Swinging Bridge Dam (P-10482) Dam Safety Remediation
Project
Project Rehabilitation Status Archive - June 15, 2006
Significant progress has been made in establishing the final remediation plan for the Swinging Bridge Sinkhole Remediation project. By letter dated June 14, 2006, FERC accepted a plan proposing the design elements of the final necessary remediation plan that is expected to address the safety issues created by the formation of the sinkhole.
In summary, the major elements of the final remediation plan include:
- Conduit Filter - A filter and drain will be constructed at the downstream end of the conduit. The conduit filter will consist of installation of filter material in a braced excavation that will extend to a level at or below the base of the conduit. The excavation will require dewatering and retention measures to maintain a stable excavation. The underside of the conduit will be grouted.
- Embankment Toe Filter - A filter and drain will be constructed along the toe of the right and left embankment sections. The filter will be installed in an unbraced excavation at the toe of the slope. In addition, a localized filter will be installed on the west side of the powerhouse.
- Penstock and Diversion Tunnel Rehabilitation - The penstock and diversion tunnel will remain open for access and monitoring during the refilling of the reservoir and the stabilization of the dam. Additional repairs of the tunnel and penstock will be undertaken depending on the final intended use of the penstock and the completion of an ongoing penstock assessment evaluation.
- Crest Seepage and Crest Wave Protection - Crest wave protection will be provided by adding parapet rock on the upstream side of the crest road. Crest seepage will be evaluated as part of the reservoir refilling operation.
- Filling of the Reservoir - The refilling of the reservoir will begin upon completion of the construction of the conduit filter and embankment toe filter, and after we authorize Mirant to do so.
The June 14, 2006 letter  authorized Mirant to proceed with selection of contractors and with the work for the conduit filter and embankment toe drain. This work will be completed by November 2006. Mirant has submitted to FERC a preliminary schedule for condition assessment of the penstock. Once the dam remediation and the penstock assessment are completed, and the reservoir refilling plan is finalized and approved, the reservoir refill will be initiated. It is anticipated that the remediation necessary to initiate a controlled refilling of the project reservoir will be completed in late fall 2006. At that time a carefully monitored reservoir filling can commence and providing the behavior of the dam is considered normal and the remediation measures are sound, the lake can be refilled to normal operating levels. The penstock and tunnel will remain open for monitoring during the reservoir refill. Additional repairs will be undertaken based on the condition assessment and on the final intended use.
Since May 9, 2006 the contractor has installed the upstream bulkhead and ventilation. The drilling for contact grouting from the penstock started on June 12, 2006. The penstock contact grouting is scheduled to start on June 19, 2006 and be completed by mid-August 2006. In addition, the dewatering wells installed at the toe, to lower the phreatic surface in the work area, were cleaned as they are expected to be used during the filter installation.
Regarding the adequacy of the Mangaup River dams spillway capacity, a supplemental hydrologic and hydraulic (H&H) analysis was completed to confirm the Inflow Design Flood (IDF) for the Mongaup Projects and determine the risk to life due to the incremental damages that may result from a dam failure. This supplemental study was submitted on March 31, 2006, and discussed in an April 19, 2006 videoconference. Additional information, clarifications, and analyses were submitted on May 4, 2006, May 17, 2006, and May 31, 2006. Following Commission staff review, the final report was submitted on June 5, 2006.
By letter dated June 13, 2006 , FERC concurred that the supplemental H&H analyses demonstrates that the current spillway capacity for all project dams is adequate to pass the IDF for each dam, It does not appear that increasing the spillway capacities of the Mongaup River dams is necessary.
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Project Rehabilitation Status Archives
January 2007
November 2006
June 2006
May 2006
March 2006
February 2006
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