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Green River levee system has weak areas, could affect flood control
Editor's note: This story has been updated to reflect additional information about Kent's levee system, as well as the overall levee system of the Green River Valley.
Recent repair projects by the U.S. Army Corps of Engineers greatly improved the strength of sections of the Green River levees in Kent. But other levees remain weak in Kent, Auburn and Tukwila, and some won't be fixed until at least 2011.
"The levees in Kent in some areas are very good," said Tim LaPorte, city public works director, in a phone interview Dec. 29. "But in other areas they are not so good."
Corps officials remain confident that the risk of flooding this winter along the Green River remains at a 1 in 33 chance after repairs to the damaged abutment next to the Howard Hanson Dam and the placement of giant sandbags along the levees.
But if a major rainstorm hits and the flow of the river exceeds 13,900 cubic feet per second (cfs), several levees could fail in the Green River Valley.
"We've never seen 13,000 (cfs) on the levees," said Doug Weber, a natural disaster safety program manager for the corps, in a phone interview Dec. 30. "We have seen 12,400 when you begin to have seepage and issues with the levees."
Crews completed major repairs last fall to the Horseshoe Bend levee on the south end of Kent near South 262nd Street and Central Avenue. That levee helps protect a stretch of the valley all the way to Interstate 405 in Renton.
"That was in one of the worst shapes in Kent," LaPorte said.
The corps and city plan additional repairs this year for the Horseshoe Bend levee.
"Horseshoe Bend is the highest priority," LaPorte said. "We will work with the corps on the next piece for completion in the summer of 2010."
The corps completed an $8.7 million project in the fall of 2008 to repair the Kent Shops, Narita and Myer's Golf levees on the river near the Riverbend Golf Course on West Meeker Street.
"Myers Golf, Horseshoe Bend are all in really good shape because of recent repairs," Weber said.
But major repairs still need to be done along the Upper Russell Road levee. That levee sits between South 231st Way and James Street, adjacent to The Lakes housing development in Kent.
The corps and city plan to repair the Upper Russell Road levee in 2011.
"To get the designs and permits we were not going to make it for this summer," Weber said. "You also have to do the work in the river when the water is low and before the fish runs, which leaves late August and September."
Kent city officials wanted a faster timeline.
"The corps has taken the lead on that," LaPorte said. "We are disappointed they could not move faster, but they said it was too ambitious to finish in 2010."
The steep slope of the Russell Road levee makes it a top priority for repairs.
"One of the main problems with a steep slope is they tend to slide," Weber said. "We can work to lay the levee back to make it more stable. When the slope is more gradual, it is less likely the slope will fail."
The corps made a steep slope more gradual as part of the repairs on the levees by the Riverbend Golf Course.
"Some areas of Horseshoe Bend and Russell Road with their steep slopes are not in as reliable condition," Weber said. "The corps is looking at Russell Road to make a more stable slope."
All of the levees in Kent stood up well to the heavy rain last January when river flows reached about 11,500 cubic feet per second.
"The levees did pretty well overall," Weber said. "That was a good test."
But if heavy rainstorms hit this winter, a weakened levee in Auburn might not be able to hold water back to protect a nearby mobile home park.
"One area of concern is the levee in Auburn," LaPorte said. "The other areas are not as big of a concern."
Other sections of levees aren't as big of a concern because they sit next to undeveloped areas where not as much damage can occur.
"We have flooding on the west part of the golf course, but that's much of a damage problem," LaPorte said of the portions of the course on the west side of the river.
Sloughing, when pieces of the levee slip into the water, can be a problem and occurs more easily on weakened levees when the river flows are high.
"Sloughing is a significant issue, but it's not a Hollywood-type failure where a levee bursts and water rushes out," LaPorte said.
If river flows reach 9,000 cfs, crews from the city, King County and corps will begin to monitor the levees to see what damage the river might be doing to the levees.
"We'll keep an eye on the seepage, the flow of material and the slopes," Weber said.
So far this winter, rainfall hasn't been heavy enough to cause any major flooding.
"One part of the threat is over - the months of November and December," LaPorte said. "Now we have January and February, but so far, so good. Each week that goes by we feel better."
But with a permanent fix to the dam anywhere from three to five years away, city officials know the sooner levees are repaired the better.
"We are looking at a long-term fix and doing a lot of work with the corps," LaPorte said. "We will not rest until the work is done to reconstruct the levees."