Green River Valley residents learned Thursday that the risk of flooding this winter has been reduced from 25 to 30 percent to as low as 4 percent.

The normal risk of flooding on the Green River, which is managed by the now weakened Howard Hanson Dam northeast of Auburn, is 1 percent in any given year.

Temporary repairs to the leaking right abutment at the dam have made a difference, said Col. Anthony Wright, Seattle district commander for the U.S. Army Corps of Engineers. Those repairs include installation of a grout wall 160 feet deep in the abutment and improvements to a drainage system inside the abutment.

The result, he said at a news briefing, is that he and a panel of outside experts believe the dam can hold more water for short periods than was felt prudent before the repairs were done.

Wright cautioned that no one in the South King County cities of Auburn, Kent, Renton and Tukwila should breathe a sigh of relief yet.

“I continue to urge people in the valley to work with their local emergency managers to determined the best path for being prepared for any contingency,” he said in published remarks.

In the event of a major Pineapple Express rainstorm or, in the worst case scenario, two of them back to back, Wright said the dam won’t be able to hold as much water, and outflows from the dam could exceed the capacity of the Green River levees.

The state, King County, cities, businesses and some 300,000 residents have been preparing for the worst. Raising levees, sandbagging and evacuation planning have been under way since spring. The Corps of Engineers has loaned more than 2.3 million sandbags, large and small.

Large sandbags greeted thousands of commuters and truckers Wednesday on a section of Highway 167 in the Green River Valley.

A state-hired contractor is working nights under lights this week to sandbag one-third of a mile of the freeway in Kent in the event of major flooding this winter from the Green River.

The sandbags are 3 feet high, and each holds 11/2 tons of sand. They will form a wall next to the southbound and northbound lanes just north of South 272nd Street. Concrete barriers will be installed to protect the bags from vehicles.

The highway has a low spot there and it may be the first section under water in a major flood, said Dave McCormick, Washington State Department of Transportation’s assistant regional administrator for maintenance and operations.

“If we can keep this section dry for even a few hours, we can get more people to safety,” he said.

The goal is to keep Highway 167 open in case the Green River overtops its banks in a major rainstorm. The highway handles more than 50,000 vehicles each day in both directions and would be a major evacuation route in the event of flooding.

Pat Moylan, the department’s Northwest Region maintenance manager, said his agency used the Corps of Engineer’s flooding maps and surveyed elevations in the valley to find the lowest point.

“We built the highway pretty high through the valley,” he said, adding that the sandbags are an insurance policy for the next few years.

After the floods last January, excessive seepage developed in the dirt abutment next to Howard Hanson Dam, leading to restrictions on how much water the dam can hold during a flood. Permanent repairs to the dam could take several years.

The cost of the Highway 167 sandbagging is less than $100,000, according to department spokeswoman Jamie Holter.

There are two other projects on the department’s flood protection to-do list. Crews will close eastbound lanes of Highway 18 over the Green River in Auburn from 5 a.m. to 2 p.m. Saturday to reinforce a bridge pier.

Crews will install 50 yards of rock to strengthen the pier footing in case of a large flow of water and debris. The footing is not dangerous now.

The bridge pier reinforcement is estimated to cost about $35,000, Holter said.

In the coming weeks, state signal crews will work on a plan to protect some two-dozen signal cabinets against flooding. The cabinets contain the equipment and electronics for everything that makes highways smart, such as traffic cameras, ramp meters, overhead freeway signs, high-occupancy toll lanes and in-road sensors used to show freeway congestion on colored online traffic maps.

One lane of traffic will be closed from 8 p.m. to 4:30 a.m. in both directions while the super sandbags are installed along the highway.

Holter said the sandbag work could be completed by tonight or early next week.

Mike Archbold: 253-597-8692

mike.archbold@thenewstribune.com