What Happened at Oroville Dam?
First, a bit of good news. After public safety officials ordered 188,000 people to evacuate communities downstream from Lake Oroville Dam on Sunday evening (see my blog from yesterday) they announced just a few hours ago that it is safe for residents to return. Example news coverage: Sacramento Bee article, Feb 14.
Perhaps because I wanted to be a civil engineer from about age 5, designing massive structures like bridges, dams, and skyscrapers, I'm always interested in situations like this. And my interest is an engineer-analyst's perspective: What went wrong, and what can we learn from it?
Two resources I've found useful to understand the situation are Vox.com's "The crisis at Oroville Dam, explained" and Wikipedia's page for the Oroville Dam.
The first thing to understand about this near-catastrophe is that the main dam itself was not in jeopardy. It was not going to "break". The problems involved the spillway and the emergency spillway.
The first problem that cropped up was a week ago, when the main spillway, which is used to release from the resevoir to keep the level safely below the top and to feed downstream rivers, developed a hole in its bed. This picture linked from the Vox article above shows what it looks like:
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Water managers wanted to keep the flow through this channel low to prevent further erosion of the bed. The problem with that was heavy rains have been filling the resevoir quickly, and water needed to be released.
That's where the emergency spillway came in. It's designed to handle overflows in a semi controlled manner. The thing is, it was designed for that- over 50 years ago- but never needed to be used until this week. And when it was used this week, the water flowing over the concrete lip rushed down the rock hill below it so hard that it started eroding the rock and dirt of the hillside. Engineers worried this would undermine the foundation of the concrete lip. If the lip were to crumble a 30 foot tall wave of water would come coursing down the side of the canyon. That is why government officials declared a huge evacuation order moving 188,000 people out of harm's way.
Engineers shifted their strategy back to the main spillway (the one in the picture above). It's damaged, but it still works. Running more water through it damages it further. But the form of damage it's suffering does not pose the catastrophic failure risk exhibited by the emergency spillway. So they sent water coursing down this channel again, causing quite a plume to shoot up into the air as nearly a million gallons of water per second flowed through.
Maxing out the spillway capacity, even at the cost of damaging it further in the process, has brought the lake's level down by 15 feet. Engineers hope to bring it down another 25 to 35 feet by the weekend, lowering it below "flood stage". Of course, weather is a variable, so we'll see how well these plans hold!
Perhaps because I wanted to be a civil engineer from about age 5, designing massive structures like bridges, dams, and skyscrapers, I'm always interested in situations like this. And my interest is an engineer-analyst's perspective: What went wrong, and what can we learn from it?
Two resources I've found useful to understand the situation are Vox.com's "The crisis at Oroville Dam, explained" and Wikipedia's page for the Oroville Dam.
The first thing to understand about this near-catastrophe is that the main dam itself was not in jeopardy. It was not going to "break". The problems involved the spillway and the emergency spillway.
The first problem that cropped up was a week ago, when the main spillway, which is used to release from the resevoir to keep the level safely below the top and to feed downstream rivers, developed a hole in its bed. This picture linked from the Vox article above shows what it looks like:
Water managers wanted to keep the flow through this channel low to prevent further erosion of the bed. The problem with that was heavy rains have been filling the resevoir quickly, and water needed to be released.
That's where the emergency spillway came in. It's designed to handle overflows in a semi controlled manner. The thing is, it was designed for that- over 50 years ago- but never needed to be used until this week. And when it was used this week, the water flowing over the concrete lip rushed down the rock hill below it so hard that it started eroding the rock and dirt of the hillside. Engineers worried this would undermine the foundation of the concrete lip. If the lip were to crumble a 30 foot tall wave of water would come coursing down the side of the canyon. That is why government officials declared a huge evacuation order moving 188,000 people out of harm's way.
Engineers shifted their strategy back to the main spillway (the one in the picture above). It's damaged, but it still works. Running more water through it damages it further. But the form of damage it's suffering does not pose the catastrophic failure risk exhibited by the emergency spillway. So they sent water coursing down this channel again, causing quite a plume to shoot up into the air as nearly a million gallons of water per second flowed through.
Maxing out the spillway capacity, even at the cost of damaging it further in the process, has brought the lake's level down by 15 feet. Engineers hope to bring it down another 25 to 35 feet by the weekend, lowering it below "flood stage". Of course, weather is a variable, so we'll see how well these plans hold!