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accuracy of any mapping exercise. Contour maps are generally only accurate to within half the contour <br />S interval. <br />The proposed postmining surface for the Seneca II-W Mine was redrawn until the proposed volume was <br />within 3.0 percent of the target volume. <br />Postmining Drainage Channel Designs <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 area. <br />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. Safely pass the peak runoff from a 100-year, 24-hour precipitation event for watersheds larger <br />than one (1) square mile; and, a 10-year, 24-hour precipitation event for watersheds less than <br />one (1) 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 within the disturbance area are shown on Exhibit 20-2. The <br />longitudinal profiles are shown on Exhibit 20-5.1, 006 Channel Profiles; Exhibit 20-5.6, Postmining Stream <br />Channel 005-E1; Exhibit 20-5.7, Postmining Stream Channel 005-Gulch; Exhibit 20-5.8, Postmining <br />Channel 005-W1; and, Exhibit 20-5.9, Postmining Channel Profiles for II-W South Area. Premining <br />channel profiles are shown on the Exhibit 7-7 series of maps. <br />Initially, the 006 reclaimed drainage was designed to safely pass the 100-year, 24-hour precipitation event. <br />All other reclaimed drainages were designed using the 10-year, 24-hour event. The peak flow was <br />determined using the SEDIMOT II computer program (Civil Software Design). Tab 17 presents the <br />methodology used in determining the input parameters for obtaining peak flows for the 006 Gulch <br />TR-59 11 Revised 08/07 <br />