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.
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.
- Seattle Times article, June 24, 2009
