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<br />• and Post Mining Cross-Sections. An examination of the cross-sections indicates that the post mining <br />conditions are similar to the premine conditions. <br />The above information demonstrates compliance with the requirements of 4.14.2. These post mining <br />slopes approximate the general nature of the pre-mining topography as demonstrated in 2.10.3(1). <br />Postmining Drainage Channels <br />All drainages that are to be disturbed by mining activities will be reclaimed to approximate the pre-mining <br />configuration and to blend with the undisturbed drainage above and below the area to be reclaimed. The <br />postmining drainage channels will be designed, constructed, and maintained to the following specifications: <br />1. Average stream gradient shall exhibit a concave longitudinal profile to the extent possible; <br />2. Remain in dynamic equilibrium with the drainage basin system without artificial structures; except <br />where necessary to eliminate or reduce erosion; <br />3. Provide separation of flow between adjacent drainages; <br />4. Safely pass the runoff from a 10-year, 24hour precipitation event (100-year, 24hour precipitation <br />• event for drainages larger than one square mile [Rule 4.05.4(2)(b)]); <br />5. Designed for long-term stability of the landscape; <br />6. The reconstructed drainage channels will eliminate or reduce the severity of nickpoints to the extent <br />possible. <br />A summary of the pre- and postmining drainage characteristics is shown on Table 20-2, Pre- and <br />Post Mining Drainage Summary. The locations of the postmine channels are shown on Exhibit 20-2, <br />Postmining Topography and the longitudinal profiles on Exhibit 20-26, Postmining Channel Profiles. The <br />postmine drainage channel designs are based on peak flows calculated by SEDCAD4 (Schwab, 1998) <br />and are designed to be stable and safely pass the 10-year, 24-hour precipitation event (100-year, 24hour <br />event for drainages larger than one square mile). The channel type (i.e., vegetated, rock riprap or <br />vegetated turf reinforcement mat [fRM]) is based on channel slope and velocity. Tab 13, Attachment 13- <br />12, Drainage and Sediment Control Plan, presents the methodology used to determine peak flows and the <br />supporting SEDCAD design demonstrations. <br />The limiting velocity for the vegetated channels is estimated using the smooth brome or mixed grass <br />channel material listed in the SEDCAD4 channel utility. Seed mixtures used at the Yoast Mine mimic <br />• that of smooth brome or mixed grass closer than other choices. Vegetated channels supplemented with <br />TRM can handle velocities up to 9.0 feet per second. The postmine channels are typically trapezoidal with <br />3H:1 V side slopes; TRM channels are designed with side slopes of 4H:1 V. <br />TR-37 8 Revised 02/08 <br />