2002-10-25_REVISION - M1986015 (5)

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Page 3 Pueblo East Pit adequacy responses M-1986-015 creates a steep slope below the water line. Expansion of the lake then occurs by infrequent mass failure of the entire bank. In a reservoir there is an additional complication -dramatic fluctuation of the water level. In the short term the same phenomena described above occurs, but because the water line is going up and down, Former cut banks are eroded away creating a comparatively smooth slope around the reservoir during low water. There may or may not be a small vertical bank at the high water line. Nevertheless, mass movement of bank material still occurs from the unconsolidated slope material into dte depths of the lake. The volume change is not significant, but in time the reservoir becomes more shallow and wider. In the case of a lake surrounded by a barrier like a slurry wall, this can eventually lead to a reduction in the distance between the slope and the slurry wall. This is a long and very slow process, but in time it is possible for the containment walls on the inside of the slurry wall to become subject to additional stresses that, in extreme cases, could lead to mass• failure. No armoring was included in the plan because it was felt this period of slope expansion was rather long for this lake. But examination of other, smaller lakes in this area shows that wave cut bank dynamics is rather strong in these soil types. Thus, it is' possible that this slope expansion process might occur faster than anticipated, even though the slope itself would not show evidence of dramatic erosion due to the illusion created by the smoothing of the slope by water level fluctuation. To correct this situation, the downwind slopes (south and east shore lines) will be armored with a waste product generated by the ready mix operation. ~Xlaste concrete is usually dumped on the ground. This creates a concrete Pattie similar in configuration to a cow pie. It has a flat bottom and a rough, domed top. These have been used with good success in shoreline stabilization along the river. The enclosed diagram "Armoring of backfill slopes - treatment" shows how these will be used on the downwind banks of the lake. They will be installed, as close as possible, edge to edge with the top of the Pattie inserted into the bank. The flat side will face the lake. These will extend down the slope as far as material will allow, but in any case much further below the maximum water level than a few feet. Whac the patties will do is reduce the amount of material loss due to the process of slope erosion as a result of fluctuating water levels. This will accomplish three things. First, it will essentially stop expansion of the lake slope toward the slurry wall. Second, it will reduce the loss of material from the slope which eventually is' deposited in the bottom of the reservoir resulting in a reduction in reservoir depth. And third, it will create a protection for the slope to help prevent the ripple patterns that normally develop on reservoir slopes. Near the top of the slope, it will also provide a place Eor plants to catch hold and protect the top of the slope from erosion that starts from above the water line. Although preference is for armoring the outside of the slope, if sufficient patties are available (or other broken concrete is available) that will be placed in the backfill stmcture to reduce the fill volume and increase particle size which will increase the strength of die slope. Item 4 - Rule 3.1.6 - (summary) Question regarding how water will be introduced into the lake when it is finished. Response: Transit Mix of Pueblo is simply creating the vessel to contain the reservoir; they will not be creating the reservoir itself. In some of these types of reservoirs the water is introduced and removed by pumping. Because this reservoir is adjacent to the river from which the water will be removed to fill the reservoir and into which the water wil[ probably be placed when discharging from the reservoir, filling and emptying the reservoir should not present serious problems. However, the actual method will be the responsibility of whoever operates the reservoir after it is completed. Transit lbtix of Pueblo will simply create the basin and if there is nobody to use the basin when completed, the basin will be reclaimed according to the alternate p]-an. It is notable though that several communities have already shown an interest in this reservoir and have said they wish it was available now rather than later With water issues reaching a high point, in part due to the severe drought of 2002, it does no[ appear there will be much problem with finding someone to us'e the reservoir. Many communities are already making plans to store more water so the severe restrictions experienced in 2002 will be less in future droughts.