Two-thirds of California residents rely on fresh water pumped from the Sacramento-San Joaquin River Delta, but the levees here are more than 100 years old and ready to give in. A failure would no doubt flood hundreds of acres across the north, but it could also compromise drinking water for an entire state already coping with “The Big Gulp.” Jeff Mount, a geology professor at the University of California, Davis, talked to PM about why the Delta is in trouble—and what we can do to save it. —Erin Scottberg
Why are the levees so important in the Delta?
The fundamental issue is that the abundant, clean water, which is particularly important for urban use, is coming out of the Sacramento River on the north—but the demand for that water is south of the Delta. Therein lies the problem: Conveyance is the bottom line.
But there’s another problem: The interior of these islands have subsided dramatically—in some cases, they’re more than 20 ft. below sea level. In the process of farming, these peat soils oxidized, meaning what was organic carbon went off into the atmosphere as CO2. Essentially, you exported a lot of carbon. If this was a smokestack, you’d regulate it.
Why are the levees in such bad shape?
The levees weren’t constructed—they evolved. They would have to continuously add on to these levees to make them bigger and stronger as the islands subside, and, frankly, the fact is things are getting worse. The inflows to the delta have been increasing over the last 50 years. When you put a foot of new material on top of a levee out there in the Delta, you have to count on the fact that you’ll loose about half of that.
You have to think about tomorrow, and there are at least four major drivers of change that are seeking to undo these levees: the continued subsidence of the islands, the increasing force on the levees, sea level rise, and the fourth one is how I really got the name Dr. Doom. The fact remains that this is going to be shaken by an earthquake. A very intense look by the state in the past year suggested that you will have catastrophic levee failure in this system even if you make all these investments in improving their performance due to high water. The reason? The foundations. To replace the foundations, to make them seismically safe, is an extraordinary cost.
How would you build an earthquake-proof levee?
There are two approaches engineers are kicking around: One is to follow the snowshoe analogy—literally try to spread the levee out so large that it can float during an earthquake. The second option is to go in and actually stabilize the foundations themselves.
What are the alternatives?
Reduce exports, because business as usual doesn’t really solve the problem-–that risk is still high. Option Two is to build an isolated facility—someway of taking the main source of water, moving it through or around the Delta in a way which isolates it from the Delta and then exporting it to the Bay Area, the San Joaquin Valley and L.A.
And this separate structure would have to be earthquake-proof, too?
Yes, but in this case, you’re building it from scratch and you have the engineering know-how. The estimates of cost of doing this are widely variable, but more than $5 billion less than $10 billion is what people are saying. You can now engineer a structure that is earthquake resistant, resistant to the effects of sea level rise, subsidence and changing inflows. That’s why this is a popular idea again. It’s back.