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Bonneville Spillway Flows Lowballed for Last 30 Years by Bill Rudolph NW Fishletter, August 13, 2004

The Corps of Engineers has determined that its estimates of daytime spill volumes at Bonneville Dam have come up significantly short of reality, especially when spillway gates are only open one or two feet.

The problem came to light after hydro operators noticed flow discrepancies between Bonneville and The Dalles dams last year, said Laurie Ebner of the Corps' Portland District, but others say it has been a puzzle for much longer. Namely, just how did the hydro system create water between the two dams?

Ebner, reporting last week to the Technical Management Team that governs real-time river operations in Portland, said the flow pattern at Bonneville was changed in 2002 after flow deflectors were installed in the spillway to reduce dissolved gas supersaturation. At that time, the spill pattern was evenly distributed throughout all eighteen bays, different from an earlier operation that concentrated spill at both ends of the spillway. That's when the discrepancies became more apparent.

But the problem probably goes back to the early 1970s, when 10 feet was added to the bottom of most of the 50-foot spillway gates, Ebner said, noting that the flat bottoms of the gates were re-engineered to sport 45-degree angles designed to reduce vibration.

Now operators think that the re-engineered gates were actually open four inches less than their measuring devices read. When they assumed they were spilling 50 kcfs, it's likely they were actually spilling about 30 percent less, or 35 kcfs.

However, at larger spill volumes the effect was less pronounced, so that a reported spill level of 100 kcfs is more likely 90 kcfs, or 10 percent less.

Ebner said the Corps is starting a program to re-calibrate the gate openings and verify that the flow discrepancies between the two dams will be cleared up.

She said there was always "inherent uncertainty" in regulating the river, with confounding factors such as contributions from tributaries and fish ladder flows. A 5-percent difference between actual and computed discharges would be considered a good agreement, Ebner said.

But with more water actually routed through daytime Bonneville powerhouses than originally thought, BPA's TMT representative Scott Bettin told NW Fishletter that the dam was actually producing about 60 MW/hr more power at spill levels of 50 kcfs than they thought.

He said a test of the dam's new corner collector for juvenile fish passage has called for 50 kcfs spill for much of the time to reduce eddies and improve fish exit time. The good news is that the collector seems to be working fine, even if spill levels are less than researchers thought.

With the agency selling more power during these spill regimes than anyone was aware, some fish managers are already grumbling about some form of payback, but there have been no estimates of how much the windfall may have been over the years.

With the BiOp's summer spill program now in place, nighttime spill at Bonneville Dam is about 150 kcfs with total flows about 200 kcfs. Daytime spill is averaging at 87 kcfs, according to the old math, that is.