The Army Corps of Engineers is making significant inroads repairing Howard Hanson Dam.

So much in fact that Col. Anthony Wright, the Corps’ Seattle District commander, told local officials Monday that the flood risk to the Green River Valley has been reduced – from a 1-in-30 chance last winter to a 1-in-60 chance this year.

Wright indicated that emergency repairs made last year to the dam have reduced the chance the Green River will flood low-lying areas of Kent, Renton, Auburn and Tukwila.

The Corps determined that the most effective risk-reduction measures to construct at the would be to rehabilitate the existing drainage tunnel, add a branch to that tunnel through the area of highest concern to intercept seepage and to install additional vertical and horizontal drains to channel more water from the abutment into the tunnel.

Once these measures are in place, the Corps believes that the dam could be operated at full capacity.

Funding recently approved for the Corps will allow construction to begin before the end of the year. The federal government approved $44 million in repairs as part of a $59 billion emergency supplemental spending bill approved by Congress and signed July 29 by President Obama.

“We’ve been working steadily trying to determine the best method to restore Howard Hanson Dam back to its design capacity,” Wright said. “We believe we have the best solution based around a series of drains that controls the water inside the embankment and the abutment of the reservoir.”

Wright said the ability to control water through drains and tunnels will keep the embankment from internal erosion and the potential collapse of the dam.

“After completion of these drains, I believe I can operate the dam at its full design capacity,” Wright said

The Corps looked at extending a previously constructed grout curtain to address the Corps’ concerns about seepage through the dam’s north abutment. But engineers and scientists found after thorough analysis that rehabilitating the existing tunnel and a tunnel extension would be more effective and durable than a grout curtain extension.

The Corps of Engineers constructed an interim seepage barrier (grout curtain) in 2009 to reduce seepage and improved the drainage of the right abutment by installing drains that more effectively direct seepage into the drainage tunnel. Data collected from testing this spring have shown these measures to be effective in controlling seepage through the abutment.

This spring, the Corps was able to hold a pool of 1,167 feet above sea level (48 percent of full) for more than two months without seeing any problems. This has provided additional confidence in the storage capacity of the dam for this coming flood season.

The reservoir at the Dam reached 1,170 feet in elevation and was held at an elevation at or above 1,167 feet May 9 through July 12, allowing engineers to test the interim seepage barrier with additional reservoir volume. During the conservation pool, the Corps conducted in-depth investigations of the dam’s current effectiveness.