After the recent failure of the Oroville dam’s main spillway, the question I keep asking myself is, “Where did the engineers go to school?”
Back in 1889, the South Fork Dam broke after heavy rains, opening up like a zipper and flooding Johnstown, Pennsylvania, killing 2,209 people. The dam had overflowed, and dams fail catastrophically when this happens. Especially earth-fill dams.
In Johnstown, the dam had been designed and built properly, opening in 1853. It acted as a reservoir for a canal system. To prevent failure, it had three large iron pipes with valves at the bottom, allowing water to be be released in high volume at no risk. The main spillway, which gave an alternate path for surplus water from the dam when it was full, was nowhere near the earth-fill portion of the dam, but was blasted out of bedrock in the surrounding hill.
The dam failed because, after the canal system went bankrupt due to the development of railroads, the the new owners of the site (rich dudes who wanted a resort with a big lake) rebuild the dam incompetently, omitting the iron discharge pipes at the base. This meant that there was no way to lower the water level in the dam. It was always full, and as soon as a series of rainstorms put in water faster than it could leave via the spillway, water poured over the top of the dam, and the dam eroded faster and faster until suddenly it was gone.
Fast-forward to Oroville, which came awfully close to a similar failure. By the standards of Johnstown’s 1853 South Fork Dam, the Oroville Dam has these flaws:
- The main spillway is a big water slide down the front of the earth-fill structure, not blasted out of solid bedrock elsewhere. This means that any failure of the spillway can easily chew away the dam and cause it to burst.
- The discharge pipes via the hydroelectric plant at the bottom of the dam are too small, with only 5% as much capacity as the main spillway. Plus, they’re so close to river level that they can’t be used in when trouble with the main spillway chokes the riverbed with debris.
- “Put not your faith in concrete.” In the 1853 South Fork dam, the important roles were played by iron pipes or channels blasted into bedrock. In the Oroville dam, these roles are played by concrete, with the results we’ve all seen.
These problems are interrelated. The dam is supposed to be able to discharge far more water from its base, using the diversion tunnels. But (surprise!) the concrete diversion tunnels are damaged.
In a structure as dangerous as a big hydroelectric dam, with a long working lifetime, and the certain expectation that many operating decisions will be made by politicians rather than engineers, the most important design criterion is this: Not only should the dam be capable of being operated by morons, it should be capable of being maintained by morons. Morons who lost the maintenance funds at the dog track.
A dam isn’t something like a railroad, where the trains stop running unless there’s continuous maintenance. It’s something that appears to be kinda-sorta working until a wall of water and debris sixty feet high wipes your city off the map.
To my way of thinking, this requires that dams be build more to be low-maintenance than to be efficient, and with lots of redundant safety features: preferably ones that work okay even if everyone but the janitor and the UPS guy have been passed out on the floor for the last 25 years.
Periodic inspection and regulation are okay to the extent that they aren’t crooked—and good luck with that. A good design needs to survive intervals of incompetence and dishonesty just much as it needs to survive natural disasters.
But no one is going to be crazy enough to declare us dictator and put our policies into practice, so enough of the big picture! Here’s my main take-away: Make sure your next home and workplace are well above your local floodplain. During the 1996 flooding here in Oregon, as I drove between Corvallis and Portland, I noticed that all the little hills that were high and dry above the floodwaters had older houses on them. Those old-timers knew a thing or two.