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• 63 <br /> of this channel is 0.083. As discussed in Section 3.1, the degradation of <br /> these channels are the primary cause for increased sediment concentrations <br /> leaving the lease area. Stabilization of these charnels is recommended for <br /> the present-mining condition. <br />' The Magpie Creek diversion channel rithin Dorchester's permit boundary is <br /> e permanent diversion. Due to this, CH-15 and CH-17 cannot be restored back <br /> to their original alignment and slope. Therefore, it is reasonable to design <br /> these two diversion channels as permanent diversions for post-mining conditions. <br />. ey designing these channels for post-mining conditions, the amount of cork that <br /> rill be required when the area rithin the G.E.C. permit boundary is reclaimed <br /> rill be reduced. <br /> L <br />L The design of permanent diversions considers channel and floodplain dimen- <br />. sions that approximate the natural configuration and blend rith the undisturbed <br />~ drainage above and below the area to be reclaimed. The charnels are designed <br /> to rnntain the 10-year, 24-hour storm event. The floodplain should be able to <br /> contain the 100-year peak discharge. The design discharges were taken from the <br /> MJLTSED results for the 10-year, 24-hour design storm and present-mining <br /> conditions. The runoff that reaches these channels is from undisturbed areas <br /> upstream of the mine site. For post-mining conditions within the Magpie Creek <br /> drainage, there is an additional contributing area to PM~ti-15 and PM-CH-17 <br /> which is the existing mine pit area. The additional runoff from this area rae <br /> estimated using the Green-Ampt infiltration equation and the procedure discussed <br /> in Section 6.1. The MOLTSED peak disccarges of 353 and 945 cfs for PM-Cft-15 <br /> and PM-CH-17 were increased by 12 and 21 cfs, respectively. However, since <br /> this area is directly adjacent to these channels, the runoff from these areas <br /> mill occur be_`ore the peak runoff from the large undisturbed areas upstream of <br /> these charnels. Therefore, adding these discharges to the peak is conservative. <br /> The runoff through these two channels is high and riprap protection is <br /> required on the channel bed and banks. Four alternatives for each channel were <br /> considered which have different combinations fo channel bed and side slopes. <br /> Trapezoidal shapes were chosen for the channels with base widths of six and <br /> ten feet for P?1-~:i-15 and PM-Cy-17, respectively. <br /> :rte design of the riprap was based on a modification of the approach pre- <br />. se^ted on panes 418-425 of Simons and Senturk (1977)_ This approach loots at <br /> ...<. fc_ces actinc o~ an individual particle, and calculates a factor of safety <br /> for its movement. A factor of safety of 1.5 vas used in the analysis. The <br />