Dam flood risk subsiding

The U.S. Army Corps of Engineers is becoming more confident in the ability of Howard Hanson Dam to control floodwater on the Green River this winter.

Col. Anthony Wright , the Corps’ Seattle District commander, told local officials Monday that the risk of a flood that would overtop levies in South King County communities is now 1 in 60, down from 1 in 30 last winter.

And by next winter, he said, the dam should be back at its planned flood risk – 1 in 140 – with the construction of a series of filtered drains inside the dam’s right abutment, as well as other improvements to start this fall.

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

Wright called the risk-assessment figure simplistic but said it’s based on a lot of work done by the Corps since the weakening of the dam was discovered following heavy rains that filled the dam reservoir in January 2009.

The federal money for the work is on its way, he said. Late last month, President Obama approved $44 million for interim measures to continue strengthening the dam.

Work on the drain pipes, estimated to cost about $15 million, will begin this fall, he said.

Additional log booms will be installed in the reservoir this fall to prevent debris from blocking the spillway, rock will be placed in key places to protect the dam from erosion and the spillway will be anchored to the bedrock.

The other major corrective action is to build a 200-foot extension of the existing drainage tunnel inside the right abutment at a cost of about $19 million. That project won’t begin until next year and will be completed by mid-2012.

He lauded the downstream communities of Kent, Auburn, Renton and Tukwila for their preparations last fall, which must be continued this winter.

Unlike last fall, when Wright set a low maximum reservoir level that he felt the dam could hold, Wright said he would determine this winter’s level based on real-time data.

Currently there are 10 times more instruments recording what is happening inside the dam than there were two years ago.

“We have a much more informed idea of water moving through the dam,” Wright said.

He said Corps officials raised the reservoir this past summer to 1,170 feet above sea level and kept it there for weeks without any serious problems.

Fortunately, last winter was a warm El Niño season that brought abnormally low rainfall and flood danger to the Green River Basin.

This winter, he said, is forecast to be a colder La Niña winter that usually brings more rain and storms to the Pacific Northwest.

There should be “no victory dances on the levies” yet, he said.

After the briefing, Renton Mayor Denis Law said what he heard was “good news. It shows the efforts that have been made have improved our chances of being able to hold more water (at the dam) and reduce the risk of flooding downstream.”

It’s smart, however, to keep sandbags in place for the coming winter, he said.

“People need to be vigilant about it; keep their flood insurance up; have a flood plan in place,” Law said.

Wright also explained that ideas about fixing the dam permanently have shifted from simply installing walls to also improving and monitoring the dam’s drainage system.

A plan for a long-term, permanent fix for the dam is set to be completed in November.