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Postmining Drainage Channels <br />All major drainages that are to be disturbed by mining activities will be reclaimed to approximate the <br />premining configuration and to blend with the undisturbed drainage above and below the area to be <br />reclaimed. The postmining drainage channels will be designed, constructed, and maintained to the <br />following specifications: <br />1. Average stream gradient shall exhibit a concave longitudinal profile; <br />2. Remain in dynamic equilibrium with the drainage basin system without artificial structures; <br />3. Establish or restore the channel to its natural meandering pattern with a stable gradient; <br />4. Provide separation of flow between adjacent drainages; <br />5. Safely pass the runoff from a 100-year, 24-hour precipitation event; <br />6. Long-term stability of the landscape; <br />7. The reconstructed drainage channels will not exhibit any knickpoints. <br /> <br />The postmining channel profiles of the major reclaimed drainages in the permit area are located on Exhibits <br />20-2 and 20-2A. The actual profiles are shown on Exhibits 20-5 series. The premining channels are <br />shown on Exhibits 7-7 series. The average postmining stream gradient will be a concave longitudinal <br />profile which approximates the premining slope. The pre- and postmining drainage areas of the reclaimed <br />channels are very similar in acreage and slope. • <br />The 006 Gulch reclaimed drainage is designed to safely pass the 100-year, 24-hour precipitation event and <br />to be stable. All other reclaimed drainages were designed using the 10-year, 24-hour event. The peak <br />flow was determined by using the SEDIMOT II computer program. Tab 17 contains the methodology used <br />in determining the input parameters for obtaining the peak flaws for the 006 Gulch reclaimed drainage <br />channel. The peak flow determination assumed the total watershed of the reclaimed channel at the time of <br />bond release. The peak flow for 006 Gulch was 26.4. The peak flow for the reclaimed drainages were <br />determined at the downstream end of the channel, so most of the reclaimed channel reach will not have to <br />handle that large a peak flow rate. This was done to ensure that the reclaimed drainage channel capacity <br />will be adequate and conservatively designed. A limiting velocity of 5.0 feet per second (fpsl was used to <br />insure that the postmining drainage channels would be nonerodible and stable. The limiting velocity was <br />estimated from the tables listed in SEDCAD as a vegetated channel using smooth brome as the seed <br />mixture. Seed mixtures used at Seneca II-W mimic <br /> <br />10 Revised 9/96 <br />