C150268 Feasibility Study - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 21 +00 and 24+93, the only erosion protection of the upstream face exists between the toe of the dam and approximately the middle of the upstream face, thus leaving the upper half of the upstream face unprotected against wave erosion. In addition, the protected upstream face of the un-recompacted section does not have a filter between the riprap and the upstream slope, the lack of a filter has led to erosion around the riprap making the riprap less effective. In addition to the seepage problem experienced in areas of the dam where recompaction has not occurred, the owner has indicated locations along the dam (~Sta. 21 +00 to 21 +80) where the dam has experienced settlement issues. The owner has repaired the settlement issues by placing soil and road base over the settled portions however based on this inspection erosion and soil movements have also occurred that can lead to further settlement of the darn. Photographs 1, 2, 3, and 4 in Appendix E show photographs ofthe darn and embankment. The intent ofthe Company is to address the above deficiencies and rehabilitate the embankment such that it meets current standards and requirements of the State Engineer. ALTERNATIVES EVALUATED A range of alternatives were considered including 1) No action, 2) Excavation and recompaction of the embankment between Sta. 21 +00 and 24+93,3) Placing a sheet pile wall along the crest ofthe dam that extends from the crest to bedrock, and 4) Placing a bentonite slurry wall along crest ofthe un-recompacted embankment that extends from the crest to bedrock. No Action Alternative One course of action considered was the no action in the rehabilitation of the dam. The results of this alternative would not necessarily result in no cost to LICRC. By not repairing the dam Rist- Benson Reservoir will become permanently restricted to gage height 10, thus resulting in the permanent loss of the 150 acre- ft of water stored between. We would expect further restrictions due to the high phreatic surface in the dam embankment to be required in the near future if seepage control improvements are not initiated. Both conditions, seepage and high phreatic surface, can cause failure of the dam and the risk and cost of the results of a failure would financially be catastrophic. While this alternative has no immediate capital expenditures, the risk and potential future costs can be very great. A problem with a slope failure in the dam embankment during the height of the irrigation season could result in the loss of the reservoir for the season. Thus the cost of not providing for the dam repair can potentially very high even without failure of the dam. A problem developing during the irrigation season can result in restrictions that could either require the reservoir be emptied or alternatively only allow limited discharges. Alternative 1 - Excavation and RecomlJaction Rist-Benson Feasibility Study Page 7