It’s no secret that a vast amount of American infrastructure is in great need of upgrades, repairs or replacements. The repairs that are desperately needed will come, and they will come in one of two ways. Either proactive repairs can be made when problems are first discovered, or repairs can be made at considerably greater cost after catastrophic failures have occurred. As was the case with the I-35 bridge collapse in Minnesota, we often pay in lives as well. Part of the problem is that infrastructure isn’t very exciting or newsworthy to many people outside of the civil engineering community which leads to complacency and apathy. As a result, it’s likely that you may not have heard about the latest struggle currently playing out in California even though it involves the largest dam in the United States and its potential failure.
Surprisingly enough, the largest dam in the US isn’t the famous Hoover Dam but the Oroville Dam at the base of the Sierra Nevada mountain range in California. At 235 meters, it is almost 15 meters taller than the Hoover Dam. It can store over four cubic kilometers of water but whether or not it will keep storing that water into the future is currently under question. In February of this year during a flood control operation damage was observed on the dam’s spillway where a massive hole had formed which only got larger as the dam was forced to continue releasing water. The hole quickly grew, and the floodwaters eroded much of the lower half of the spillway embankment, forming a canyon.
The greater threat to the dam itself wasn’t simply the damage to the main spillway, but the use of the dam’s emergency spillway. It was used for the first time after the main spillway had to be shut down, but once the water started flowing, the amount of erosion behind the emergency spillway was much higher than anticipated. It was thought at one point that the erosion might undermine the strength of the dam itself which would have let loose a 9-meter-high wall of water down the Feather River, destroying many communities in its path. An evacuation order was issued for residents of the area during these series of events, but luckily the main spillway stabilized (although heavily damaged) and was able to allow Lake Oroville to drain enough to alleviate concerns of a total dam failure. The snowmelt in the Sierra Nevada isn’t finished yet, however, so the dam and the engineers working on it aren’t quite out of the woods.
As of this writing, the damaged main spillway has just been reopened and is expected to handle 35,000 cubic feet per second for a few weeks. Engineers have made great strides to make sure that the spillway can handle the outflow and have been given some gifts from the surrounding topology to make that a reality. First, rather than the entire mountainside eroding beneath the damaged concrete spillway, a large “splash pool” formed that serves to dissipate much of the energy of the moving water before flowing through the newly-created canyon. This has helped keep erosion to a minimum. Oddly enough, increasing the amount of water flowing down the spillway decreases the amount of erosion (to a point) because it lands further down the mountainside, limiting erosion under the good section of spillway further up. The addition of shotcrete to this area has also helped to stabilize the spillway. Additionally, the erosion towards the bottom of the spillway formed a canyon after the water scoured the landscape down to bare bedrock. This rock is a lot less prone to erosion and has thus created a relatively working spillway that bypasses the bottom half of the man-made spillway.
While the investigation as to the cause is ongoing, there have been a few other American dams that have had similar spillway issues that engineers have had to learn from. The most notable was at Glen Canyon Dam in Arizona where, in the early 1980s, cavitation issues in the main spillway tunnel led to the formation of a giant hole inside the tunnel that was rapidly eroding the canyon walls and threatening the integrity of the dam itself. Cavitation is a phenomenon where changing pressure in a liquid can create bubbles which later collapse under higher pressure and generate a damaging shockwave.
The Glen Canyon Dam was constructed between 1956 and 1966 before the invention of “air slots” that prevent damage caused by cavitation in high-velocity flows. The repairs to Glen Canyon Dam included these air slots and the dam has been operational since. Potential repairs to the Oroville main spillway include similar designs to mitigate cavitation, but further investigation is needed. This will likely not be completed anytime soon, though, as the main spillway is required to operate until the risk of flooding has passed after the spring snowmelt in the Sierra Nevada has stopped.
Future repairs include plans to reinforce the rear of the emergency spillway. During its first-ever use this year, erosion behind the emergency spillway was much more rapid than predicted, also raising concerns that the dam may fail. This risk has largely been mitigated for now, but it is something that will need to be addressed in the immediate future. Transmission lines that run across the emergency spillway will likely need to be repaired as they may have experienced foundation issues from the rapid erosion.
While repairs are ongoing, its likely that this dam will be on the radar for the foreseeable future. Unfortunately, this isn’t an isolated problem for dams or with infrastructure in general. Most of the big-ticket infrastructure items were built decades (or, almost a century) ago and have been minimally maintained in the interim. The future of this infrastructure, and the people who depend on it, rests on the shoulders of engineers willing to try new and innovative solutions to keep it all together.
You can keep up with the issues at Oroville Dam via the California Department of Water Resources, the organization responsible for the upkeep of the dam, or via some fantastic reporting by Juan Browne via YouTube. If there is another piece of infrastructure near you that isn’t receiving the attention it deserves, tell us about it in the comments or email the author directly at firstname.lastname@example.org.
Featured image by Dale Kolke / California Department of Water Resources (California Department of Water Resources) [Public domain], via Wikimedia Commons