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Postmining Drainage Channels <br />All major drainages that are to be disturbed by mining activities will be reclaimed to an approved • <br />postmining configuration and will blend with the undisturbed drainage above and below the disturbed <br />area. The postmining drainage channels will be designed, constructed, and maintained to the following <br />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 <br />except where necessary to eliminate or reduce erosion. <br />3. Provide separation of flow between adjacent drainages. <br />4. Saiely pass the runoff from a 100-year, 24-hour precipitation event for watersheds larger than <br />one 11) square mile; and a 10-year, 24-hour precipitation event for watersheds less than one <br />11) square mile. <br />5. Designed for long-term stability of the landscape. <br />6. The reconstructed drainage channels will eliminate or reduce the severity of nick points to the <br />extent possible. <br />The locations of the postmining channels in the permit area are shown on Exhibits 20-2 and 20-2A. • <br />The longitudinal profiles are shown on Exhibit 20-5.1, 006 Channel Profiles, Exhibit 20-5.6, Postmining <br />Stream Channel 005-E7, Exhibit 20-5.7, Postmining Stream Channel 005 Gulch, and Exhibit 20-5.9, <br />Postmining Channel Profiles for II-W South. Pre-mining channel profiles are shown on Exhibit 7-7 <br />series. <br />Initially the 006 Gulch reclaimed drainage was designed to safely pass the 100-year, 24-hour <br />precipitation event and to be stable. All other reclaimed drainages were designed using the 10- <br />year, 24-hour event. The peak flow was determined by using the SEDIMOT II computer <br />program. Tab 17 contains the methodology used in determining the input parameters for <br />obtaining the peak flows for the 006 Gulch reclaimed drainage channel. The peak flow <br />determination assumed the total watershed of the reclaimed channel at the time of bond release. <br />The peak flow for 006 Gulch was 26.4. <br />In 2003, as the final backfilling and grading has been conducted and the final postmining <br />topography determined, the 005 and 006 Gulch postmine drainage channels have been <br />redesigned to meet actual postmine configuration. The redesigns have been based on peak <br />flows calculated by SEDCAD4 computer program. This program is a more sophisticated method <br />of calculating resulting flows. All of the postmining channels in the 005 and 006 watersheds <br />have been redesigned using the 10-yr/24-hr storm event. The resulting type of channel (i.e., <br />vegetated or riprapped) is based on channel slope and velocity. The peak flow for the reclaimed <br />TR-45 10 Revised 3104 <br />