Weir Science Tries to Offer
by Joe Rojas-Burke
A retractable vessel is designed to ease the fishes' journey over Columbia River dams,
but critics doubt its ability to improve runs
One of the strangest vessels ever to travel the Columbia River is nearing its launch. The boxlike hulk, made of 1.7 million pounds of steel, sports a giant water slide and a submarine's innards to make it submerge and resurface on command.
Its one-time, tug boat-powered voyage will bring it to a permanent destination, the Ice Harbor Dam, the last hydropower project on the Snake River.
The $12.8 million device is the latest in technical wizardry devised to protect salmon from death and injury at they traverse a system of eight federal hydropower dams. The structure -- called a removable spillway weir, or RSW -- helps juvenile salmon move downstream without getting sucked into power-generating turbines. Operators are moving to outfit all the dams with the structures within 10 years.
But critics, including conservation groups and Native American tribes with treaty rights to salmon, say there is reason to doubt the spillway weirs are capable of improving survival of migrating salmon and steelhead. Some say the most likely benefit is the potential for dams to generate more energy for the Bonneville Power Administration, the federal energy wholesaler.
"We see this as a boondoggle," said Bob Heinith, hydro program coordinator for the Columbia River Inter-Tribal Fish Commission, which manages fisheries for the Warm Springs, Yakama, Umatilla and Nez Perce tribes. He insists the hugely expensive structures are unlikely to boost overall salmon runs.
"What this really is is another vehicle for Bonneville to make more money out of the river," said Liz Hamilton of the Northwest Sportfishing Industry Association.
Federal fisheries officials, however, are so confident in the spillway weirs that they are counting on them to increase fish survival rates. The expected increase helped officials to conclude the dams will no longer jeopardize the Columbia Basin's 13 salmon populations protected under the Endangered Species Act
"The current biological opinion depends very much on them (the weirs)," said Bob Lohn, regional administrator of the National Marine Fisheries Service, the agency responsible for the recovery of listed salmon stocks.
Taking the plunge
The spillway weir is the result of a decade of trial and error, much of it at Lower Granite Dam, the first federal hydro project that upper Snake River salmon encounter on their migration to the sea. None of it proved practical as a means to pass large numbers of fish until the U.S. Army Corps of Engineers developed the prototype removable spillway weir, installed at Lower Granite in 2002.
The safest way for fish to cross dams is over spillways rather than through turbines. But spillways -- designed to release heavy flood surges -- can also be deadly.
"From a fish's perspective, it means diving 40 to 45 feet, undergoing an instantaneous pressure change of one to two atmospheres, and being squirted out onto an abrasive concrete spillway," said Lohn.
Tests of the weir design show its shallow flow over the dam attracts young salmon, allowing the fish to swim past in the upper 10 feet or so of the water in a giant slide. The juveniles, some no bigger than fingers, move swiftly, potentially avoiding predatory birds and bigger fish.
Part of the engineering challenge was making the weir retractable to handle flood surges. Designers put submarine-style tanks inside the structure. Flooding the tanks sinks the weir on two hinges in a controlled descent, tucking it away so floodwater can pass. Filling the tanks with air floats the weir back into position to move fish over the dam.
"It is an absolute engineering marvel," said Lt. Col. Randy Glaeser of the Army Corps of Engineers during an unveiling last week of the weir that will be installed in February at Ice Harbor Dam. Designers also built it to float like a barge for its trip upriver from the building site at Thompson Metal Fab Inc. in Vancouver.
Two routes, same results
Studies at Lower Granite make one thing clear: The prototype achieves fish survival levels at least as good as regular spillways, but using a fourth of the river flow volume to pass the same number of fish. Thus, spillway weirs could allow dams to divert more flow to turbines to generate more energy.
Less certain is whether the removable weirs can improve survival compared with the current program of releasing large volumes of flow over spillways.
"There is no statistically significant difference between the two routes," said Noah Adams, a fisheries biologist with the U.S. Geological Survey, which conducted the study at Lower Granite for the Army Corps. Researchers, using transmitters implanted in salmon, found the survival rate was 98 percent with the removable weir. That compared with 93 percent survival with the regular spillway. But a statistical margin of error of 6 percentage points made the difference insignificant.
The independent study team confirmed that fish using the weir took less time to pass the dam and were less likely to wander upstream. Lohn, the federal fisheries service administrator, said the weir's surface flow attracts young salmon, which have been tracked crossing the river to reach the passage structure.
"We have confidence that there is a real increase in survival," said Lohn. The fisheries service estimates the structures could improve survival by 3 percent to 5 percent at each dam. "If fish are passing through eight dams or five dams, those repeated effects are huge," Lohn said.
The 'Worst of Years' per dam survival is 97.2% (97.2)8 = 80% or 20% mortality.
To 'improve survival by 3 percent to 5 percent at each dam' would provide (more than ?) 100% survival.
|Direct Mortality||Fall Chinook||Spring Chinook||Steelhead|
(Mortality from 8 dams)
(Mortality from 8 reservoirs)
A better use of money-- National Marine Fisheries Service, December 21, 2000 (more)
"Where current operations are providing very good survival, its almost impossible for an RSW to improve on survival," said Tom Lorz, a hydraulic engineer with the Columbia River Inter-Tribal Fish Commission.
Heinith said funding for the weirs could accomplish more for salmon recovery if it were used on other projects, such as more extensive releases of water over existing spillways.
Steve Wright, BPA administrator, said the technology, while not a panacea, "is a significant step forward." Bonneville, he said, is obligated to repay about three-quarters of the construction costs to the federal Treasury.
Wright said the economic benefits will go to Northwest residents in the form of lower energy costs. At Ice Harbor, spillway releases for salmon passages cost more than $30 million a year in foregone revenues from power generation, Wright said.
"What we want is something that can achieve our twin goals," Wright said: the production of affordable energy and the protection of salmon runs.
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