The U.S. Army Corps of Engineers, Seattle District, has awarded two projects, funded by the American Recovery and Reinvestment Act, to improve dam safety at Howard Hanson Dam.
The Corps awarded a contract July 2 to construct a seepage barrier to Nicholson Construction for $8.87 million. On July 31, the Corps awarded a $916,874 contract to improve drainage tunnel function to Jensen Drilling Company.
“Seepage concerns at Howard Hanson Dam are among the Corps’ highest priorities. The project to construct a grout curtain is expected to reduce seepage through the abutment, and the improvements in drainage to the tunnel should help direct remaining seepage through a safe pathway,” said Seattle District Commander, Col. Anthony O. Wright. “This work should increase our confidence in the dam’s ability to reduce downstream flood risk.”
Following a record high level of water behind Howard Hanson Dam in January, the Corps restricted flood storage behind the dam after observing two depressions on the right abutment, increased water levels in groundwater monitoring wells, and silty water entering the abutment drainage tunnel.
Since that time, engineers have excavated the depressions, installed additional monitoring equipment and conducted tests, while a summer conservation pool was stored at the dam.
There is no imminent risk of dam failure. However, what the Corps has found so far hasn’t yet increased confidence in using the full flood storage capacity of the dam. Should a major flood event occur with a limited flood storage capacity, it is possible that levees in the lower valley could be overtopped.
Preparations for the upcoming flood season include constructing an interim seepage barrier wall and improving the drainage tunnel to control seepage through the most critical part of the right abutment.
The seepage barrier project will entail the Corps installing of a 450-foot-long grout curtain into the right abutment. The grout curtain will vary from 90 to 170 feet deep. The contract requires real-time collection of water-pressure testing and grouting responses and near real-time adjustments to grout mixes. The anticipated volume of grout to be placed is 20,185 cubic feet. The work also requires the design and construction of a work surface and a stormwater collection system to prevent runoff into the reservoir, which in the summer contains the drinking water supply for the City of Tacoma. The purpose of the seepage barrier is to significantly reduce the amount of seepage into the right abutment and its potentially detrimental effects.
The drainage tunnel project calls for installing additional drains within the right abutment. Two 6-inch vertical drains will be installed to replace an improperly functioning vertical drain. Thirteen 3.7-inch horizontal drains will be installed from within the existing drainage tunnel. The purpose of the drains is to safely control and manage seepage entering the right abutment from the reservoir.
In addition to constructing these two projects, the Corps is continuing to monitor the abutment and repairing segments of levee along the Green River. Simultaneously, the Corps has initiated test borings, geotechnical modeling and analysis to support the grouting operation, as well as planning for a long term repair project to address seepage.
The Corps has been working closely with King County and the downstream cities of Auburn, Kent, Renton, Tukwila and Seattle to prepare for flood season, should higher-than-standard flows be necessary from the dam.
The Corps of Engineers is continuously reassessing the integrity of the right abutment to determine the amount of flood storage space available for the upcoming flood season.
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