DAYTON, Wash. – Significant river debris caused by unusually high Snake River flows damaged vertical barrier fish screens (VBS) in Little Goose Lock and Dam’s juvenile bypass system (JBS). Several damaged VBS screens entrapped juvenile steelhead and salmon between the VBS mesh and metal “porosity plates,” which control the flow of water.” Debris damage caused 1,800 juvenile fish to perish as they were exposed to excessively turbulent water conditions. Of those 1,800 fish lost, 1,438 were hatchery steelhead. About 10 million juvenile fish migrate downstream this time of year at Little Goose.
During routine inspection and trash raking of two of the dam’s six hydropower units beginning May 1, Corps staff discovered river debris had torn mesh in numerous VBS panels. Some torn panels had trapped juvenile fish. The Corps immediately began preparation to repair the torn mesh, first performing a safety inspection of the emergency bulkhead and also obtaining additional equipment and supplies.
A Corps crew began working 12 hours per day on May 4 to unwater the system and remove the large VBS panels from their operating position. After removing these large panels, staff removed accumulated debris, counted juvenile fish lost, and began repair of the damaged mesh panels.
Because of the extensive effort required to repair VBS panels, a second 12-hour-a-day crew was added on Wednesday, May 10, expanding work effort to 24 hours per day. Crews completed the work on Wednesday, May 17.
Fish encounter the VBS screens early in the juvenile bypass system as they are guided up “gatewell slots” with river flow. When functioning normally, VBS screens keep migrating fish from contact with porosity plates. Many of the VBS mesh panels were originally installed in the mid-1990s.
The juvenile bypass system helps juvenile fish avoid hydropower turbine passage by routing them through a series of JBS water-filled channels and flumes. This bypass system allows for subsequent fish sampling, barging or release back into the river.
Little Goose Dam has experienced several river debris issues and juvenile fish losses due to this year’s high Snake River flows. The dam’s 875-foot-long debris barrier or “trash shear boom” 2.5-inch diameter wire cable broke unexpectedly in spring 2014 due to wind and wave action, debris load, and accumulated wear. The debris barrier’s floats and attached fence, which extends about 4 feet underwater, protect the powerhouse and fish bypass system from floating river debris.
The debris barrier will be reinstalled this coming winter after several years of planning, budgeting, design, and contracting efforts.
The debris barrier’s cable links together twenty 42-foot-long concrete-exterior, foam-filled floats stretching across the front of the powerhouse from the dam’s navigation lock to the edge of the spillway. A large crane was required to remove the barrier components after it was damaged in 2014, and refurbishment and replacement of the debris barrier components will also require heavy equipment and labor.
Snake River flows have been very high this year due to heavy precipitation and runoff. Corps’ dams on the lower Snake River have seen higher-than-normal river debris come through the area.