Green River needs higher levees — quickly

It's hard to fathom what rain and bad timing might do to this lazy scratch of water.

Look down at the newly planted banks of one of the highest levees on the Green River in Kent and a thin stream floats by at a languid 300 cubic feet per second — a good two stories below the lush fairways of the Riverbend Golf Course.

But if the federal government releases more water than normal from the troubled Howard Hanson Dam this fall, the resulting flood might actually swamp the golf course and the homes and business nearby.

Emergency-management officials suspect floodwaters could hit an unprecedented 17,600 cubic feet per second — about 40 percent higher than the river has been in at least 50 years, with about 60 times more volume than the river is carrying now.

And very few sections of the Green River from Highway 18 to the Duwamish River have banks anywhere near as high as those near Riverbend. One, near a cluster of apartments in Auburn, is less than half as high.

As winter approaches, emergency managers now agree they need to quickly raise the levees along the lower river — all 40 miles of them — before the rains begin in earnest.

But they aren't entirely sure how to do it. And no one knows how much it would cost or how to pay for it, though King County Executive Kurt Triplett asked the County Council earlier this week for $8 million for limited levee work.

"In the time that we have, we've decided that the most important thing we can do is build them all up," said Steve Bleifuhs, who oversees flood plains with King County. "But there's some concern among some engineers that we might make the problem worse."

Because an abutment to the Howard Hanson dam was damaged during a storm last winter, Army Corps Col. Anthony Wright has said he may release water during heavy rains this winter to prevent the reservoir from filling up too high and stressing the dam. His fear is that too much water could cause the dam to collapse, which in turn could cause a far worse flood.

Wright said there are continuing repairs to the dam that could make all these preparations moot, but if those repairs don't work the odds of him releasing enough water this winter to cause a major flood probably are as high as 1 in 3.

The water could submerge vast swaths of southern King County, including parts of Kent, Auburn and Tukwila.

Since 2007, a new county taxing district has raised money and started a 10-year project to rebuild the older, weaker levees, including the stretch along the golf course that was finished last year. The work is slow and expensive: It typically costs between $1,500 and $2,500 a linear foot.

But even the river's newest levees are only designed to handle a 100-year flood, which means the channel can take about 12,400 cfs of water.

If Wright releases water during a brutal winter storm, engineers suspect the combined flows could easily reach 17,600 cfs, perhaps significantly more.

What to do about the levees has been one of the few points of contention among elected officials trying to maintain a unified front in the face of a threat they aren't entirely sure how to explain. Leading the charge to build up the levees has been Kent Mayor Suzette Cooke.

"We've done all this talking about preparing people for a flood," she said. "Where I'd like to see our focus is on what to do to prevent a flood."

She and her city's engineers have urged emergency managers to try a simple approach: Line the riverbanks with "super sacks," — doorway-size plastic and fiber bags filled with rock and gravel, or flexible pontoonlike plastic tubes, that could add several feet in height to the riverbank.

For some areas, there is vast agreement. Emergency managers next week plan to start building up an 800-foot section of levee near 22nd Way in Auburn, which is now about 15 feet shorter than the levee at Riverbend Golf Course.

"That is one of the most significant low spots," said Brian Murray, another flood-plain manager with the county. "We haven't done anything with it before now because the apartment complexes are right up against the back side."

But some engineers said adding thousands of pounds of bags on top of earthen river banks supersaturated with rain after a long winter could cause other portions of river bank to collapse, making any flooding worse.

"Some of the levees are not structurally sound enough to throw more weight on them," Wright said during a meeting last week.

Late Thursday, emergency managers agreed they would attempt to build up the entire lower river. But they would look for additional ideas for tackling the sections where the levees are the weakest.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com