High Flows Bring Debris to Bonneville Dam,
by CBB Staff
High Columbia River flows have brought high levels of debris into Bonneville Dam's smolt bypass screens, resulting last week in a severe rate of descaling in young migrating fish.
In response, salmon and hydro managers last week agreed that "submersible traveling screens" guiding fish -- and therefore flow and debris -- into the bypass system be removed at Powerhouse II.
That led to most fish moving past the dam by spill or the "corner collector”, with the remainder through the turbines or finding their way into the collection system orifice without the STS guidance. From the orifice, smolts enter a pipe that sends them into the river below the dam. (Even when the STS/smolt bypass system is operating, most in-river spring migrants pass Bonneville through spill or the corner collector. The bypass system is intended to keep the remainder of the fish from using turbine passage.)
This year's smolt migration season is the first the new STSs at Bonneville Dam have been fully operational, except for one important component. A "turbine intake extension" crane needed to properly clean debris from bypass screens is broken.
So with the high flows up to about 400,000 cfs in the past few days -- and the crane out of action, debris began to build up faster than the Corps could clean it, just as large numbers of fish were reaching the dam.
The larger debris material such as logs, are stopped by trash racks in front of the dam. But smaller debris, such as sticks and leaves, move with the fish and flow into the STS guidance screens and then toward the vertical barrier screens at the gatewells. The VBS screens trap the debris, but there are "hot spots" where debris has not collected. These clear areas on the screen create a hard flow (like a drain) which impinges the young fish against the VBS screen, resulting in descaling.
Sampling on May 20 showed descaling in smolts at Bonneville Dam had reached 19 per cent in yearling chinook. Though it's obvious such descaling is stressful for fish, it is not known how descaling relates to mortality rates.
With flows at the dam dropping somewhat in the past week, the river operators at this week's multi-agency Technical Management Team meeting decided to attempt a partial return to using the bypass system so more fish can avoid the impacts of spill (and associated gas levels) and turbine passage.
TMT members agreed that the STS should be reinstalled at one unit to see if a more aggressive way of cleaning the VBS can be achieved, and if debris accumulation was lessening. (Three submersible traveling screens are needed to cover one unit.)
The STS for the unit was put in place Wednesday afternoon with evaluations taking place Thursday. However, debris built up fast again on the VBS screens, faster than the Corps could adequately clean it without the TIE crane.
With high flow expected again in the coming days (flows at Bonneville were about 400,000 cfs this morning and are expected to peak higher than that Monday) the Corps decided to again pull out the STS, said Dan Feil of the Corps this morning.
He said the Corps is planning to reinstall the STS at one unit again Monday for another assessment. Though flows will still be high, the primary issue would be whether enough "debris flushing" has taken place to allow for further installations of STSs with the aim of getting the smolt bypass system fully operational.
The subject will again be discussed at next Wednesday's TMT meeting.
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