Hanson Dam s mysteries solved

Transcription

1 Hanson Dam s mysteries solved Hanson: Earth dams are expected to leak MIKE ARCHBOLD; STAFF WRITER STAFF WRITER LAST UPDATED: APRIL 27TH, :34 AM (PDT) 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 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. STORM-FED RESERVOIR A storm laden with heavy rains roared into the Puget Sound area in early January 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.

2 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. Wright even climbed down into one of the depressions to get a firsthand look. He wasn t pleased with what he found. Engineers soon discovered muddy water flowing from the drainage tunnel, an indication of possibly serious trouble in the embankment. Had the dam sprung a leak from a phenomenon called piping that had weakened other earthen dams across the country and led to their catastrophic failures? The Corps launched an all-out effort to prepare Green River Valley residents for the potential of major flooding. Levees were repaired and raised; sandbags appeared around homes and businesses; evacuation plans were drawn up. Meanwhile, the engineers and other scientists went to work to figure out what was going wrong. DETECTIVE WORK Richard Smith, an engineering geologist in the Corps Seattle District office, was a member of the core team of 15 engineers and scientists the team at times numbered up to 85 who investigated the dam. Members poked and prodded the embankment with sensitive monitors and instruments. They dug into the abutment and brought up core samples of the material to better understand its composition. Smith, who was trained in rock mechanics and groundwater seepage, said recently that he felt like a forensic detective as he studied decades-old construction documents and handwritten notes to see if they shed light on what was ailing the dam. It was while researching the documents that Smith and the others solved the mystery of the depressions.

3 In 1956, dam designers called for two shafts 38 to 55 feet long 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, Smith said. They collapsed under the weight of the record-high water pooling behind the dam during the 2009 storm, he said. The depressions turned out not to be dangerous to the dam, he explained. They were relatively small and short and didn t affect the strength of the abutment. Other scientists concentrated on the problem of the muddy water. They worried that it might be the result of a phenomenon called hydraulic piping, where seeping water opens a hole on the downstream side of the dam. Over time the seepage can gather speed and the hole or pipe grows upstream toward the reservoir. As it grows, it expands, and, when large enough, can collapse from above. That s what led to the collapse of the Teton Dam near Rexburg, Idaho, on June 5, Eleven people drowned when the dam failed as it was being filled for the first time. The cause of the muddy water flowing at Howard Hanson was not that serious. The 11 vertical drains that divert seepage into a drainage tunnel were built without filter material to stop sediment from entering the drain along with the ground water. The immediate solution to that problem turned out to be relatively easy. Engineers simply plugged one of the 11 drain pipes, and the water coming out of the dam s drainage tunnel began flowing clear again. That was only a temporary fix, Smith said. The long-term solution requires installing more vertical drains and making sure all drains, new and old, are properly filtered.

4 Although the depressions and the hydraulic piping turned out not to be the disasters engineers feared, Smith said the grout curtain, or injected concrete barrier that the Corps installed in 2009, still was necessary and not a waste of money. Parts of the surface between the abutment and the dam had never been properly covered, Smith said, and the grout curtain protected them from erosion. THINKING AHEAD The events two years ago also prompted long-range planning for the dam. A study is under way with results expected in It will recommend a risk-reduction plan for the dam and will look at structural and nonstructural alternatives. Some of the study s preliminary conclusions are that a full concrete wall along the quarter-mile-wide right abutment is not needed. More filtered drainage pipes are necessary and are being installed. Extending the drain tunnel still is under discussion. The dam has the attention of the Corps not only in Seattle, but also in Washington, D.C., according to district spokeswoman Casondra Brewster. What happened in January 2009 pushed Howard Hanson higher on the funding list of projects nationally, she said recently. Dams like Howard Hanson are designed for a 50-year lifespan, Brewster said. What is happening now, she said, is a reinvestment for the next 50 years. Fortunately, Mother Nature hasn t tested the dam since The winter was warm and dry, and this past flood season brought a lot of rain but not to the Green River basin. By fall, Brewster said, the dam is expected to be back to its authorized operational capacity before the holes and muddy water appeared.

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