As part of our routine monitoring efforts, EWEB conducted a bathymetric survey of Trail Bridge Reservoir in May 2021. The reservoir is one of three that make up our Carmen-Smith Hydroelectric Project located about 70 miles east of Eugene off of Highway 126 near the headwaters of the McKenzie River.
The results of the survey, which is essentially a map of the topographic features of the reservoir bottom, indicated three "depressions" that required further investigation. One of the depressions was apparent during re-examination of a similar survey performed in 2010.
EWEB conducted follow-up inspections with a remotely operated underwater vehicle along with dye testing by divers in early June 2021. Based on the dye tests, the two larger depressions are actively taking water and are considered sinkholes. Subsequent dye testing and geophysical investigations in July and August indicated that there is no concentrated seepage flow through or under Trail Bridge Dam.
Public safety is EWEB's top priority, and staff are working diligently to manage the safety risk to the public downstream of the dam while actively investigating sinkhole conditions and cause. EWEB's Dam Safety team, along with Generation Operations and Engineering, have been working closely with experienced consultants and the Division of Dam Inspection and Safety of the Federal Energy Regulatory Commission to address the issue and develop a risk mitigation plan.
We have implemented several risk-reduction measures to date, including lowering the Trail Bridge Reservoir level approximately 7 feet below the normal operational elevation, increasing monitoring and surveillance at the dam and downstream, and ensuring effective notification and communication with Emergency Action Plan (EAP) partners.
The decision to lower the reservoir level was made out of an abundance of caution.
EWEB has multiple monitoring devices in place to detect anomalies or any unexpected changes. These include stream flow gauges, reservoir level sensors, groundwater level sensors, turbidity monitoring sensors, remote video cameras with night vision capability and seepage weirs at the toe of Trail Bridge Dam.
"Based on our understanding and the data we have gathered so far, the risk of a catastrophic dam failure is low," said Daniel Huang, EWEB's Chief Dam Safety Engineer. "We will continue our investigation and evaluation, and that could take a year or longer."
Carmen-Smith duty operators conduct daily inspections of the dam, and can further lower the reservoir if any anomalies are detected visually or by the monitoring equipment.
There are plans in place to notify the public, public safety agencies and the U.S. Forest Service should the situation change. There is also a siren at Olallie Campground, downstream of the dam, that EWEB staff can activate should the status of the dam change. The campground was closed in August for the remainder of 2021 due to its close proximity to the Knoll Fire.
The steps EWEB has taken with the increased monitoring and lowering the reservoir level by 7 feet reduce the likelihood of a dam failure. Staff, including engineers, are confident they can quickly intervene if anomalies are detected, which further lowers the likelihood of dam failure.
We will continue to monitor and assess the sinkholes to determine the cause and potential remedies.
Anyone with questions or concerns can contact Joe Harwood at joe.harwood@eweb.org.
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