California’s Tallest Bridge Has Nothing Underneath — Full Transcript

Forest Hill Bridge soarses across the valley of the North Fork of the American River just outside Auburn, California at more than 700 ft or 200 m above the canyon floor. It’s the fourth tallest bridge in the United States. When it opened in 1973, crowds cheered for the impressive new structure. But if you take a closer look, it doesn’t really make any sense. This isn’t an interstate highway or even a major thoroughfare. The road sees only a few thousand vehicles a day, connecting Auburn, an exurb of Sacramento with a population just shy of 14,000 to scattered rural communities and recreation areas in the western foothills of the Sierra Nevadas. And while the American River does occasionally flood, it doesn’t flood 700 ft. Before this, the crossing was basically a low-water bridge. A structure of this magnitude just looks out of place. But it wasn’t just a boondoggle, at least not at the outset. It was built that way for a reason. And the story behind it is not only pretty wild, but it also sits at the hinge point of a major chapter in American infrastructure. I’m Grady and this is Practical Engineering.

[00:01:00] California’s central valley is one of the world’s greatest agricultural regions. Over 400 mi long, more than 50 mi wide, this remarkably fertile area is nearly half the size of England. The city of Sacramento sits near its center, right where the Sacramento and American rivers meet. To manage and distribute water across this enormous landscape, the federal government launched the Central Valley Project in 1933. A sweeping effort by the US Bureau of Reclamation to store water in the wetter northern part of the valley and distribute it to the drier south. In the process, the system would also generate hydropower and reduce flood risk for growing urban centers. I’m glossing over a lot here. The history of California is steeped in water issues and even just the central valley project is nearly a century of details, but critically Folsom Dam was one of the first big components of the plan.

[00:02:00] Built in 1955 on the American River, the concrete gravity dam provided significant flood protection to the city of Sacramento. However, it was constructed relatively early in our understanding of basin scale hydrology and the uncertainty surrounding the frequency and magnitude of flooding over long periods of time. It became clear pretty quickly that Folsom Dam didn’t offer quite as much flood protection as was originally promised. Plus, because Folsom Dam had to keep its flood pool empty to handle potential inflows, its ability to store water for irrigation or municipal supply purposes was somewhat limited.

[00:03:00] The answer to these problems, at least according to the federal government, was Auburn Dam, authorized by Congress in 1968. The new structure would sit upstream of Folsom and control the variable flows of the north and middle forks of the American River. It would be the tallest dam in California and one of the tallest in the country and work began in earnest in the early 1970s. One of the first steps in the process was rerouting the American River. Crews built a large coffer dam and carved a diversion tunnel through the canyon wall. With the water redirected, they could begin drying out the bend in the river where the huge new dam would eventually sit. Once the site was dried out, crews began exploring the underlying geology more thoroughly. They drilled bore holes, excavated tunnels and shafts, and surveyed the rock that would serve as the dam’s foundation.

[00:04:00] The site’s geology turned out to be more complex than expected. Some zones of rock were more compressible than others, which could lead to dangerous stress concentrations in the dam. And there were a lot of joints and fissures in the rock mass, making it more challenging to predict how they would behave under extreme loads in addition to creating paths for water. So the next phase of the project was a major foundation treatment program starting in 1974. This mainly involved pressure grouting fractures to reinforce weak zones against the enormous weight of the structure and to make the geology more watertight, preventing seepage from flowing under the dam. With major construction works underway, anticipation for the reservoir was growing. Around the future rim, land values soared and developers rushed to stake claims. Lakefront homes were planned. Entire communities emerged, built on the promise of a shining new shoreline.

[00:05:00] Then in August 1975, a magnitude 5.9 earthquake struck near Oroville Dam, only about 50 mi or 80 km away from the site. The quake only caused minor damage to structures in the area, but it rattled confidence in the Auburn project. The geology of the western Sierra Nevadas had long been considered stable, but the Oroville earthquake introduced a troubling possibility that the loading and filling of large reservoirs could trigger seismic events in the area. This phenomenon known as reservoir induced seismicity is still not well understood even to this day. The pressure of water infiltrating bedrock and the weight of a reservoir can change the balance of forces along faults potentially triggering movement. You know, when Oroville is full, that’s roughly 10 billion pounds of force or 4 billion kg of mass. It’s a staggering amount.

[00:06:01] You can imagine how that might affect the underlying geology. The Auburn Dam as a thin concrete arch in contrast to the concrete gravity dam at Folsom or the earthfill embankment at Oroville would be especially vulnerable to earthquakes. Thin arch dams rely on the canyon walls to resist the thrust of the structure. In fact, I’ve made a video all about the topic you can check out after this. If one side shifts even a little during a quake, the results could be catastrophic. In April 1976, a report by the Association of Engineering Geologists concluded that an earthquake like the one at Oroville could cause the proposed Auburn Dam to catastrophically fail. It was back to the drawing board for the project even as the foundation grouting program continued. And then the project was shaken again. That same year, the newly completed Teton Dam in Idaho collapsed during its first filling, killing 11 people and causing billions in damage. It had been built by the same agency, the Bureau of Reclamation.

[00:07:01] Concern continued to mount about the safety of Auburn Dam, which would have devastating consequences for the thousands of Californians downstream if it were to fail. It was all enough to bring Auburn’s momentum to a halt. While dam construction paused, one aspect of the project had already been finished. Forest Hill Bridge. With a coffer dam on the river and the diversion tunnel only sized for smaller floods, there was a risk of overtopping the existing bridge, cutting off access between Auburn and the Sierra foothills. So, the Bureau of Reclamation decided to get a head start on a project that would eventually be inevitable. A new bridge, permanent and high enough to span the reservoir once it filled. If they were going to build a new bridge, they figured they might as well build it right the first time. The result was a striking steel cantilever bridge with two slender concrete piers soaring skyward from the canyon floor.

[00:08:01] Actually, there was another bridge planned over the middle fork of the American River, the Ruck-a-Chucky Bridge. It was a wild idea, a curved cable-stayed bridge where all the cables are anchored in the hillsides rather than tall towers. But while that project was shelved, Forest Hill made it all the way through design and construction. At the time of its opening in 1973, it was the second highest bridge in the United States. But as time went on, it became increasingly clear they had jumped the gun. By 1980, engineers floated two new dam designs that could withstand potential earthquakes. Both would be shifted slightly downstream from the original site. But by then, the tide of public and government support for the dam had turned. Construction costs had ballooned, and Auburn Dam was looking less feasible every day. As originally proposed, the structure would be even larger than Hoover Dam in size, but store less than 10% of Lake Mead’s volume.

[00:09:00] Meanwhile, upgrades to Folsom Dam and improved levees around Sacramento offered far cheaper ways to reduce the flood risk that was the major impetus for the dam in the first place. New hydrologic data also suggested that earlier flow estimates had been overly optimistic, reducing its value for conservation. The benefits of Auburn Dam were shrinking as the cost grew. It was turning into an incredibly expensive solution in search of a problem. At the same time, environmental and advocacy groups were gaining momentum. The project would flood canyons used for whitewater rafting and kayaking. It would drown ecosystems, inundate archaeological sites, and destroy long segments of the wild and scenic forks of the American River. It became clearer and clearer that the ends simply couldn’t justify the means. And yet, the idea never fully went away.

[00:10:01] In 1986, a massive flood hit the area. Water backed up at the diversion tunnel at Auburn, overtopped the coffer dam, and caused it to fail. Downstream levees were breached and much of Sacramento flooded. For a moment, the momentum behind Auburn Dam and its promise of flood protection returned. But it later became clear that the flood wasn’t entirely a natural disaster. The bureau hadn’t followed the operating guidelines at Folsom Dam, worsening conditions downstream. And by then, the grassroots opposition, cost concerns, and shifting priorities had all but put the Auburn Dam project to bed. Various proposals resurfaced over the years, including the idea of a dry dam that would only hold water during floods, but none gained much traction. With its many iterations and proposals, the project became known as the dam that wouldn’t die. But in 2008, the state of California revoked the bureau’s water rights permit for the project. Maybe not sealing its fate completely, but at least burying it several feet deeper.

[00:11:00] This story really gets to the heart of the challenge with large-scale public works projects. No matter how you configure them, there are big losers and big winners. There’s no doubt that a dam across the American River upstream of Folsom could provide significant benefits to the public. Flood control, water supply, hydropower, recreational opportunities, or some combination of them all. But those benefits have to be weighed against real costs. Environmental damage, staggering capital investment, long-term maintenance, the inherent risk of catastrophic failure, and the social toll of displacement and disruption. The mid 20th century was the heyday of American dam building, an era driven by ambition and optimism, but also by uncertainty. We didn’t have enough historical data to fully understand river systems.

[00:12:01] We couldn’t yet grasp the long-term consequences of altering them, and we couldn’t see into the future to know what the true impacts of these structures would be or what the cost of keeping them in good shape might amount to. Since then, we have a lot more experience with huge multi-purpose reservoirs. And it seems in general that the more we learn, the more the answer to whether they’re worth it seems to be maybe not. And that maybe turns into a probably when you consider that all the best sites are already taken. New Melones Dam completed by the Bureau of Reclamation in 1979, not too far from Auburn, faced a lot of similar controversy and pushback. Although the project was eventually completed, the fight was bitter and its legacy so far is mixed. The project is widely considered to be the last great American dam, at least great in size, if not in public sentiment.

[00:13:01] No other reservoir of that scale has been built in the US since. And with the Auburn Dam project mostly dead, it seems doubtful that there ever will be. The American River continued flowing through the diversion tunnel until 2007 when a new pump station and restoration project returned the river to its original channel. Kayakers can now navigate downstream and even have some new features at the pump station to choose from. The artificial rapids on the left or the screen channel on the right. After more than three decades, the river was back in its place, tying a bow on a dam that was never built. And yet, just a few miles upstream, the Forest Hill Bridge still stands, dramatic, overbuilt, and strangely out of sync with its surroundings. And we’re still kind of stuck taking care of this bridge whose scale is so out of proportion with its purpose. In the 2010s, the bridge underwent a major seismic retrofit to improve its safety and make future inspections easier.

[00:14:01] More recently, it was part of a nationwide program inspecting bridges built with T1 steel, an alloy that in some cases has shown concerning cracks at welds. The I40 bridge crack in Memphis, which I covered in an earlier video, triggered the effort. And there have been quite a few defects found in bridges since then. So, here’s hoping that Forest Hill doesn’t make the list. It’s a cool structure in its own right, but it stands for more than just an engineering achievement. Auburn Dam left a lot of scars, both on the physical landscape and the political one. But it also left this bridge that has become more than just an out-of-place oddity. In a sense, it’s become a monument to the end of an era in US major public works projects and hopefully a tribute to the caution and care that will shape the next one.

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