By Fraser Perkins —
Though the Mesopotamians invented the arch, the Romans refined, perfected, and incorporated it into their bridges, aqueducts, and buildings. The utility of the arch comes from its ability to distribute a load along its entire span, not just in the middle of the arch. The same principle applies also to dam construction. Flip an arch on its side with the convex surface facing a force and it works just as well on its side, as it does when oriented vertically. The Bureau of Reclamation finished Hoover Dam in 1935 on the Colorado River with just this arrangement – convex side facing upstream and concave side facing downstream.
Regrettably, cement arch dams have a major flaw – they fare poorly in major earthquakes and risk failure, making them unsuitable in California. Enter the embankment dam. An embankment dam, though far less elegant than the delicate, graceful curve of an arch dam, does well in earthquakes. In its usual form an embankment dam has a core of clay buried under rock, gravel, or earth, and covered with concrete. Oroville Dam is such an embankment dam. Sadly, embankment dams like Oroville also have a potential flaw. If water flows over the top, called overtopping in dam-speak, the entire structure is at risk of catastrophic collapse – like a sandcastle disintegrating before a wavelet.
Oroville Dam, completed in 1961 to impound the Feather River, contains both a primary spillway and an emergency spillway to prevent overtopping. It was also designed to withstand likely floods. Sadly, since 1961 several factors have undermined confidence in Oroville Dam.
Design – The original contingency plans for Oroville Dam did not include data from the Flood of 1861-1862. This little heralded event devastated California with an endless series of storms battering the Sierra Nevada. Normally placid rivers became avalanches of water which killed 4,000 people, roughly 1% of the population, and washed away 25% of the buildings. Sacramento became Venice with boats (but no gondolas) traveling along makeshift canals, formerly known as streets.
Placement of Spillways – Both spillways are located at the top of the dam. Though a reasonable common-sense plan, this design prevents preventive release of water until the very last moment. If thousands of cubic feet of water/second cascade into Lake Oroville, as occurred during the Flood of 1861, the spillways may not release sufficient water to prevent overtopping. Sacramento might once again become Venice.
Spillway Durability – During the 2017 California storm season, Oroville Dam suffered a major structural failure. Water gushing down the main spillway gouged a 30-foot hole in the center and promptly started eroding the spillway back toward the top of the dam. To avert overtopping and disaster, dam managers opened the emergency spillway for the first time…and weren’t entirely sure what would happen. It’s always exciting to test an emergency system for the first time during an actual emergency. Fortunately, we all got lucky, overtopping never occurred and dam managers later repaired the spillways.
Technology – Over the past 30 years, satellite imaging has given a clearer picture of what happens during storms. Major currents of water vapor, called atmospheric rivers and containing as much water as the Mississippi River, collide with our mountainous backdrop releasing prodigious amounts of rain.
Lax Permitting – Simply put, under intense pressure we have built extensively on former lake beds putting entire communities at peril. This past winter Lake Tulare suddenly sprang to life for the first time in memory, surprising us all.
This is the “good” news, and now for the bad. As dicey as our dams may be, downstream from our dams, levees, some dating from the Gold Rush era, will likely fail well before our dams do.
Now, add in Climate Change… While most years are dry, summer droughts year after year should not blind us to the risk of catastrophic flooding. Our atmosphere is warming, and warmer air has greater capacity for holding moisture. Future storms and atmospheric rivers will likely contain more water vapor and rain. Warmer ground temperatures will also melt snow more quickly, accelerating run-off, flooding, and the potential for dam and levee failure.
Potential flooding is not confined to California north of the Tehachapi Mountains. Despite our drier climate, Southern California is also at risk from flooding. Seven Oaks Dam, a dry* embankment dam providing protection from the Santa Ana River, failed a tunnel release test in 2005 when the reservoir was only one-third full. Fortunately, after repairs the tunnel passed a retest in 2011.
The message is clear: we need to upgrade our water infrastructure to not only conserve water during droughts, but also protect against flooding. It is especially challenging to guard against a potential threat, one which may not come in our lifetime, but which will eventually occur with devastating consequences. Though not as exciting as many projects, upgrading our infrastructure to withstand flooding is a civic duty that can only be done by all of us acting together; in short, by government. Democrats control all branches of state government – we should act proactively on infrastructure upgrades.
I invite you to watch a short video provided by the USGS: Arkstorm. Ignore the sensationalism, eerie music, and period costuming and focus on the brief atmospheric river sequence. The popcorn is on you!
*Dry means there is usually no water in the reservoir