So, California is at serious risk of flooding.
As readers may be aware, the Oroville Dam’s emergency spillway came very near to failing a few days ago, triggering massive evacuations for people who live downstream. The situation has been stabilized, and people may now return home, but more storms are due in tomorrow, and so everything is very precarious.
Before I get to my point (climate change), I offer a brief synopsis of the situation and of the dam.
The Dam Situation
The Oroville Dam* blocks the Feather River, a tributary of the Sacramento River, and thus creates Oroville Lake, near Oroville, California. The dam was first proposed in 1951 and was finally completed in 1968 (and dedicated by then-governor, Ronald Reagan). It is the tallest dam in the United States. Its primary purpose purpose is flood control, though it also collects water for both municipal use and for irrigating the San Joaquin Valley, and generates hydroelectric power.
Under normal circumstances, water released by the dam flows out through pipes at its base, to rejoin the Feather River. During floods, a spillway off to the right of the dam (that is the dam’s right, river-right) can be opened. Since the spillway gate is high up, close to the level of the top of the dam, water must flow from the gate down a long, concrete-lined path, to get back to the Feather River. So far so good. But on February 7th, a week ago today, the rushing water eroded a hole in the floor of the spillway. The concern was that if the hole grew large enough, it could undermine the spillway gate and cause it to fail–so the gate was partially closed. And the water started to rise.
Fortunately, there was a plan B in place. The earthen embankment to the right of the spillway is topped by a secondary dam that is lower than the primary dam. If the spillway system fails, water will spill over this secondary dam–called the emergency spillway–instead of over-topping the main dam (and possibly damaging it). On February 11th, for the first time in the facility’s history, water did flow over the emergency spillway–and began eroding the embankment.
Had the erosion gone on long enough, it would have undercut the secondary dam, washing it away, and sending a giant wall of water down over the homes of almost 190,000 people. Hence the evacuations.
Officials were able to drop the lake level enough to stop the flow over the emergency spillway and to make emergency repairs, but California’s rainy season still has at least two more months to run. A catastrophic failure at Oroville is still not out of the question.
Dam Climate Change
The Oroville Dam, like much of the rest of California’s water infrastructure, suffers from several problems.
American infrastructure generally is in poor shape, largely because it is politically much easier to fund new construction than to fund repair (I wonder, too, whether changes in the tax structure have starved public works–much of our infrastructure dates back to a time when America was much more civics-minded than it is today). So many dams are past due for maintenance. Oroville specifically might have gotten a concrete lining for its emergency spillway–as was suggested and rejected in 2005–had more funding been available. In that case, erosion would not have been a threat.
But Oroville was also designed for much smaller flood volumes than are now considered likely. Part of that is simply that the modeling is more accurate now, but part of it is that floods are bigger. There is more impermeable surface, preventing water from seeping into the ground before it reaches a river, and there are more extreme weather events, thanks to climate change. Droughts, like the one California just came out of, are deeper and longer, while rainy periods are wetter than ever before, too. The state is currently having its wettest year on record–2017 has topped the region’s typical annual rainfall already. The system just wasn’t designed for this.
Scientific American (as seems to be its usual) cautions that it’s too soon to tell whether there’s any link between Oroville’s dam problem and climate change, but acknowledges that problems like this will occur more frequently because of climate change. However, a study published six years ago explains that storms associated with “atmospheric rivers” do hit California more intensely in certain climate change scenarios–specifically, while average atmospheric river (AR) activity stays the same, the extremes become more so, with more storms, more intense storms, and warmer storms in some years. The recent storms have been AR storms, and at least some have been notably warm. That seems like a pretty clear link to me.
Variation in precipitation is not the only factor, either.
Snowmelt in California’s mountains has been getting earlier and earlier since the 1940’s. Regional, and possibly natural patterns are involved, and back in 1994, when this paper was published, researchers weren’t sure anthropogenic climate change was a factor. But that was 23 years ago. I bet they’re sure, now, I just haven’t tracked down a more recent paper on the subject, yet. Earlier snowmelt and warmer winters (in which more moisture falls as rain rather than snow) together mean that more water runs off the land without having time to soak into the ground–or be used by agriculture. That means both more trouble with flooding and with California’s aging dams and more serious droughts, potentially in the same year.
In fact, parts of California (though not Oroville or, as far as I can tell, the watershed that feeds Oroville Lake) are still in a drought, according to the US Drought Monitor (unfortunately, you won’t be able to find this week’s report is you visit the monitor after it next updates).
What does all of this mean for Californians? It means they need us to stop causing climate change, obviously. But the state will also need to make decisions about its infrastructure, its water-use plans, and its development patterns that are more in keeping with the climate change we’ve already locked in–and those decisions depend on accurate and up-to-date data and analysis.
Next time anyone asks you if it’s really important for state and Federal governments to have access to accurate climate science, you can talk about the thousands of people who might drown this winter if the Oroville dam fails after all.
We already know that the evacuation clogged the highways. Had the spillway failed, some people could have been overtaken by water in their cars.
*Yes, I linked to Wikipedia, even though I generally consider it an unreliable source. In this case, the details of the Wikipedia article are consistent with, and largely seconded by, what I’ve read elsewhere, but I’d have to cite a half-dozen other articles at once if I wanted to avoid Wikipedia in this case.-C.