2016-09-16_PERMIT FILE - M2016010

Home Browse Search

Transit Mix Concrete Co. 4. Sediment Control Structures Norwest developed a site drainage plan that covers the access road and the site facilities. For each operational phase and for the re-sloped reclamation phase the site facilities drainage changes as the mining pits develop and are contemporaneously reclaimed. This section describes the sediment control structures necessary to meet the water management design criteria for each of these periods. Figures G-6 through G-11 show the drainage plans for the different phases of operational periods of the mine, and Figure G-12 shows the final re-sloped surface. Each figure includes the network of culverts, drop culverts, terrace ditches, down drain channels, and sediment ponds. The entire drainage plan is shown for the first phase. After the first phase, only those portions that change (i.e. quarry area) for the subsequent phases are shown. The presented designs of the ditches, culverts, and sediment ponds are based on the worst case drainage conditions from the various phases. Thus, the structures will be able to function for all of the other cases. For the reclaimed phase/configuration on Figure G-12, this represents the final configuration. Between the Phase VI and the final reclaimed state, the sediment pond and associated ditches will be left in place until the reclamation standards are met. Once that occurs, then the ditches and sediment ponds will be removed and the area treated with temporary BMPs. a. Ditches The water management plan includes four types of channels: clean water diversion ditches, terrace ditches, road ditches, and down drain ditches. Clean water diversion ditches divert un- impacted runoff to existing stream channels. The terrace and down drain ditches have been designed to collect runoff from disturbed areas and convey it to sediment ponds. Road ditches will collect the runoff from the access road and convey it to a series of sediment ponds. Runoff from native areas between the clean water diversion ditches and the disturbed areas will collect in the down drain ditches and is included in the sediment-laden water volumes. i. Terrace Ditches Terrace ditches will intercept runoff from the disturbed areas, minimizing local erosion and assisting the shedding of water off the surface. The terrace ditches have been spaced to limit the excess fine stockpile slope lengths to approximately 200 feet. This slope length is typical for stockpiles in the mining industry and will help reduce the sediment volume that erodes from these areas. During the operational period, terraces are included on each of the benches at the toe of the slope. For reclamation purposes, terraces will be built into the slope as benches. The terrace ditches will be triangularly shaped with 3H:1 V and 1 OH:1 V side slopes. These ditches have gradients of approximately 1% to 2% and do not require armoring. Terrace ditches have been designed to convey runoff from the 100-year, 24-hour storm with a minimum of one-foot Hitch Rack Ranch Quarry Permit Application M-2016-010 September 15, 2016 G-14