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Memo to Steve Renner - 4 - January 4, 1989 D.9 - Diversions and Sediment Control and Drainage Crossings 17. In Appendix H, it is difficult to correlate the watersheds shown on Figure H-1 with the watershed name used to identify each model run. Please provide a summary table which correlates the ditch segment name, model run watershed name, watershed shown on Figure H-1 and peak flow in english units. 18. A curve number of 70 was used in all runoff calculations. Although this appears to be within the range expected, what parameter values were used to derive this curve number based on the SCS curve number method referenced. 19. All of the hydraulic lengths appear to be longer than expected. What estimation method was used to derive these parameter values? If these were underestimated, please provide revised peak flows as appropriate. 20. The capacity of the ditches are acceptable at the shallow slopes used in the calculations and indeed may be oversized. However, no evaluation of channel-bottom stability was performed as a part of ditch design. At 100-year, 24-hour peak flows the velocities are erosive (e .g. Type 2 channel at 228 cfs equals 10 fps at 1.7 foot flow depth). The stability of the ditches should be evaluated and channel-bottom treatments specified for the slope conditions to be encountered along the di*. ch. Riprap sizing or other treatment criteria should be shown. For temporary channels a 10-year, 24-hour peak flow would be appropriate, for permanent channels a 100-year, 24-hour design storm should be used for estimating peak flow, 21. A segment of upland diversion ditch is shown to traverse the actual pit face in the north area of the West Pit. This does not appear to be a preferred design due to stability concerns. Can this segment be redesigned? If not, what measures will be taken to insure stability? Wi11 this ditch be a permanent structure? 22. For the ditches below the waste rock disposal facilities no design storm runoff calculations were provided. What method was chosen for sizing ditches and how was the adequacy of this design evaluated? 23. No peak flow calculations are provided in Appendix H for the South Waste Rock Disposal Facility haul road culvert. Please provide. 24. Aheadwater-to-diameter ratio of about 2 was used in sizing culverts. This design is acceptable if the downstream face of the embankment and a sufficient length of channel bottom are armored with riprap. A design for riprap size and extent should be submitted. The riprap specifications required for the low-velocity drainage swale on the river haul road crossing should also be noted. As these drainage crossings are temporary structures, riprapping the upstream face and cutoff collars will not be required in the design.