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Army Corps expects to know by fall whether Hanson Dam repairs will further reduce Green River flood risk
The U.S. Army Corps of Engineers expects to know in September whether or not the recently completed repairs will return the Howard Hanson Dam to its normal operating capacity and reduce the heightened risk of flooding that has threatened the Green River Valley the past two winters.
Crews have installed additional drains and wells at the dam. But until they complete testing this summer after the reservoir pool behind the dam reaches full capacity, they won't know how well the repairs worked to protect the cities of Auburn, Kent, Renton and Tukwila from flooding next winter and many winters to come.
"We will evaluate the testing and make a determination to what level we will operate it," said Col. Anthony Wright, commander of the Seattle district of the Army Corps, at a stakeholders meeting Monday at corps headquarters in Seattle. "It is not fixed yet because we don't know whether our fix worked."
Wright compared the fix to repairing an engine.
"We've put new pistons and rings in and put some oil in there it needs a little bit of gas before we start the engine," he said. "We have to fill the pool and then start testing it to see what's happening. We don't know yet if we have fixed it."
The odds of flooding in the Green River Valley remain at 1 in 60, Wright said. The odds of flooding are 1 in 140 when the dam operates at full capacity. A winter storm in 2009 damaged an abutment next to the dam and reduced the capacity of the flood-control reservoir.
"I'm optimistic it's going to be better than we have now but until I test these wells and see it they are working as designed, I'm not prepared to change the risk level," Wright said.
City officials from Kent, Auburn, Renton and Tukwila as well as King County representatives attended the meeting to hear Wright's udpate about the dam.
"It's about the same information we've heard in the past that they're still working on it and still need to get it tested," said Mike Mactutis, city of Kent environmental engineering manager, who added Wright shared a lot more details about the repair work. "It looks like the dam is relatively full now at about 40 percent full so they need to take it a little bit higher before they start doing all of the real testing."
Wright said he expects the reservoir behind the dam to reach capacity by mid-June when crews can begin to test the results of additional drains and wells installed in the abutment next to the dam.
The abutment next to the dam was formed nearly 10,000 years ago by a landslide. The federal government built the rock-and earth-fill Hanson dam in 1961 next to the abutment to control major flooding in the Green River Valley. The dam is about 25 miles east of Kent.
Problems with water storage behind the dam were discovered by the corps when a 10-foot-wide depression formed on the embankment next to the dam after heavy rain in January 2009. The Corps stored a record amount of water in the reservoir during that storm to prevent flooding.
"I'm very optimistic about how these drains are going to perform but I don't know exactly how it's going to be until we test it and see whether or not what we did and what we designed worked," Wright said. "We are going to confirm that the actions we have taken will result in an improved performance of the structure."
Auburn Mayor Pete Lewis said he expects residents will have to go through one more winter at a heightened risk of flooding until crews complete all of their work and tests.
"They are going to continue to work on these repairs throughout the summer and they will have some body of evidence by the end of this year that will give them a better idea of how we might fare this year," Lewis said. "But we won't have full confidence in the system until next year. So we go through one more storm season before we know for sure."
Mactutis said Kent city officials sure hope the repairs work. The cities have avoided major flooding because of mild winters the last two years, but a similar storm to 2009 could cause major damage with the dam not at full capacity.
"We are eagerly anticipating what the results will be," Mactutis said. "It sounds like they've considered all of the options and have looked nationwide and probably worldwide as well as far as what other people have done with dams. It was a very comprehensive look and a good result is what their goal is."
Crews from Kent placed giant sandbags two years ago along about 12 miles of the Green River Trail to boost the capacity of the levees because of the reduced capacity of the Hanson Dam. The city would like to remove those sandbags.
"We'll get the results in September," Mactutis said. "We'll take a look at what they need to do at the dam as well as what we need to do to prepare for the winter and whether we are OK with removing the sandbags off of the levees at that point or not.
"Once the flood control part is under control, the goal is take the giant sandbags down. But it's going to be very expensive to take them off."
Mactutis said city and King County Flood Control District officials are starting to study how much it would cost to remove the sandbags.
"I do not have an estimate," Mactutis said of the cost. "It's going to depend on what you do with the sand. You need to haul the sandbags somewhere and we do not have a disposal site set up yet. That would be a big part of the cost is the gas prices to haul to the disposal site or whether we will be able to use it in the city for another project would be ideal."
Kent spent about $2.5 million to install the sandbags along the levee. The King County Flood Control District then reimbursed the city the full cost of the project.
The Corps received $44 million from the federal government for the Hanson Dam repairs. Wright said about $30 million of that has been spent and the remaining funds will cover additional repairs to complete in an effort to handle a 1 in 10,000-year flood. That work includes installation of booms to block debris from entering the dam's spillway. Officials would only use the spillway if a flood is expected to exceed the design capacity of the dam.
"We are still OK on money," Wright said. "We have enough money to finish the other three phases."
Wright said he wanted to remind everyone that the Green River Valley is a floodplain.
"Over centuries it got flat because it flooded all of the time," Wright said. "I can't remove that risk. We can reduce it. The purpose of Howard Hanson Dam is flood-risk reduction and we are trying to get that back up where it should be."
The Corps spent $8.9 million to install a grout curtain in the fall of 2009 that increased the storage capacity in the reservoir behind the dam.
Wright remained confident the additional drains and wells will work to repair the leak through the damaged abutment next to the dam. The ability to control water through drains and tunnels will keep the embankment from internal erosion and the potential collapse of the dam.
"Once we have the wells tested and we've made a determination of what the design capacity is, we will announce that," Wright said. "I'm pleased with the results. I'm very proud of the (design) team's accomplishments. They have been tireless in trying to come up with innovative ways to reduce the risk of flooding in the Green River Valley and I think we're headed on the right path."
For more information, go to www.nws.usace.army.mil/.