Laserfiche WebLink
<br />... <br /> <br />The engineer plans to re-establish an operational outlet on the left abutment of the north dike, <br />either a concrete-lined slot channel or a concrete conduit, with a low-head slide gate. The invert of this <br />arrangement would be placed such that the top 5 feet of the normal storage in the reservoir could be <br />drained off through it. Tim believes that bedrocK is close to the ground surface in that area. We <br />discussed the desirability of having the capacity to lower the reservoir by 5 feet in 5 days, if possible. The <br />owner is confident that the hydraulic connection between the main reservoir area and the portion which <br />wraps around to contact the north dike is low enough to allow withdrawal of the top 5 feet through that <br />arm of the reservoir. <br /> <br />Seepage along the lower slope and toe of the main dam would be addressed by the placement of <br />an aggregate filter and drain system, with a perforated collection pipe. A two-stage aggregate filter/drain <br />system is proposed, without geotextiles which might tend to clog under the presence of observed iron <br />bacteria in the area. We indicated the need for a non-perforated outfall pipe and a device at the <br />downstream end for measuring seepage. Also, it is the intent of the design to include the downstream end <br />of the grouted outlet pipe within the toe filter/drain, to ensure that any seepage from or around the old <br />outlet is filtered and picked up by the drain. <br /> <br />Stability of the downstream slope of the main dam would be enhanced by the pl~cement of a <br />granular filter material on the stripped downstream slope of the dam, followed by suitable embankment <br />materials used to flatten the slope to an acceptable angle. Tom McDaniel has proposed the rather <br />controversial concept of wintertime stripping of the downstream toe area in leapfrogging panels after the <br />organic materials in place at the toe have frozen. This would, presumably, permit dealing with the soft, <br />wet materials when they are easier to work with. The problem then becomes placement of suitable fill <br />materials back in the excavation under cold, wet weather conditions, which would be very difficult, at <br />best, to say nothing of the possibilities of finding a contractor who would be willing to work under such <br />conditions. The problems with this approach were discussed in detail. <br /> <br />The proposed work at the north dike would be to merely add to its downstream face to achieve a <br />suitable embankment. I indicated that seepage conditions are bad along the toe of this dike, as well, and <br />that the excessive aspen growth on all surfaces of this small dike would have to be removed. Removing <br />tree stumps and chasing roots might require such extensive excavation that it would be more economical <br />to simply remove this dike and rebuild it, which would also allow suitable foundation preparation to be <br />performed. This would, of course, require lowering of the reservoir to below the foundation surface for <br />the dike, which could be a problem. <br /> <br />The question of spillway adequacy was also brought up. I informed them of the possibility of the <br />upgrading of the dam to a Class 2 structure, based on the downstream hazards I noted in 1995. That <br />would require a greater flood routing capacity than now exists, and it would probably make sense to <br />incorporate a larger emergency spillway with adequate freeboard (minimum of 3 feet) into the current <br />design and construction work, as it could be included at minimal cost. The enlarged spillway would still <br />be on the left abutment of the north dike, probably contained within the area used for construction of the <br />new outlet. The owner controls all of the land downstream of the dam for about Y:z mile, so location of the <br />spillway is not a flood easement issue. The watershed for the reservoir is small, so development of a <br />spillway size which would allow routing of the Y:z PMP flood should not be overly difficult. It was <br />indicated that the invert of the emergency spillway channel should provide the minimum freeboard to all <br />points on the dam and dike crests; this would require raising the existing dike crest somewhat. It is <br />intended by the owner that the existing pipe service spillways at each end of the main dam be retained <br />(although the spillway channel for the left pipe would be rerouted away from the dam toe), so the <br />emergency spillway could be designed as an earth section with a simple grade control/crest structure to <br />prevent backcutting to the reservoir during its design flood discharge. <br /> <br />2 <br />