Here’s an in-depth report about the time the left spillway failed at Glen Canyon Dam from John D’Anna writing for The Arizona Republic. It’s quite a tale. Click through and read the whole thing, here’s a excerpt:
…water in Lake Powell would come within inches of topping the dam’s massive spillway gates as engineers frantically tried everything they could think of, rigging 4-by-8 sheets of plywood to extend the top of the gates and releasing more than half a million gallons per second into the Colorado River.
Before it was over, the force of the water releases would gouge house-size holes in the dam’s crippled concrete spillways. The white water would tinge red from the bedrock sandstone, and ominous rumbling sounds would be heard as boulders the size of cars belched from one of the spillways into the river.
The more water the engineers released, the more damage they did. But they had no choice.
“We were sitting on a pretty good catastrophe waiting to happen,” said Art Grosch, an electrician who worked at the dam and ran electrical cable into the mangled spillways.
“That lake (Powell) is 190 miles long and has something like 2,300 miles of shoreline,” he said. “And it was rising a foot a day.”
[…]
“Even if Lake Powell and Lake Mead remain low, megaflood risk persists and is likely to be increasing. Precipitation intensity, and the amount of precipitation falling in the most intense events, are increasing globally and across the United States, in large part because sea surface temperatures and atmospheric water vapor content are both rising, increasing the odds of more extreme precipitation events. These trends will continue as long as emissions of greenhouse gases to the atmosphere continue.”
Such a flood would be a longshot, what’s called a “black swan” event — something so incredibly rare as to be almost unimaginable. But the thing about a black swan is that you never see one until you see one, and in the late spring of 1983, the engineers at the U.S. Bureau of Reclamation were about to get a close-up look…
The 1982-83 El Niño was the strongest ever recorded at the time. The tales of its fury were recounted in newspaper headlines of the day — and later in books and scientific journals — as it unleashed disaster on virtually every continent, from searing droughts in Australia, Africa and south Asia, to violent floods in South America…
In a normal spring, the last major snowfall in the Rockies occurs around the end of March. As temperatures rise in the lower elevations, the snowpack melts slowly and creates a steady stream of runoff as temperatures rise into the higher elevations.
But there was nothing normal about the spring of 1983.
The late-winter snowfall of January and February was actually below normal, but in early March it accelerated and didn’t quit until May. And then, instead of increasing gradually, temperatures skyrocketed.
Instead of a slow, steady flow down from the mountains, the runoff gushed…
In the Rockies, every tributary of the Colorado River was running high and fast, and the Colorado itself was running at 100,000 cubic feet per second — enough water to cover the entire 517 square miles of Phoenix in 6 inches of water in less than a day.
And all that water was barreling into Lake Powell toward a choke point only a quarter of a mile wide. The only thing in its way was Glen Canyon Dam, which had only been completed 20 years before and had only been filled close to capacity once…
Gamble and his team were already releasing the maximum amount of water possible, about 40,000 cfs, through the eight massive turbines in the dam’s power plant. But as more and more water gushed into Lake Powell, Gamble knew he would have to begin releasing water through the dam’s two spillways, two massive tunnels bored into the sandstone on either side of the dam and running parallel to the river.
The spillways are 41 feet in diameter — picture the height of a Boeing 737 from the runway to the tip of the tail — and extend more than half a mile, beginning about 600 feet upstream of the dam and emptying out a few hundred feet on the downstream side.
Each tunnel is lined with concrete 3 feet thick and is controlled by colossal steel gates at the top — 52 feet tall and weighing 350,000 pounds — that control how much water is released. At the bottom of each is a “flip bucket,” a sort of 40-foot ski jump that sends the water into the air before it hits the river, dissipating its energy and controlling erosion.
The first third of each spillway tunnel drops more than 500 feet in elevation from lake level at a 55-degree angle beforeintersecting with horizontal tunnels that were originally drilled to divert water around the dam while it was being built.
The upstream portions of the diversion tunnels, which connected directly with the reservoir, were sealed with giant concrete plugs more than 150 feet long. Using the original diversion tunnels allowed the bureau to save time and millions of dollars by not having to drill thousands of more feet and line the new tunnels with concrete.
When the spillways are wide open, they are capable of releasing 276,000 cubic feet of water per second, more than 2 million gallons a second, from the reservoir. That number was based on detailed studies of peak flows down the Colorado through history, both the 100 years of recorded history by man and the eons of geologic history recorded in the strata of the canyon walls.
According to a design report from 1961, the 276,000 cfs number was 1.7 times the highest flow ever recorded on the river. In other words, the spillways were built to handle 70 percent more water than had ever been seen before on the Colorado.
It was little wonder then that Tom Gamble’s faith in his dam was unshakable…
On June 2, as the water surged toward the dam, it was just inches below the spillway gates.
Engineers opened the left gate and began releasing 10,000 cfs, enough to fill 450 backyard swimming pools every minute, but still only a fraction of capacity. Three days later, they doubled the flow to 20,000 cfs and planned to open the gates even more.
But early the next morning, on June 6, engineers began to hear strange rumbling noises from somewhere deep in the dam works…
As the sun came up, Gamble stood on a platform above the spillway, which for two days had been sending an elegant arc of white water into the river below.
But in the early morning light, Gamble could see large chunks of something — probably rocks or chunks of concrete the size of office chairs — being ejected into the river. And the white water had taken on a reddish tint, a hint that the spillway’s 3-foot concrete lining had somehow been breached and the native red sandstone that gave the Colorado its name was washing into the river.
Update: Post corrected to include the link to my Coyote Gulch post.
Here’s a Coyote Gulch post from a while back with video that tells the story about the spillway failures in 1983. I also include Seldom Seen Smith’s prayer from the, “Monkey Wrench Gang.”