WALLA WALLA, Wash. — Balancing hydropower and salmon populations is a delicate balance. To safely detect underwater maintenance needs at McNary Dam, the Army Corps of Engineers uses diving robots in the early winter, a slow time for migrating juvenile salmon.
According to Army Corps spokesman Joe Saxon, the majority of dives by remotely operated underwater vehicles and humans is between Dec. 15 and March 1 to minimize the effect on migrating fish. He said all diving work is coordinated with other agencies like NOAA and the EPA to assess environmental impacts on the river and endangered species.
“The in-water work window is the time span that has minimal effect on endangered species like steelhead, bull trout, so it is a better time for us to perform dive work,” Saxon said. “An example would be the installation of sensors on the turbine intake and spillway gates that are used in fish research projects.”
The robots also inspect damage to trash racks — wide metal grills stacked on top of each other in front of the dam’s intake gates that stop debris from being sucked into the turbines and damaging them. For the safety of divers and robots, Saxon said when divers or robots are inspecting the trash racks the turbine intake gates are locked out to mitigate the differential pressure danger, suction caused by turbines as water flows through the dam. If the intake gates are not locked, the pressure could pull in an ROV or diver, trapping them.
“ROVs are the most cost-effective way to inspect underwater structures, so we try to use them as much as possible when no construction or repair work is required,” he said.
One of the most vital inspections by the robots are of vertical barrier screens in front of every turbine, Saxon said.
“These screens protect juvenile fish from going through the power house and direct the fish up into a collection channel where they are sent to the juvenile fish facility,” he said. “The juvenile fish facility either discharges the fish below the dam or sorts the fish by species, and collects the fish and transports them below Bonneville Dam.”
The fish also go through passive integrated transponder tag detectors where data of origin is collected. If any of the screens are found to be defective, the turbine is taken out of service and the screens must be repaired or replaced with a spare, Saxon said.
The trash racks are cleared with a rake attached to a crane throughout the year, but especially in the spring during high water runoff that carries logs downstream.
Saxon said robots are used as an initial inspection tool when there is an issue with equipment below the waterline and out of sight, and used in conjunction with sonar that helps locate structures in deep, dark, murky, turbid water and to take a look at structures from a greater distance.
“If the sonar shows an area where something looks out of place, we can move the ROV in closer for a more detailed look,” Saxon said.
Some examples he listed are inspecting wire ropes anchored to the bottom of the river — some as deep as 120 feet — and the underwater condition of coffer cells, piers and other structures below the water surface.
“The ROV provides us with a safer means of collecting data and is a more cost-efficient way to gather this data,” he said.
While robots are safer, more efficient and cost effective, Saxon said human divers are used when there is a need for cutting, welding, cable installation, concrete work or underwater placement of equipment.