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• b~3- z (6.10) <br />d <br />It should ba noted that the preceding empirical formulas are simply <br />guidelines. These equations do not apply to all conceivable floc conditions, <br />not do they differentiate between practical and impractical channel <br />configurations. <br />Channel designs having width-to-depth (b/d) ratios significantly dif- <br />ferent from the es~irical ruler (Equations 6.7-6.10) should be evaluated <br />further. It may ba possible to improve the channel design by using a properly <br />designed lining or imtallinq grade control structures. Thees methods rill be <br />discussed in the follwinq section. <br />6.6.3 Svaluatlon of the Need for Rock Riprap or Crade Control Structures <br />If the cross section determined lrom the stable channel design procedure <br />(Section 6.4.1) is not economical or acceptable according to the b/d ratio <br />(Section 6.4.7), then a sore practical croee section can be designed by using <br />a channel lining and/or grade control structures. 1- channel lining allws <br />designing for a larger parsiesible velocity without scour or erosion of the <br />• channel. For instance, if the bedrock o[ the natural ground la a poor sedi- <br />mentary rock strata with a lw permissible velocity, a smaller channel lined <br />with a durable riprap may be more economical and etsble. Additiomlly, chan- <br />nel linings can be used to reduce or eliminate seepage losses from the chan- <br />nel. 1fie reduction of seepage is not usually a major concern in a surface <br />sine operation; hwever, 1t may become important In areas of the mine site <br />where seepage could cause rater quality oz stability problems. Possible sta- <br />bility problems [was •eepaga include slippage along backlill arena, mass or <br />•urlacs sloughage of waste sites and bank sloughing in otharviae stable chan- <br />nels due to seepage pore pressure. Ideally, channel lininge for diversion <br />structures •hould be saintenanea Cree and have a long design life, since they <br />rill Rava to remain 'forever' after bond is released. <br />Grade control structures can reduce the velocity upstream of the etruc- <br />lure to a nonerosive value. Multiple grads control etructurae can be used to <br />control long reaches of a stream. The design procedures for channel lininge <br />and grade co~.trol structures follow. <br />• <br />