Since early spring the U.S. Army Corps of Engineers has been testing the troubled right abutment at Howard Hanson while it raises the level of the conservation pool.

But with new discoveries and observations at higher levels in the reservoir pool, Seattle district corps engineers now fret that the abutment is showing signs of internal erosion.

Last week, the pool reached an elevation of 1,169.2 feet, and engineers saw fresh evidence that the abutment is still weakened.

Engineers are particularly concerned about recent dye testing that shows water swiftly moving through the right abutment when the pool is higher. These tests indicate that instead of meandering through the abutment as it should, the water has established flow paths within the natural materials that could lead to internal erosion if water is held at these higher elevations for long periods of time.

“This phenomenon continues to be troubling,” said Mamie Brouwer, Program Manager, but she added that no visible distress of the right abutment has been observed.

Meanwhile Seattle District Commander Col. Anthony O. Wright wants downstream community residents to know that the risk for higher flood levels is significantly increased compared to what they are typically used to, and will remain so until issues with the dam’s right abutment can be resolved.

“I can’t stress enough our number-one 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.”

“We hope that the people of the Green River Valley are getting themselves prepared for any contingency this fall,” said corps Spokeswoman Cassandra Brewster. “At this point in time, we know that the operational capacity of the Howard Hanson dam will not be at full capacity, though in fact we have not seen it at full capacity in its history. It still has a problem, but we need to see how it manifests and how to fix it.”

Brewster emphasized the corps’ concern is for a major flooding event.

“We’re talking about …when we go into flood operation, not the spurts here and there,” Brewster said. “If we get hit hard, people need to be keyed in with the City and emergency management and know what to do.”

“What this really says is they have been doing tests to see if filling up the depressions made a difference and, nah, it didn’t make a difference,” said Auburn Mayor Pete Lewis. “What we’re getting from them is ‘get ready.’ But I will be jumping up and down for a dryer winter.”

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.

For this reason, the corps has decided to release water from the dam over the course of the next two weeks, to bring the pool level down to 1,155 feet above sea level. This operation will result in reservoir outflows that are not expected to exceed 1,200 cubic feet per second based on current weather and hydrologic forecasts, well within established thresholds. Residents along the river will not see significant changes in the river flows over the next two weeks.

Around-the-clock monitoring and other data-collection processes will continue during the pool draft.

“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, and continuing to monitor the pool throughout the draft. At the same time, the team has initiated test borings as well as geotechnical modeling and analysis to support planning for a long term repair project to address seepage.

“Regardless of the possibility of increased flow rates from the dam,” said Dam Safety Program Manager, Rob Romocki, “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. So, it’s easy to see the risk increasing when the dam has a lowered operational capacity.”

Information on flood preparedness may be found at: http://www.kingcounty.gov/safety/prepare/FloodPlan_GRiverBasin.aspx

Updates regarding the dam may be found at: http://www.nws.usace.army.mil.