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<br />culvert with an open channel would require considerable <br />• surface disturbance, and an open channel in this area may not <br />be a practical alternative due to excessive grades. Given that <br />culvert 2A has and continues to function with little or no <br />maintenance, redisturbance of this area is not justified and <br />replacement with a steep open channel would probably result in <br />undesirable erosion and water quality degradation. <br />13. Channel SIIPM-1 below SIIPM-5 has been redesigned to pass the <br />peak discharge from the 100-year, 24-hour event. Revised <br />design documentation is included with these responses for <br />replacement in the permit document. <br />14. The postmining topography was previously approved by the <br />Division. This TR does not propose to change the approved <br />postmining topography, especially since reclamation regrading <br />for much of the mining disturbance is close to completion in <br />accordance with the approved plan. <br />• <br />• <br />SCC submits that the proposed channel configurations are <br />similar to natural stream channels in the upper portions of <br />area watersheds that are generally steep and straight. The <br />natural streams don't establish significant meanders until <br />they reach the lower portion of the watersheds where stream <br />velocities decrease and channel and floodplain areas expand <br />sufficiently to allow meanders to develop. To the extent <br />reasonably feasible and as constrained by grade and terrain, <br />SCC will establish some lateral variation (bends and meanders) <br />in the reestablished channels during construction. <br />15. The available mapping accuracy of 1" to 500' with a 10' <br />contour interval is marginal relative to effective drainage <br />design. As indicated on the channel profile and in the design <br />documentation, the grade of channel SIIPM-4 will not exceed <br />33~. The constructed location and alignment of the channel may <br />vary slightly to minimize gradient and take advantage of <br />localized variations in terrain that may not be indicated by <br />the mapping. <br />16. A line was added on Exhibit 7-19 indicating the end of the <br />temporary wadge Diversion. The revised exhibit accompanies <br />these responses for replacement in the permit document. <br />17. The D50 of the granular filter material should be equal to the <br />Riprap D50 divided by 40. The following table indicates the <br />minimum D50 for the granular filter material for a give riprap <br />size. <br />Riprap D50 <br />(inches) <br />15 <br />12 <br />9 <br />6 <br />Filter D50 <br />(inches) <br />.37 <br />.30 <br />.23 <br />.15 <br />