Corps of Engineers speak on next phase of dam construction

Photo by Jake Lee Green
Construction of a 440 feet long, 17 and a half feet high, wall will begin now that the ‘S’ curve detour has been opened for public use.

By Kathe Malouf
Special to the Sun

Now that vehicle traffic has been diverted onto the Highway 155 detour, construction can get underway on the next phase of the Isabella Dam Safety Modification Project: a 17-foot-tall wall that will be built at the crest of the Main Dam.

The Isabella Dam Safety Modification Project (DSMP) will raise both the Main and Auxiliary Dams by 16 feet. In order to accommodate the increased height of the Main Dam, a 17-foot tall wall will be constructed at the Main Dam’s right abutment to stabilize and support the material that will be needed for the height increase.

According to Rick Brown, Corps of Engineer’s spokesperson, the wall on the dam side will be approximately 440 feet in length and 17½ feet in height. A second, shorter wall of about 6½ feet, will be constructed on the mountain side of Highway 155.

With the highway detour open to traffic, construction activities are expected to begin soon, Brown said. The initial work will consist of foundation excavation and preparing the foundation for concrete placement.

Brown said that the highway detour had to be in place and open to traffic before work could begin on the wall and foundation. He noted that the foundation will be placed underneath Highway 155 and crews will be tearing up a section of the highway for the foundation footing.
The actual construction of the wall will likely follow a pattern of one section of foundation and one section of wall, followed by another section of foundation and another section of wall, rather than placing 440 feet of foundation and then moving on to construction.
Brown said that construction of the right abutment wall will take approximately one year to complete.

The right abutment is not a single wall, Brown said, noting that it includes three components of the wall phase: the 17-foot tall wall on the dam side, the shorter wall on the mountain side and the foundation. According to Brown, the foundation and the mountain side wall of the abutment will form sort of a U-shaped chute, when viewed from Highway 155.
“You might hear some folks talking about this being another spillway,” Brown said. “It will form a tertiary spillway, but would likely only see action in the event of a probable maximum flood. What we don’t want people to think is that Hwy. 155 will become a primary spillway and cut them off anytime we have to release water from the reservoir. That is not the case.”
According to Brown, the primary function of the right abutment wall is to stabilize and support the Main Dam height increase, adding that the creation of a tertiary spillway is considered by the Corps as an added benefit in the event of a worst-case scenario flood event.
Once the wall has been completed, Hwy. 155 will be restored and returned to its current alignment.

Because crews will be working on the abutment wall while traffic is moving, motorists will be subject to traffic control. For the majority of construction, the traffic detour will be two-way; however, traffic will be reduced to one-way for a short duration of about six weeks. During that time, a 24-hour traffic signal will be installed for traffic control.
Once the dam is raised, the upstream (lake side) detour, will remain in place and serve as the access road to the top of the dam. The downstream side of the detour will be removed once construction is completed.
Along with the increased height to both Main and Auxiliary dams, the $600 million DSMP will include rebuilding of the walls of the existing service spillway. A new emergency spillway and labyrinth weir will be constructed between the existing service spillway and Auxiliary Dam and several recreation facilities around Isabella Lake will be upgraded.

Brown noted that work at Auxiliary dam will be halted for the season, adding, that the stop in construction is to ensure dam safety during the wet season. He added that a significant amount of the earthwork occurring on the auxiliary dam is restricted to April through October for safety reasons.
Brown noted that the contractor has completed most of this year’s seasonal work and will restart the Auxiliary Dam work for the next construction season that will start no earlier than April, 2020.

Construction on the project will be completed by the end of 2022.