2008-12-04_PERMIT FILE - C1994082A (12)

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• and Post Mining Cross-Sections. An examination of the cross-sections indicates that the post mining conditions are similar to the premine conditions. The above information demonstrates compliance with the requirements of 4.14.2. These post mining slopes approximate the general nature of the pre-mining 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 pre-mining 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, 24hour precipitation event (100-year, 24hour 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 Post Mining Drainage Summary. The locations of the postmine channels are shown on Exhibit 20-2, Postmining Topography and the longitudinal profiles on Exhibit 20-26, Postmining Channel Profiles. The postmine drainage channel designs are based on peak flows calculated by SEDCAD4 (Schwab, 1998) and are designed to be stable and safely pass the 10-year, 24-hour precipitation event (100-year, 24hour event for drainages larger than one square mile). The channel type (i.e., vegetated, rock riprap or vegetated turf reinforcement mat [fRM]) 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 SEDCAD4 channel utility. Seed mixtures used at the Yoast Mine mimic • that of smooth brome or mixed grass closer than 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:1 V side slopes; TRM channels are designed with side slopes of 4H:1 V. TR-37 8 Revised 02/08