Water is seeping more rapidly through a flank of the Howard Hanson Dam, and the Army Corps of Engineers doesn't know why.

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The speed at which water is seeping through a flank of the Howard Hanson Dam has, by one key measure, increased since January, and the people who operate the dam don’t know why.

Officials with the Army Corps of Engineers said in a news release Tuesday that water is flowing through the 48-year-old dam’s right abutment “very fast” and may mean the earth-and-rock structure could erode if too much water is stored behind the dam 32 miles upstream from Auburn.

Nobody’s saying there will be large-scale floods for the first time since the dam was built, but the weakness in the dam abutment — the side of the valley against which the dam was built — means the Corps of Engineers may have to severely restrict how much stormwater the dam can hold back for the next several winters.

And that could mean more water flowing through the valley below, raising the risk of flooding for the cities of Kent, Renton, Tukwila and Auburn.

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Built in 1961, the Hanson Dam transformed the Green River Valley below it. Once an agricultural area that routinely flooded during heavy storms, the valley became one of the nation’s largest warehouse districts, along with thousands of homes, stores, factories, hotels and restaurants.

Seepage through the dam’s right abutment has caused concerns for much of its life. Improvements were made in 1965 and again in 2002, but engineers are worried the problem may have worsened.

The Corps of Engineers publicly acknowledged new concerns about the stability of the dam abutment in January, when staffers found several “anomalies” during a storm that dumped 15 inches of rain in 24 hours behind the dam.

Surprising readings at water-pressure gauges; fast-flowing, muddy water in a drainage well; and a dam-safety engineer’s discovery of a sinkholelike depression suggested water was moving through the abutment too fast and possibly taking soil with it. The reservoir, at its highest level ever, was quickly drawn down to a lower, safer level.

The Corps is, again, in the process of lowering the reservoir. The Corps said in its news release that the water won’t be released fast enough to endanger swimmers, boaters and other people on the river.

Meanwhile, cities in the Green River Valley already have been bracing for what could be the first massive flooding there since the 1950s. Officials in Kent, Renton, Tukwila and Auburn have been making flood-contingency plans since January.

“It confirms what we’ve already known, that we need to be extra alert — and hope that it doesn’t rain so much this fall,” said Kent city spokeswoman Michele Witham.

“For us there’s no real change. We did not expect any less,” said Auburn Mayor Pete Lewis.

Tukwila Public Works Director Jim Morrow said, “We have been assuming all along that there is a very high potential that we’ll see flooding come the winter. … This latest announcement in no way changes our approach to the problem nor does it in any way give us pause that we may be going in a different direction.”

As the Corps cautiously raised the water level in the reservoir behind the dam in recent weeks — while keeping a close eye for any abnormalities — things seemed normal by most measures.

But in the past few days, results of a dye test came back with surprising results. A small amount of dye moved through the abutment into a drainage tunnel far faster than it had during a January test after the storm.

Asked why, Hanson Dam program manager Mamie Brouwer replied, “That’s what we don’t understand yet.”

More dye-test results will be done in the coming weeks, and contractors are currently drilling into the abutment to help engineers understand what’s happening.

By November the Corps will install a “grout curtain” to reduce seepage, and will drill more vertical and horizontal drains. The measures are expected to cost more than $20 million.

Planning will then begin on a permanent solution, which is likely several years away. Dam-safety experts from around the country are advising the Corps on those interim measures and possible long-term solutions.

Bolstering the right abutment and determining how much water the dam can hold next winter are among the Corps of Engineers’ top priorities, district officials say.

“I can’t stress enough our No. 1 mission here is public safety,” Seattle District Commander Anthony Wright said in a statement Tuesday. But in order to avoid the possibility of catastrophic dam failure, safety may mean — depending on rainfall — releasing so much water from the dam that the valley floods once again.

Keith Ervin: 206-464-2105 or kervin@seattletimes.com

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