April 2, 2008 - Commission Approves Reopening of the Swinging Bridge, Mongaup and Rio Reservoirs (P-10482, P-10481, P-9690) Letter
July 19, 2007 - Commission Approves Reopening of Swinging Bridge Reservoir (P-10482, P-10481, P-9690)
On May 5, 2005 a sinkhole was discovered in the crest of the Swinging Bridge Dam. Swinging Bridge Dam is located in the state of New York. An investigative program commenced to assess the condition of the dam, to determine the cause of the sinkhole, and to determine actions necessary to correct the problem. FERC has required the licensee, to retain the services of a Board of Consultants (BOC) to oversee investigations and the design and the construction of remedial measures needed to correct the deficiency. With the urgency of the situation and concern for dam safety the reservoir has been drawn down to a safe level. All project remedial work was completed by mid-July 2007. The reservoir was refilled at a controlled rate while the repaired embankment was monitored. The 24th BOC report dated December 20, 2007 noted that the Swinging Bridge Dam performed as expected during the refilling stage. However, the dam is still under close monitoring and the results are periodically reviewed by the Licensee, by their consultants, by the BOC and FERC. A March 26, 2008 telephone conference did not find any adverse trends.
The Swinging Bridge Reservoir was open for recreation on July 21, 2007. A Commission letter dated April 2, 2008 approved the downstream Rio and Mongaup Reservoirs to be opened for recreational activity.
Cause of Sinkhole
- Long-term settlement of the penstock/tunnel through the dam has caused it to crack.
- Water leakage from the joints and cracks in the pipe (penstock) escaped through cracks in a hollow concrete tunnel located beneath the pipe, carrying soil with it.
- The loss of soil into the tunnel caused a cavity in the center of the dam.
- The cavity grew in size until the soil overhead collapsed, creating a sinkhole.
- The crest in the area of the sinkhole has been repaired.
- Reservoir was lowered to reduce loading on dam and reduce downstream impacts in the event of an emergency.
- With the lowered reservoir the erosion and seepage in and along the pipe and tunnel have essentially been eliminated.
- The penstock has been drained and filled with concrete to remove the source of water believed to be the cause of the problem.
- With the penstock drained, the internal erosion of the dam appears stopped.
- Presently the main sinkhole and the minor sinkholes on the upstream slope have been repaired and the dam crest restored.
Project Rehabilitation Status - Current Actions
July 28, 2007–April 3, 2008
- Filling of the Reservoir - The Final Reservoir Refill Plan, which was submitted on December 22, 2006, was accepted by the FERC on February 1, 2007. The refilling of the reservoir in accordance with the Reservoir Refill Plan began on February 2, 2007, when the dewatering well pumps were turned off. The refilling process requires gradually filling the reservoir to predetermined hold points to allow the instrumentation to stabilize and to perform dam safety assessments. During the controlled filling, the dam was closely monitored and assessed to make sure the remedial measures are effective up to the normal maximum reservoir elevation of 1070 feet. The reservoir refilling has been completed. The maximum elevation (1069.5 feet) was reached on October 13, 2007.
- The 24th BOC report dated December 20, 2007 noted that the Swinging Bridge Dam performed as expected during the refilling stage. A Surveillance and Monitoring Program (SMP) proposed by the Licensee has been reviewed by the FERC and found to be adequate. The dam is under close monitoring for another year and the results are periodically reviewed by the Licensee, by their consultants, by the BOC and FERC. A March 26, 2008 telephone conference between the above parties did not find any adverse trends.