Hanson Dam's mysteries solved

Green River: Engineers turn detective, find and fix threatening conditions

April 27, 2011 

Hanson Dam's mysteries solved

A mysterious sinkhole is marked with tape on the earth abutment next to Howard Hanson Dam. The depression was found after the reservoir was lowered after flooding in January 2009. This photo was taken in February of that year.


It had the makings of a doomsday scenario.

Two holes suddenly appeared in the dirt-and-rock abutment of Howard Hanson Dam after a drenching “Pineapple Express” in 2009.

Even scarier was when muddy water began flowing from the dam’s drainage tunnel – an indication that something might be terribly wrong inside the abutment.

For a few days, the problems startled U.S. Army Corps of Engineers’ dam engineers, geologists and safety officers who monitor and care for the dam, which has protected the Green River Valley for decades.

Engineers didn’t know what was causing either problem, and feared the dam might fail and send a devastating wall of water sweeping through Auburn and Kent and farther downstream.

Two years later, those who monitor the dam say they’ve solved both mysteries and begun the planning and work to ensure that the dam provides flood control for the region for years to come.

Col. Anthony Wright, commander of the Corps’ Seattle District, told downstream officials and residents that what happened was a wake-up call.

“I can’t eliminate all risk (of any flooding),” he said. “But we can get back to what it was before.”

A storm laden with heavy rains roared into the Puget Sound area in early January 2009.

It dumped enough water that operators decided to let the Eagle Gorge Reservoir behind the dam rise to prevent flooding downstream. In two days the reservoir rose to a record-high level.

When the storm ended, operators emptied the reservoir as usual.

What followed was an ominous surprise: Two depressions – one large, one small – appeared on the face of the sloping wall of the dam’s right embankment.

The Corps scrambled to learn what was happening.

Engineers soon discovered muddy water flowing from the drainage tunnel, an indication of possibly serious trouble in the embankment.

It was while researching the documents that engineers solved the mystery of the depressions.

In 1956, dam designers called for two shafts to be dug into the ancient landslide that would form the right abutment of the dam. They wanted to see what was inside the landslide.

Like mine shafts, the tunnels included milled timbers for support. But the tunnels were never filled and compacted; their openings simply were covered over.

They collapsed under the weight of the record-high water pooling behind the dam during the 2009.

The muddy water also was not as serious a problem as thought. It stemmed from the fact that filtering material was not used in the 11 vertical drains that divert seepage into a drainage tunnel. Engineers plugged one of the 11 drain pipes, and the water coming out of the dam’s drainage tunnel began flowing clear again.

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