Contractors finished the last of the scheduled dam safety construction at Howard Hanson Dam in August, making improvements to the log boom and adding rock to the upstream embankment slope.
The dam received many upgrades and repairs in the past few years after seepage issues were observed in the right abutment during a flood in January 2009. Most of the construction was completed by 2011.
"In 2009, immediately after we made the improvements, further analysis indicated we needed to reduce the risk further to acceptable levels," said Richard Smith, U.S. Army Corps of Engineers, Seattle District, dam safety program manager. "As part of the Corps’ dam safety risk-based approach, a comprehensive review of the entire project was conducted and additional measures were identified that needed to be implemented."
The existing log boom needed upgrading because it was anchored at the 1,206 foot elevation. This was too low to keep debris from clogging the spillway in a 10,000-year flood event. The upgraded log booms are anchored higher up on the slope and more robust. A secondary boom was also installed to provide more protection.
"There’s a lot of woody debris in the reservoir that could easily clog up the spillway," Smith said. "Without a log boom that could rise with higher pool elevations, the woody debris from heavily forested areas present a hazard, potentially impacting our ability to manage water."
Another upgrade completed earlier this year was replacing the rock on the upstream face of the dam for better slope protection.
"Though an epic flood is unlikely, we didn’t want to risk erosion of the dam embankment, which could lead to failure," said Mamie Brouwer, Howard Hanson Dam project manager. "We replaced the degraded, weathered rocks with larger, more-durable riprap designed to resist the higher flow velocities like those expected in a 10,000-year event."
The repairs and additional safety measures reduce the risk of dam failure, and the downstream community once again has the same level of protection as when the dam was originally designed.
"After we discovered the issues in 2009, the Corps and local governments worked very closely to ensure people living downstream understood they are vulnerable," said Brouwer. "The dam is fixed and is operating to its designed capability, however it was only designed to reduce the risk of flooding and its impact; there are still risks associated with living in a flood plain. With that in mind, we expect the dam to protect the valley for decades in the future, the same way it has since it was built."