U.S. installs spillway weir at 810-MW Little Goose

The Corps of Engineers has installed a spillway weir at the 810-MW Little Goose hydroelectric project on the Snake River to benefit juvenile salmon and steelhead migrating downstream past the dam.

The temporary spillway weir, a surface-bypass system for fish passage, was installed Feb. 20. (HNN 10/8/08) With its installation, surface fish passage facilities are in place at all Corps dams on the lower Snake River.

A spillway weir fits inside a dam’s spillway, raising the opening for water to pass and allowing juvenile fish to pass near the surface of the water rather than diving 50 to 60 feet to pass through a spillway. Fish pass the dam near the water’s surface under lower accelerations and lower pressures, providing a more efficient and less stressful route, the Corps said.

The Little Goose weir differs from massive one-piece surface bypass structures previously installed at three other lower Snake River projects: 810-MW Lower Granite, 810-MW Lower Monumental, and 634.6-MW Ice Harbor. It was shipped in sections and assembled during installation, in Spillway Bay 1. Initial biological testing of the system is scheduled for mid-March, Project Manager Jack Sands said.

The new weir is modeled after weirs at 980-MW McNary and 2,160-MW John Day dams, both on the Columbia River.

Unlike the first prototype weir, called a removable spillway weir, installed at Lower Granite in 2001, the new temporary spillway weir is smaller and less expensive to build, the Corps said. However, it provides a similar benefit in creating surface passage through a spillway.

Temporary spillway weirs do not feature the pump-operated ballast system used in removable spillway weirs to lower the structure to the bottom of the dam forebay during flood operations. Temporary spillway weirs can be removed using the dam’s gantry crane.

The temporary spillway weir is about 35 feet tall and 50 feet wide. The steel structure weighs nearly 150 tons. It features a low relative cost, is easier to implement than other weirs, and allows for flexible biological testing, the Corps said.

Several contractors worked on the Little Goose project, including Advanced American Construction, Portland, Ore., which won a $2.06 million contract to manufacture and install the new weir. The Corps awarded a separate $3.2 million contract to Advanced American to install spillway deflectors that reduce dissolved gases in the water and direct fish more quickly out of the stilling basin.


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