An independent review by three engineers has concluded the U.S. Army Corps of Engineers is taking the right steps for temporary and permanent fixes to stop seepage through a damaged abutment next to the Howard Hanson Dam.
The corps plans to extend a grout curtain along the abutment this year or next year as a temporary fix, until a concrete cutoff wall can be constructed by 2014 or 2015. The dam helps protect the cities of Kent, Auburn, Renton and Tukwila from Green River flooding. Crews need to fix the leak in order to prevent the dam from failing.
“The most reliable long-term solution would be a concrete cutoff wall in a portion of the right abutment,” according to the 117-page report by engineers from Ohio-based Battelle Memorial Institute that the corps released Wednesday.
“We wanted to know if we are challenging the risk correctly and acting urgently enough and they said yes to that,” said Eric Halpin, a corps special assistant for dam safety, during a media teleconference call Wednesday.
The corps still needs to acquire federal funds for the proposed $44 million project that would add 650 feet in length to the grout curtain installed last fall to help stop a leak in the right abutment. The leak showed up after a heavy rainstorm in January 2009.
But the independent engineers agreed with the corps that while the grout curtain can be installed quicker, the curtain would not work as well as a concrete cutoff wall to stop seepage through the abutment.
The corps expects to have the design for a concrete cutoff barrier finished by the end of June in order to apply for funding from Congress in 2012 to start the project for the permanent fix.
Lt. Col. James Rollins, deputy commander of Howard Hanson Dam for the corps, said during the teleconference that a concrete cutoff wall has worked to stop leaks at other earthen dams across the nation.
“Construction (of a concrete wall) could start as early as the end of next year,” said Rollins, if Congress approves funding for the project in the 2012 fiscal year. “It would take an estimated two to three years to construct.”
Steve Poulos, one of the engineers that visited the dam and analyzed reports as part of the independent review, said the corps is constructing a concrete cutoff wall at the Clearwater Dam in Missouri to stop a similar seepage problem faced at the Hanson Dam.
“There has probably been about 20 others,” Poulos said during the teleconference about leaks at earthen dams across the nation.
Halpin said about 80 percent of the dams in the nation are earthen structures where internal erosion looms as the most dominant risk for failure of those dams.
Although the report concluded seepage through the right abutment ranks as the highest risk to failure of the Hanson Dam, it also states that the corps needs to evaluate the potential impact of an earthquake on the stability of the abutment.
The corps will conduct seismic analysis to determine potential damage to the abutment by an earthquake, Rollins said.
The abutment was formed nearly 10,000 years ago by a landslide. The federal government built the rock-and earth-fill Hanson Dam in 1961 next to the abutment to control major flooding in the Green River Valley. The dam is about 25 miles east of Kent.
The risk of flooding this winter sat at a 1 in 33 chance because the leak in the abutment reduced the storage capacity behind the dam. The extended grout curtain could return the odds of flooding in the Green River Valley to the 1 in 140 chance when the dam operates at full capacity.