So What Happens When Engineers Get It Wrong?

Folsom Dam Spillway-gate Failure – 17 July 1995. The design for the gate didn’t account for corrosion over time. Folsom Dam (California, 1995). And it doesn’t help matters that they dropped the ball on maintenance. (The dam looked fine right up until the point of failure.)

Actually this is more a case engineers not taking time and the elements into account. Steel rusts.

The reservoir was at full capacity on the day in question, and so one of the spillway gates was opened to draw down the water levels. Should have been just another day…

As the radial gate was raised water began flowing down the spillway chute as designed. It was not until the gate opening reached approximately 2.4 feet that the operator felt an “unusual vibration” accompanied by harsh grinding noises. No more than five seconds passed between the time the vibrations and sounds began and when the operator turned to look at the gate. The radial gate swung completely open releasing approximately 40,000 cfs.

While 40,000 cubic-feet per second is a lot of water, it is only about 1/3 of the amount of flow that the Folsom Dam spillway and downstream dams were engineered to handle. So proper setting of other spillway gates in the downstream dams avoided over-topping of those dams. It could have been so much worse.

So. What happened to the gate that failed?

The forensic report documenting the failure of the Folsom Dam gate revealed the cause of the malfunction to be excessive friction at the 32-inch diameter trunnion pin, or pivoting mechanism. Unaccounted for in the gate design, this friction was caused by the corrosion of the pin over time. Due to the additional friction forces, the loads experienced by the trunnion pin caused increased loading in the gate struts and braces of the gate.

It caused enough increase in the loads to cause a failure of one of the diagonal braces. If you look at the photos that accompany the link above, it appears that one whole side of the gate tore free from its pivot resulting in uncontrolled release of water. About 40 percent of the reservoir was drained before the water level was below that of the gate in question.

Repairs cost 20 million dollars.

So the design of a steel dam-gate was done in by corrosion that wasn’t accounted for in the initial design. (“Rust never sleeps.”) The truly sad thing is that they seem to acknowledge that they were ignoring maintenance, because everything “looked fine.”

A renewed focus was placed on maintenance and monitoring of radial gates, many of which were retrofitted to strengthen struts and bracing and ensure sufficient lubrication.

In other words, this was probably avoidable, had they been managing the infrastructure.

Stuff needs maintenance. When 2 bits of metal are supposed to move relative to one another, like this trunnion pin, lubrication is required. And when you are designing something with a life-span that is expected to be decades, you should consider the impacts of the elements. Why is it that time again, this stuff gets ignored? Because the the dam looked fine, right up until the point that if failed.

A part of me doesn’t want to include this video, but hey… “Rust never sleeps,” was a saying around the docks that got repeated a lot. Because it’s true. “Water always wins,” was another.

That is the “Out of the Blue” version of “My My, Hey Hey” by Canadian singer-songwriter Neil Young and the American band Crazy Horse from the album Rust Never Sleeps. For a harder-edged version see the “Into the Black” version. I’m not sure I ever liked either version. One is too clean, and one is too distorted. Needs some middle ground.