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New flooding concerns surface at Howard Hanson Dam
Troubles continue to plague Howard Hanson Dam, a crucial structure built on the Green River northeast of Enumclaw that provides flood protection for valley cities like Auburn, Kent and Renton.
Engineers are concerned that the right abutment is showing signs of internal erosion.
The U.S. Army Corps of Engineers has been in the midst of testing during the traditional summertime rise in the pool behind the dam. The pool reached an elevation last week of 1,169.2 feet above sea level and engineers saw renewed reasons to believe that the abutment is still weakened.
Of particular concern is the recent dye testing that shows that water is moving through the right abutment very fast at higher pool elevations. These tests indicate that there are flows within the natural materials of the right abutment that could lead to internal erosion within the right abutment if water is held at higher elevations for extended periods of time.
“This phenomenon continues to be troubling,” Program Manager Mamie Brouwer said. However, there is no visual distress of the right abutment, she said.
Although data collected below 1,157 feet seems to stay within normal ranges, elevations going higher seem to cause more seepage and uncertain water paths through the abutment, according to a Corps of Engineers press release.
Therefore, the Corps decided to release water from the dam during a two-week period, to bring the pool level down to 1,155 feet above sea level. Residents along the river will not see significant changes in the river flows, according to information released by the Corps.
“We do not understand how the water is traveling through the abutment,” Brouwer said. “We know that what we may be seeing fits the traditional definition of internal erosion.”
The Seattle District’s immediate objective is preparing for the upcoming fall/winter flood season. Preparations include continuing with plans to construct an interim seepage barrier wall and improving the drainage tunnel to control seepage through the most critical part of the right abutment. Simultaneously, the Corps has initiated test borings, geotechnical modeling and analysis to to help plan for long-term repairs.
The Corps’ Seattle District commander, Col. Anthony O. Wright, wants downstream residents to know that the risk for higher flood levels is significantly increased compared to what they are typically used to, until the issues with the dam’s right abutment can be resolved.
“I can’t stress enough our No. 1 mission here is public safety,” Wright said. “We will continue to keep Green River Valley leaders and first responders informed. We ask that residents contact their community leaders and get as much information as possible on how to prepare for such a contingency.”
Wright was referring to the Corps’ efforts of working closely with King County and the downstream cities of Auburn, Kent, Renton and Tukwila to prepare for the fall/winter flood season with the increased risk of higher-than-standard flows from the dam.
Dam Safety Program Manager Rob Romocki reminds residents that flood dangers exist no matter what occurs at the dam.
“It truly is important for the communities to understand that this is a river valley and the potential for flooding exists even when Howard Hanson Dam has full operational capacity and the levees work as they should,” he said. “So, it’s easy to see the risk increasing when the dam has a lowered operational capacity.”