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,;: .' <br />all Post Mining drainage channels peak velocities and total runoff designs. In addition to this, it takes <br />a number of years before a grass cover of 70% or greater can be achieved and past experiences have <br />shown that designing to optimal conditions rather than conservative conditions can have disastrous <br />and costly results. <br />Page 7, paragraph 6.0 Post Mining Channels. Vegetated channel designs utilize two retardant <br />classes to determine channel stability and channel capacity. The classes range from A to E, where B <br />is for short grass conditions and D is for tall grass conditions. SCC uses C and D respectively. Please <br />justify the non-standard use of C-D rather than the normal B-D condition, otherwise the Division <br />recommends using B conditions to determine channel velocities for stability purposes. <br />8. Page 9, Ditch Design Schedule. The typical roadside ditch is a triangular ditch with 2:1 and 10:1 <br />side slopes. Hydraulically speaking, this is a good design due to the large hydraulic radius/area ratio. <br />However, maintaining this design in steep terrain is often very difficult and costly at times. If the <br />10:1 side slope design is not maintained and allowed to approach steeper side slopes, the flow <br />characteristics will inherently increase thereby compromising [he integrity of [he design. In lieu of <br />the fact that many of the roadside ditches have profiles ranging in the 10-15%, considerations should <br />be given to redesigning portions of these ditches. <br />The design criteria for most of the roadside ditches call for lining the channel with gravel. According <br />to most hydraulic literature sources and the Sedcad 4 modeling program, permissible velocities for <br />coarse gravel is 6 fps and the corresponding Manning's coefficient is .025. If(graded silts to cobbles <br />when colloidal~is used instead of coarse gravel to achieve a Manning's coeffictent o , <br />permissible velocity of 5.50 fps must be used in the design. Eithe condition is acceptable, but the <br />coarse gravel Manning's coefficient of .030 and a permissible v ocity of 6.0 fps used by SCC, is not. <br />All of the steeper portions of the following ditches need t be redesigned due to an exceedance of <br />permissible velocities: <br />B-1 C-3 D-1 ( t~,,~ ~~~ <br />B-2 D-2 U So r <br />B-~ p 4 0~ 1~1N fi,N~ cbtr2 ~w7s h i s Tyr) e loo t,c <br />D-5 <br />D-6 <br />9. Appendix 13-12.3 Post Mining Channels. Due to the issues noted above with the Ilydrograph <br />Response Shape, Curve Numbers associated with reclaimed C and D soils, and the retardant classes <br />for vegetated channels, it is recommended that all Post Mining Channels designs be re-evaluated. <br />