2018-01-03_PERMIT FILE - C1994082 (12)

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and Postmining Cross -Sections. An examination of the cross-sections indicates that the postmining conditions are similar to the premining conditions. The above information demonstrates compliance with the requirements of 4.14.2. These postmining slopes approximate the general nature of the premining topography as demonstrated in 2.10.3(1). Postmining Drainage Channels All drainages that are to be disturbed by mining activities will be reclaimed to approximate the premining configuration and to blend with the undisturbed drainage above and below the area to be reclaimed. The postmining drainage channels will be designed, constructed, and maintained to the following specifications: 1. Average stream gradient shall exhibit a concave longitudinal profile to the extent possible 2. Remain in dynamic equilibrium with the drainage basin system without artificial structures, except where necessary to eliminate or reduce erosion 3. Provide separation of flow between adjacent drainages 4. Safely pass the runoff from a 10 -year, 24-hour precipitation event (100 -year, 24-hour precipitation event for drainages larger than one square mile (Rule 4.05.4 (2)(b)) 5. Designed for long-term stability of the landscape 6. The reconstructed drainage channels will eliminate or reduce the severity of nickpoints to the extent possible A summary of the pre- and postmining drainage characteristics is shown on Table 20-2, Pre- and Postmining Drainage Summary. The locations of the postmine channels are shown on Exhibit 20.2, Postmining Topography, and the longitudinal profiles on Exhibit 20-2B, Postmining Channel Profiles. The postmining drainage channel designs are based on peak flows calculated using SEDCAD (Schwab, 1998) and are designed to be stable and safely pass the flows from a 10 -year, 24-hour precipitation event (100 -year, 24-hour event for drainages larger than one square mile). The channel type (ie: vegetated, rock riprap, or vegetated turf reinforcement may (TRM)) is based on channel slope and velocity. Tab 13, Attachment 13-12, Drainage and Sediment Control Plan, presents the methodology used to determine peak flows and the supporting SEDCAD design demonstrations. The limiting velocity for the vegetated channels is estimated using the smooth brome or mixed grass channel material listed in the SEDCAD channel utility. Seed mixtures used at the Yoast Mine mimic that of smooth brome or ixed grass closer than the other choices. Vegetated channels supplemented with TRM can handle velocities up to 9.0 feet per second. The postmine channels are typically trapezoidal with 3H:1V sideslopes. TRM channels are designed with sideslopes of 4H:1V. TR -37 8 Revised 02/08