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• 68 The design depths for the channels are a summation of normal depth, super- elevation, wave height and required freeboard. The resulting design depths are presented in Table 19. Design depths based on both the 10-year and 25-year storm are provided. Since there is little difference in the 10-year and 25- year design depths, it is suggested that the 25-year depths be adopted. This allows a much larger margin of safety. The riprap vas designed for the 10-year event, and may experience some damage during the 25-year and 100-year events. The riprap requirements for a 25-year or 100-year event would be impractical. However, by not letting the flow overtop the riprap for the 25-year event, dam- age will be minimized during the larger event. This is why choosing the 25- year depths in the lined channels is reasonable. Tha alternatives that are recommeeded and were used in the MULTSED post- mining analysis are 15-D and 17-D. The selection of these alternatives is r- based on several reasons. Alternatives 15-A and 15-H were rejected because of their steep slopes, high velocities, and large required riprap sizes. In addition, the runoff • would impinge on the opposite bank of the Magpie Creek diversion channel in- ducing bank instability. Alternatives 15-C and 15-D both have a smooth tran- sition into the Magpie diversion channel which minimizes this effect. In addition, these two alternatives have channel slopes close to the natural chan- nel slope. Side .s lopes of 3:1 were chosen in preference to 2:1 because of increased bank stability, since the channel is designed for permanent restoration. in addition, alternative 15-D has a lower velocity and, therefore, smaller re- quired riprap size than alternative 15-C. For permanent restoration of thin channel, special considerations need to be made where this channel joins the Magpie Creek diversion channel. At the confluence, the Magpie Creek diversion banks should also be protected with riprap. Even though this is not within the G.E.C. permit boundary, this will ensure that PM-CH-15 will remain stable should the banks of the Magpie Creek diversion begin sloughing and become unstable. The recommended channel alignment is shown on Hydrolocy Map 3, Appendix A. Alternative 15-D was used in the MULTSED model for post-mining conditions. The results showed the sediment concentrations entering and leaving this channel to be als.ost ev_ual. . Alternative 17-D is recoaanended because of its milder channel and side slopes. By using side slopes of 4:1, the bank stability is increased and the channel becomes more representative of the natural channel. The design channel