1989-12-22_REVISION - M1988112 (3)

Home Browse Search

Memo - Steve Renner - 3 - December 22, 1989 16. A pan evaporation coefficient of 0.70 was used in the water balance whereas 0.74 was given in Exhibit K of the original permit. Please clarify. 17. The diversion ditches located upslope from the collection pond are shown in cross-section on Detail 2 of Figure C-8 and in plan form on Figures C-3 and C-4. No peak flow calculations are provided to substantiate their design, however the size appears to be conservative. Please provide the following information on each diversion: a. The upland drainage area, b. The minimum slope to insure drainage, c. An estimate of the size of runoff event which would overtop the berm, d. The erosion control measures to be utilized at the ditch outlet downslope to the natural drainageway. 18. A small area without upland runoff control exists between tY~e east and south diversions. How will runoff from this small ridge area above the tailings be handled? 19. On page D-61 of the original permit application, the statement is made that details of the erosion protectionJrunoff control for the mill site area will be provided upon completion of the mill design. Heave these been developed and, if not, what is the expected timing of tihis submittal? 20. Please describe briefly the diversion, sediment control, and drainage crossing maintenance plan. 21. The final topography on the tailings surface is described as being created by selective deposition at the end of operations sucn that only minor grading is expected prior to topsoil placement and revegetation. It is noted in Appendix L (Spillway Design Calculations) that "..., the reclamation plan may spigot clean material and seed, rather than using earth movers to grade the surface." Please clarify the method proposed for topsoil distribution and seeding on the tailings surface. 22. No post-mine channels have been proposed for the top of the reclaimed tailings other than those resulting from final deposition as shown on Figure F-3. Flow paths will approximate the pre-mining cond~Rtion as noted. A significant reduction in channel slope will occur toetween pre and post-mine conditions. Although the reduction in slope will tend to reduce the potential for erosion, it will not alleviate the potential as stated on page E-6. The broad flow zones (channels) created on the tailings surface and shallow slopes will result in low velocities during flood conditions. However, during smaller runoff events, channelized flow from upstream areas may tend to cut a channel thru the topsoil which has been respread on the tailings. It may be advisable to direct this flow in a constructed low-flow trickle channel rather than allow the topsoil to be eroded away in finding its preferred path. Please address the stability of these channels and any channel bottom treatment required for low-flow conditions.