Kent city officials are keeping a close watch on the drop in storage capacity at the Howard Hanson Dam because of the potential impact of valley flooding from the Green River if a severe rainstorm strikes.
In fact, Kent Valley residents, businesses and property owners are slated to receive an informational letter this week from Mayor Suzette Cooke in connection with the decision by the U.S. Army Corps of Engineers to lower the storage level because of a damaged abutment at the dam.
The Army Corps operates the dam, about 20 miles east of Kent, to help control the level of the Green River as it flows through Auburn, Kent, Tukwila and Renton. Corps engineers found a damaged abutment at the dam after the heavy rainfall in early January and decided to lower storage capacity at the dam’s reservoir until they can fix the problem.
“There is no immediate danger to people or property below the dam,” said Casondra Brewster, spokeswoman for the Seattle division of the U.S. Army Corps of Engineers, in a phone interview Thursday. “The damage is not in the dam itself, but the abutment.”
Even without the immediate danger, city officials want to be sure residents are aware of the dam’s role to protect the city from flooding as well as what it might mean if its ability to store rainwater remains at one-third capacity.
“We don’t want people freaking out when it rains,” said Michelle Witham, city community and public affairs manager, of the reason for the letter. “But if there is a severe rain, it could mean a higher flood alert and we want people to be aware of the situation because the dam will not be able to hold as much water.”
The federal government built the dam in 1961 to control major flooding in the Green River Valley.
The dam served its purpose during the heavy rain in early January as the Corps held back water to help keep the Kent Valley from flooding. The reservoir reached more than 75 percent of its capacity.
But in order to assess and fix the damaged abutment, Army Corps officials decided to keep the storage level low over the next couple of months until the problem is fixed.
“We’re still investigating and evaluating the depression on the abutment and where we are at,” Brewster said. “The preliminary tests were inconclusive.”
Engineers discovered a sinkhole of about 10 feet in diameter in early January on the right side of the dam, said Tim LaPorte, city deputy public works director, in a report Feb. 3 to the City Council. LaPorte said that it is small area compared to the mass of the dam.
“The Corps told us it’s not a huge concern, but they want the cities to be prepared,” LaPorte said.
Currently, the reservoir behind the dam sits empty, LaPorte said. The reservoir is not filled until heavy rainfall in the winter or in the summer to serve as a water source for Tacoma Water. Kent gets part of its water supply from Tacoma Water.
Engineers are conducting secondary tests at the dam to figure out what is going on and how to fix the problem, Brewster said.
“We have no definitive timeline,” Brewster said on when the problem might be resolved. “We hope to try to have everything ready before the next flood season.”
Typically, heavy rainfall in the Green River area diminishes greatly after the month of February, LaPorte said. Heavy rainfall starts again in November.
“The weather forecast is being really kind, and we hope that continues,” Brewster said. “But if we have another storm like early January, we would re-evaluate the restriction and determine whether to provide more storage.”
The emergency management services division of the Kent Fire Department will work with the Army Corps, Red Cross, King County and the cities of Auburn, Renton and Tukwila to prepare for a major flooding event, said Dominic Marzano, Kent assistant fire chief, in a report to the City Council.
Marzano said the cities will develop a flood-warning system, figure out a potential evacuation of residents and where to shelter those people.
“The most important thing will be consistent messaging and awareness,” Marzano said. “There is a lot of open land that would be impacted, but it might not impact that many citizens. It is very contingent on the disaster. We’ll plan for the worst, but it will depend on the situation.”
Kent officials do not expect the Army Corps to have a good idea on what needs to be done to repair the damage next to the dam until spring, Witham said.
Corps engineers plan to gradually refill the reservoir in the spring and monitor the damaged abutment.
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