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~ stable rock to cut a 100-yNar spill•.+ay. >[aintenance w111 be iLLL. ~ performed at least annually, or more requently whenever the w.`' , ~Y accumulated sediment has reached 50 per cent of the sediment ~ J,w ~, volume. Onsite equip!~ent, ~:onsisting of dozers, wheeled loaders ~ i and haul trucks will b? used to dr?dge the accumulated sediment, ~ Coarse material will b? sal:.;ged and sold as aggregate. The fine ~ ~ ~ grained gnat?rial will be sa'_vaged as soil and stockpiled with they ~~~'1.~ soils stripp?d from th? wcr}; areas. Those materials that are y~Od'. stockpiled for une yro~.ainy season or lunyer will be planted with a,~. a perennial sod-forming grass cover. ;,ri alternative to the built-up embankment is being considered. ~r - This alternative would be to depress the settling basin below grade creating a sump in the same vicinity of the proposed pond. Since the floor of the pi r_ is planned to be lowered as part of the mining plan, this sump could easily be accommodated. The ~. ~1'~`'~ configuration of the sump would be the same as the pond. The crest of the sump would be excavated into competent bedrock and function as weir spillway. 13. a. Bench Width to Accomodate the Plunge Pool The bench width in the vicinity of the East tributary drainage channel will be increased to accommodate the plunge pool as described in section 13. c. 13. b. Channel Design Characteristics The design flows for the recreated channels are the 100-year, 24- hour discharges as reported in the December 2, 1988 surface water responses: Waldo Canyon above the confluence with the East tributary, 850 cfs; East tributary above the confluence with Waldo Canyon, 14s cfs; Waldo Canyon at Highway US 24, 990 cfs. The recreated channel slope for all channels is 2.5 per cent (the existing average channel slope for all of Waldo Canyon is 13 per cent), The velocities in the channels cut into the granite bedrock are 10 feet ,per second (fps) in the East tributary and 16 fps in the Waldo Canyon channel. 13. c. Plunge Basin and Drop Structure This comment is concerned with potential headcutting in the recreated drainage channels that may ultimately lead to offsite erosion in the channels. The place where this potential exists is in the northeast corner of the site as the East tributary and, possibly, where the main Waldo channel, crosses the permit boundary and "cascades" down a series of plunge basins to reach the floor of the reclaimed pit. The design of t][e plunge basin is shown in the following drawing. This type of plunge basin has been successfully used by the Bureau of Reclamation at many free-falling outlets of dams to dissipate the erosive energy of the water. Based upor. critera 2