ARRA Funds Safety Projects at Howard Hanson Dam
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, according to an Aug. 6 press release.
A project to construct a seepage barrier was awarded July 2 to Nicholson Construction for $8,878,380. A contract to improve drainage tunnel function was awarded July 31 for $916,874 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 they observed two depressions on the right abutment, increased water levels in groundwater monitoring wells, and silty water entering the abutment drainage tunnel. To date, 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.
The seepage barrier contract is for installation 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 drainage tunnel improvements contract is for installation of 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.