Weather permitting, the U.S. Army Corps of Engineers could begin this month on the annual spring fill of the pool behind Howard Hanson Dam, a critical step toward measuring the effectiveness of interim flood control measures instituted after the flooding of January 2009.

“I don’t think we have determined a date for the pool raise,” said Patricia Graesser, public affairs supervisor for the Seattle District of the U.S. Army Corps of Engineers in Seattle. “I know that we have a plan that needs to be approved for the pool. We are planning on doing that this spring, but we don’t have an exact date yet.”

The January 2009 flooding opened up two holes in the face of the right abutment of the dam, a natural support structure made of 20,000-year-old landslide material. The appearance of those depressions and the simultaneous appearance of rock and dirt in the drainage tunnels underneath prompted concerns about the integrity of the abutment and finally of the dam it supports.

Those concerns are responsible for the sandbags and barriers that now line the entire Green River from Auburn through Renton. Those flood-control measures significantly lowered the risk of flooding in the Green River Valley during the rainy season.

Filling the pool is essential to test the effectiveness of the seepage barrier, or grout curtain, that was completed on the abutment last November.

“We have done additional grouting beyond what was completed in November, and we’ll be doing an analysis of how effective that grout curtain is,” Graesser said. “We also did some additional drilling to get a better idea of the geology. Fortunately for us, we had a very mild winter, so we didn’t have a pool elevation at all. The highest we had it was up probably 10 feet. We didn’t see any troubling signs this winter. Spring will give us a little more information.”

U.S. Sen. Patty Murray has asked that Col. Anthony Wright, commander of the Seattle District of the Army Corps of Engineers, and Brig. Gen. McMahon of the Seattle District to Washington D.C., to provide information about the dam.

The corps is in the study-and-design phase of a permanent solution that could include a barrier wall that that would restore the dam to its full capacity.

“Everything we said in November is still true, that the grout curtain in our option has improved the odds from 1 in 3 to 1 in 32. That’s a much higher risk than is acceptable,” Graesser said. We working toward a study to determine the best alternative and design of that alternative and construction. We are still saying it could be three to seven years out.”

Graesser said the corps will release additional information on the Howard Hanson Dam March 18.