Tukwila Flood Watch Information

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Howard Hanson Dam Flood Watch Information

Background:

Following a record high level of water behind Howard Hanson Dam in January 2009, the U.S. Army Corps of Engineers became concerned after discovery of two depressions on the right abutment, increased water levels in groundwater monitoring wells, and the appearance of sediment-laden water entering the abutment drainage tunnel. These concerns compelled the Corps to limit the dams operational capacity, in order to not put it at further risk. While the dam itself is not in immediate danger of failing, there is increased risk to downstream communities until seepage concerns with the right abutment have been resolved. The Corps of Engineers placed restrictions on flood storage and established an aggressive monitoring program, in addition to other risk reduction measures. Reduced flood storage capacity results in more frequent and larger volume releases during flood events, which could increase the probability that levees in the lower valley could be overtopped. Physical actions implemented to reduce risk include construction of a seepage barrier (grout curtain) and improvements to drainage in the abutment. Extensive coordination with local communities has occurred to aid emergency preparedness.

Situation:

New investigations, testing, and engineering analyses have been performed. The Corps now has more confidence that we can safely store water to the summer conservation pool level (48 percent of full). However, the Corps is not yet assured that Howard Hanson Dam can safely store large flood inflows. The Corps is conducting a dam safety study to determine the best solutions to allow normal operation. The Corps has reviewed and concurred with the proposed solutions identified below. The final report will be completed in November 2010.

Project Status Update:

8/3/10

On Friday, July 30th, the U.S. Army Corps of Engineers briefed U.S. Senator Patty Murray and the Valley Cities on the status of the Howard Hanson Dam. It's good news. The Corps has been conducting a Dam Safety Study to determine the best solution to allow normal operation. The final report will be completed this November, but the Corps has already determined the additional corrective actions necessary to restore the dam to a safe condition and maintain water storage to 100 percent of full. The Corps has determined that instead of expanding the grout curtain as a temporary fix to the Dam, they will get better results by installing a new drainage and erosion control system inside the dam and abutment. This will let them to control the seepage and keep it from eroding the fine aggregate from the abutment and will let them monitor and protect the current grout curtain better. They will start on construction soon after getting the funding (which was included in an emergency supplemental appropriations bill that was signed by the President on July 29th). These corrective actions will be in place well in time for next flood season (but not this flood season). At that time, these actions should bring the Dam back to it's designed capacity and the risk of overtopping the levees to 1 in 147. This fix is not permanent but would be much more durable than the expanded grout curtain. The Corps' announcement is currently available on their website.

This past spring the Corps tested the grout curtain that they installed in 2009. The testing showed the grout curtain and other interim measures were effective in controlling seepage through the abutment. This has provided the Corps additional confidence in the storage capacity of the dam for this coming flood season. Later in August, the Corps expects to complete their operational plan for this winter's rainy season. It's possible the operational plan will be based on the Dam holding more water than last rainy season which would lower our risk of flooding. We will update you when the Corps has announced their operational plan.

In summary, this is good news. However, we are still at higher than normal risk of flooding and need to continue to be prepared.

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Train-the-Trainer Program

The City of Tukwila is conducting several meetings for business community members regarding the most current conditions at the Green River’s Howard Hanson Dam. These meetings will be in a “Train-the-trainer” format to familiarize someone from each agency with the materials provided and allow them to return to their business and train employees and colleagues in the 5 Flood Safety Steps.

Use this page for information on future meetings and to download materials for use when presenting the 5 Flood Safety Steps to your business or neighborhood.

View the Train-the-Trainer Page

Flood Mitigation Photos

News Updates

8/16/10

Howard Hanson Dam Risk To Be Cut In Half

Preliminary work on the Green River’s Howard Hanson dam will halve the upcoming winter’s risk of a Kent Valley flood — to one in 60 during a normal rainfall year, compared to the risk in 2009. This was the essence of a report from Army Corps of Engineers Seattle District Commander Col. Anthony Wright, during a press event Monday. Read the entire article...

5/13/10

Murray Delivers Funding For Howard Hanson Dam Fix

Today, U.S. Senator Patty Murray (D-WA), a senior member of the Senate Appropriations Committee, announced that she has secured $44 million in emergency funding for critical repairs to the Howard Hanson Dam. Murray included the funds, which will go to the Army Corps of Engineers, in the Fiscal Year 2010 Emergency Supplemental Appropriations bill. Read Entire Press Release...

4/30/10

Pool Level At Howard Hanson Dam Reaches 1,157-Foot Benchmark

The reservoir at Howard Hanson Dam will reach elevation 1,157 feet above sea level within the next 24 hours. Since April 21, when the reservoir reached the benchmark of 1,147 feet, the U.S. Army Corps of Engineers has conducted intensive monitoring, including scientists on the ground around the clock. Since early March, the Seattle District of the Corps has been slowly filling Howard Hanson Dam’s reservoir for its annual conservation pool. Read Entire Press Release...

4/22/10

Western Washington Lawmakers Push for Funding to Reduce Risk of Flooding in Green River Valley

Today, Senator Patty Murray, Senator Maria Cantwell, Congressman Adam Smith, Congressman Dave Reichert, Congressman Norm Dicks, Congressman Jim McDermott, Congressman Jay Inslee, Congressman Brian Baird and Congressman Rick Larsen sent a letter to Lieutenant General Robert Van Antwerp urging the U.S. Army Corps of Engineers to make unused funding available for short term fixes to protect the tens of thousands of residents and 95,000 jobs of the Green River Valley from catastrophic flooding while permanent fixes to the Howard Hanson Dam are underway. Read the Entire Press Release...

You may also view the entire letter sent from our Valley mayors to the Congressional Delegation. The letter was sent to Congressman Adam Smith and a list of other officials.

Mayor Haggerton's Flood Awareness Public Service Announcement