2008-01-16_PERMIT FILE - C1994082A

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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). Post mining 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 post mining 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. The locations of the postmine channels are shown on Exhibit 20-2, Post-Mining Topography and the longitudinal profiles on Exhibit 20-2B, Post-Mining Channel Profiles. The reclaimed drainage channels are designed to be stable and safely pass the 10-year, 24-hour precipitation event (100-year, 24-hour event for drainages larger than one square mile). The designs are based on peak flows calculated by SEDCAD4 software program. The resulting type of channel (i.e., vegetated, rock riprap or vegetated turf reinforcement mat [TRM]) is based on channel slope and velocity. Tab 13, Attachment 13-12, Drainage and Sediment Control, contains the methodology used in determining peak flows and the SEDCAD results for design. The limiting velocity for the vegetated channels is estimated using the tables listed in SEDCAD4 based on a vegetated channel using smooth brome as the vegetation type. Seed mixtures used at the Yoast Mine mimic that of smooth brome 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:1V side slopes. The TRM channels are designed with side slopes of 4H:1V. TR-36 8 Revised 08/07