as the river stage rises above the invert of the inflow/outflow structure based on HEC- RAS modeling of the inflow/outflow structures. For the 200-ft channel with a bottom width of 20 feet, the inflow would be 30 cfs, 60 cfs, and 120 cfs at river stages of 1.0 feet, 1.5 feet, and 2.0 feet above the channel invert, respectively. The 200-ft channel with a bottom width of 50 feet would convey inflows of 60 cfs, 140 cfs, and 270 cfs at similar stages of 1.0 feet, 1.5 feet, and 2.0 feet, respectively. For the 300-ft channel with a bottom width of 50 feet, these inflows would decrease slightly to 50 cfs, 120 cfs, and 220 cfs at similar stages of 1.0 feet, 1.5 feet, and 2.0 feet, respectively. The inflow through the 200- ft channel with a bottom width of 100 feet would be 120 cfs, 280 cfs, and 510 cfs at stages of 1.0 feet, 1.5 feet, and 2.0 feet respectively. At these inflow rates, the cells will fill from the normal water line to the invert of the inflow/outflow channel in approximately 5 to 14 hours at a river stage of 1.0 foot above the channel invert. Fill time decreases to 3 to 6 hours with the stage at 1.5 feet above the channel invert and further decreases to 1 to 3 hours at 2.0 feet above the channel invert. Based on the 2003 floodplain study of the Cache la Poudre River by the Corps of Engineers, these inflow/outflow channels will convey flood flows into the cells more frequent that a 10-year event and possibly more frequent than a 5-year event. Inflows to the mined cells will be conveyed through the grass-lined channels at non-erosive velocities less than 5 feet per second. In the vicinity of the mined cell, higher velocities would be expected as the inflow runs down the 4:1 side slope of the reclaimed cell. Slope protection such as a turf reinforcement mat or an articulating concrete block revetment is recommended to protect the rundown portion of these reclaimed slopes. Final design of this slope protection will be prepared prior to construction. P10138 Lafarge\0138.013 Parsons Permit\DRMS Exhibds\Parsons Stabil¢y Exhib@.doc Parsons Mine DRMS 112 Permit Page SE8 of SE8