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The postmining reclaimed drainages in the permit area are located on Exhibit 242, Postniining <br />Topography Map. The average postmining stream gradient will be a concave longitudinal profile vvhich • <br />approximates the premining slope. The pre- and postmining drainage areas of the reclaimed channels <br />are very similar in acreage and slope. <br />The reclaimed drainages are designed t.o safely pass the t0-year, 24-hour precipitation event and '[o be <br />stable. The peak flow was determined by using the SEDCAD 4 computer model. Tab 13, Facilities, <br />contains the methodology used in determining the input parameters for obtaining [he peak flows for <br />the reclaimed drainage channels. The peak flow determination assumed one year of revegetation of <br />the total watershed. The peak flow for the reclaimed drainages were determined at the downstream <br />end of the channel, so most of the reclaimed channel reach will not have to handle the estimated peak <br />flow. This was done to ensure that the reclaimed drainage channel capacity will be adequate and <br />conservatively designed. For channels requiring riprap, the CSU/OSM method was used riprap design <br />For further protection of the channel bed, a layer of Mirafi 700X geotextile fabric will be installed prior <br />to placement of the riprap. The shape of the reclaimed channels was assumed to be trapezoidal with <br />3H:1 V side slopes as a worst case condition. The side slopes will be no steeper than 3H:1 V <br />The channel bottom width was also calculated using SEDCAD 4. Attachment 13-12, Drainage and <br />Sediment Control Plan, shows all input and output information. All reclaimed stream channels will be • <br />conservatively constructed to these measurements. Typical post mining channel profiles are shown on <br />Exhibit 20-2, Post Mining Topography. The typical section will be used the entire length of the <br />reclaimed channel. Figure 13-12.3 presents is a summary of the post mining channel designs. <br />After the watershed has been graded and the location of the drainage channel determined, the channel <br />will be constructed to the required shape. All drainages will be inspected for stability according to the <br />rill and gully plan in lab 22, Revegetation Plan. Maintenance or monitoring that may be necessary as <br />a result of the inspections will be submitted to the Division for review and, if necessary, approval. The <br />side slopes of the meanders of the reclaimed drainage channels will be protected from erosion by <br />revegetation with drainage seed mix and shrub plantings according to Tab 22. Revegetation Plan. <br />Maintenance of reclaimed drainage channels will be identified according to the Rill and Gully Plan. A <br />summary of the pre- and post mining drainage characteristics is shown on Table 20-2, Pre- and Post <br />Mining Drainage Summary. <br /> <br />10 Revised 2199 <br />