An abutment that anchors one side of Howard Hanson Dam on the Green River has leaked for decades. Engineers have monitored it, worried about it, debated what to do with it and repeatedly made improvements to control it. A leak detected in January prompted the U.S. Army Corps of Engineers to reclassify the dam as...
In 1949, the Army Corps of Engineers warned that water might seep through the north abutment of a dam being contemplated on the Green River, but the leak “would be negligible and could be easily controlled.”
The abutment — a hillside that anchors one side of the Howard Hanson Dam — has leaked almost since the dam began operating in 1961. Engineers have monitored it, worried about it, debated what to do with it and made a series of improvements to control it.
The latest leak, detected after a huge rainstorm last January filled the reservoir higher than ever, prompted the corps to reclassify Hanson as “unsafe” with an “urgent and compelling” need for immediate action. Only 10 of the corps’ 650-plus dams fall into that high-risk category.
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“I know they keep saying the dam is not in danger,” said Doug Johnson, Dam Safety Office leader in the state Department of Ecology. “That’s true, it’s not. But a failure of the abutment could be as catastrophic or nearly as much as the failure of the dam.”
Contractors are working feverishly to staunch the leak while the corps designs a more permanent fix — most likely a concrete wall to seal the abutment once and for all.
But until the abutment is shored up, the corps will store less water behind the earth-and-rock dam, meaning that in a big storm it might have to release enough water to overtop downstream levees in Kent, Renton, Auburn and Tukwila.
The chance of a major flood this fall or winter is as high as one in three. That has local governments scrambling to move critical operations out of the flood plain, beef up levees, and call on businesses and the 26,000 residents downstream to prepare for the worst.
Seepage through a dam or abutment isn’t necessarily a problem.
But if dirt and rock begin to wash away — as may have happened in January — the result could be catastrophic.
Failure of the dam would send a wall of water careening toward homes, schools, warehouses and factories. Six cities would flood, from Pacific to Renton, with peak depths of some 20 feet in downtown Auburn.
“Public safety is the number one priority,” said Mamie Brouwer, the Hanson Dam program manager. “We’ve never lost a dam — the corps hasn’t — and we don’t intend to lose one now.”
Choosing site for dam
Before the dam was built, floods were routine in the Green River Valley, where early pea and lettuce farmers built their homes on stilts. During a 1946 flood, the Army was called in to rescue people and livestock.
Local and state officials — most notably Seattle attorney, state legislator and Seattle Chamber of Commerce activist Howard Hanson — prodded the federal government to build a dam that would protect the valley and foster economic development.
The Corps of Engineers studied possible sites in the 1930s and ’40s, recommending in 1949 a dam be built 33 miles upstream from Auburn. Eagle Gorge had the right shape for a dam and would do less damage to declining salmon runs and to a Tacoma municipal water intake than two alternative sites downstream, the corps noted.
The dam’s primary role is flood control. Water also is released in the summer to help salmon, and the dam supplies municipal water systems in Tacoma and parts of South King County.
The dam, nestled in a forested mountain watershed, is 235 feet high, 450 feet long and 960 feet thick at the base. Its connection to the bedrock foundation and bedrock left abutment appear solid. The right abutment, on the north side, is another matter.
The sand, gravel, rock and boulders that compose the north abutment were deposited there some 10,000 years ago when a mountainside slid into the Green River.
A 1949 report by the corps said water might seep through the hillside if a dam were built there, but it shouldn’t cause serious problems.
Brouwer called the siting decision “a calculated risk on the early engineers’ side.” Because the slide material stayed in place so long, “they said, ‘Well, it’s been doing its job for 10,000 years — we think we’ll just go with it.’ “
When the corps was designing the dam in 1956, consulting engineer James Growdon wrote that Eagle Gorge “is not a particularly good site for a dam and reservoir. However, it is reported to be the best site available on the Green River.”
To control seepage, Growdon recommended building a drainage tunnel, covering the reservoir side of the abutment with impermeable material, and digging a trench in the abutment that would be filled with impermeable material.
District Engineer Col. R.J.B. Page rejected the tunnel and trench, and opted for a more permeable sand and gravel blanket on the abutment. If seepage occurred, he wrote, it could be dealt with “when the problem arises. … In any event, there appears to be no danger of a failure occurring in the right abutment which would affect the dam.”
1965 storm damage
Problems became apparent during a 1965 storm, when water suddenly gushed from the north abutment downstream from the dam. Concerned the spring could cause the hill to slide, the corps covered it with gravel, held in place by a wood structure.
The agency began taking a closer look at seepage and ways to keep it from weakening the abutment.
Horizontal drains were drilled in 1967, but they didn’t work and were abandoned later. The corps in 1968 installed a 640-foot-long drainage tunnel fed by smaller wells and drains to carry off excess water within the abutment.
That didn’t solve the problem. Engineers saw “significant seepage” from the lower abutment and sediment-clouded water in the drainage tunnel in late 1995 and early 1996.
In 2002, in an effort to control leaks, the corps built a “grout curtain” within the abutment. Workers drilled a line of vertical holes deep into the abutment and pumped in cement to fill gaps in the rock and sand. But the cement was too thin to set up in place.
“Some locations just took and took and took grout,” Brouwer said. “It just took grout, and we don’t know where it went.”
The result: The seeping continued.
Alarm after heavy rain
Last January, a few days after a storm dumped 15 inches of rain in the upper Green River Valley in a day, workers noticed water in the drainage tunnel had turned brown with dirt. Groundwater levels in the abutment rose surprisingly high.
A few days later, concern turned to alarm when Dam Safety Project Manager Rob Romocki found a 10-foot-wide, 6-foot-deep depression in the abutment near the dam. Was water washing soil away within the abutment?
The corps began lowering the reservoir as quickly as possible.
It turned out the depression was caused by the collapse of an old tunnel dug before the dam was built, not by piping — internal erosion caused by a leak that can lead to catastrophic failure.
Teton Dam disaster
Piping is the last thing the corps wants to see.
In 1976, piping in an abutment of the federal Bureau of Reclamation’s Teton Dam in Idaho led to a collapse two days after seepage was seen. Eleven people died in the disaster.
Two days after the depression was discovered at Hanson Dam, scientists released dye on the reservoir side of the abutment, expecting it to take at least 11 hours to seep through to the drainage tunnel. It reached the tunnel in less than five hours.
The corps cautiously began raising the pool last spring to see what would happen at various levels. Above a certain point, the water flowed into the tunnel at a surprisingly fast rate.
And when scientists introduced dye, it reached the tunnel in only 90 minutes — suggesting that piping might be occurring somewhere in the abutment. Problems were most severe near the dam, along what engineers call “the short path” of flow from the reservoir to the river.
The district commander, Col. Anthony Wright, ordered the reservoir lowered again.
A contractor now is installing a double-thick grout curtain along both sides of the old curtain. Scheduled to be finished by Nov. 1, the $8.9 million curtain will be longer and provide a more solid barrier.
Tests now under way will give a partial indication of the curtain’s effectiveness and help Wright decide how high he will allow the reservoir to rise this winter.
Meanwhile, the corps has begun a three- to five-year process of designing and building a permanent fix — likely a concrete wall up to 2,000 feet long and 350 feet deep built within the abutment — at a cost of hundreds of millions of dollars.
If a dam were built today at Eagle Gorge, the corps probably would install a concrete wall at the outset, Romocki said.
Even though the site isn’t ideal, he said, “You can engineer anything. As an engineer, that’s the way we think. There is no problem too big.
“We will come up with a fix.”
Keith Ervin: 206-464-2105 or email@example.com