A dam on the Green River in western Washington state remains one of the nation’s most unsafe, despite $15 million of work that has been completed over the past year. But the Army Corps of Engineers told local officials on March 18 that an additional $44 million in upgrades is needed to lower the structure’s failure risk to acceptable levels.

Photo: Seattle District, U.S. Army Corps of Engineers

Leaks in the Hanson dam landed it on the Corps’ danger list. The Corp says it needs a $44-million repair.

“We can manage the risk at the dam, but doing so puts those downriver at risk. That’s what makes this dam unique,” says Mamie Brouwer, program manager for the Howard A. Hanson Dam.

The dam was opened in 1962 to ease flooding in the Green River valley. But it is one of a handful in the U.S. that are classified at the highest risk because of the potential loss of life combined with the economic consequences should the dam fail. The Hanson dam is one of 12 Corps dam projects the agency has classified “urgent and compelling and unsafe.”

In January 2009, after heavy rains and a high-water event in the reservoir, officials in the Corps’ Seattle District noticed seepage and depressions in an abutment of the earthen-engineered dam. The Corps immediately lowered the reservoir and initiated improvements to shore up the abutment, which is the result of a 10,000-year-old landslide.

With money from the American Recovery and Reinvestment Act, the Corps granted a $13-million contract to Nicholson Construction, Cuddy, Pa., to install a 450-ft-long grout wall, consisting of two 10-ft-thick walls. An additional $1-million contract was awarded to Jensen Drilling, Eugene, Ore., to improve a drainage tunnel in the abutment.

The dam is critical to industry downstream on the Green River. Since the earthen dam was built, local residents have become accustomed to living and operating in the floodplain without risk, says Dana Hinman, a spokeswoman for the city of Auburn, Wash. The area, near Tacoma and southwest of Seattle, has grown into one of the nation’s largest warehouse and goods distribution centers, with an estimated $4-billion value.

Even with the fixes finished last month, the area faces the risk of flooding of up to 10 feet in a one-in-25-year storm event. The planned $44-million project would provide protection for up to a one-in-140-year storm event by extending the grout wall 650 ft to the east and deepening the entire 1,100-ft wall another 200 ft. It is now from 90 ft to 170 ft deep. A Corps official says the agency’s report and recommended solutions will be presented to Congress by June. If funding for the additional work is approved, the job will be let for competitive bid. A permanent fix, likely to include a concrete cutoff wall and possibly costing up to $500 million, is now being studied.