Projects
Swinging Bridge Dam (P-10482) Dam Safety Remediation Project
Reservoir Drawdown as Interim Safety Measure
During the public meeting held on May 25, 2005, FERC representatives discussed the necessity to lower the reservoir behind Swinging Bridge Dam to provide adequate safety until engineers can assess the integrity and safety of the dam and complete necessary repairs. At that time, the reservoir was in the process of being lowered and a decision would be made as to the final extent of lowering required for safety.
During the first Board of Consultants (BOC) meeting held at the site on June 2, 2005, the BOC concluded that the water surface elevation of the Swinging Bridge reservoir should be lowered an additional 17 feet to elevation 1030 feet and the downstream Mongaup and Rio reservoirs should be lowered 10 feet below the spillway crest of those dams. During the second BOC meeting held June 16, 2005 it was agreed to lower the reservoir even below the 1030 foot level as hydraulics would permit.
The additional drawdowns were proposed by Mirant's consultant engineer as a further necessary short term measure for the safety of the dam until a thorough evaluation of the dam can be completed and remediation of the dam occurs. Downstream flood studies presented at the 3rd BOC meeting on July 29 and 30, 2005 evaluated the expected impacts of relatively frequent flood events. The BOC concurred in the decision to; lower Swinging Bridge Reservoir to elevation 1020 or below.
- With Swinging Bridge Reservoir at elevation 1020 feet or lower, an unexpected sunny day failure of Swinging Bridge Dam, while still releasing substantial flows, is not expected to overtop the downstream Mongaup or Rio dams;
- A relatively minor rain event (2-year storm or greater) would still raise the Swinging Bridge Reservoir to a level that could pose a risk to the downstream Mongaup and Rio dams. The need for careful monitoring of Swinging Bridge reservoir continues throughout the remediation effort;
- During the week of October 9, 2005, heavy rains raised the Swinging Bridge Lake back to elevation 1036.3 feet. By use of the Unit #2 turbine, the reservoir was pulled back down to elevation 1025; and
- On July 25, 2007, the Swinging Bridge Reservoir level was recorded at elevation 1057.14 feet. The downstream reservoirs levels of Mongaup and Rio were about 926.52 feet and 800.8 feet, respectively.
The necessary coordination with the resource agencies, the affected public and the public officials continues as long as the drawdowns are considered necessary.
|
 |
|
