California Levees In Trouble From Drought

What's worse than months or years without rain? Rain, after months or years, at least under some circumstances.

For instance ... it gets try, plants become vulnerable to fire. Fires happen denuding the dry landscape. Then it rains, and you get more severe floods together with landslides. You know the story because for years this has been the pattern in California.

But there is another roughly similar, or at least analogous, problem that is now being discussed. The levees that are mean to keep floodwaters contained in California were already in fairly bad shape. Prior to the drought, a significant number of levees were known to be at risk of failure should they actually get used. Many are thought unable to handle earthquakes as well.

But with the drought, several factors have probably made the levees weaker. This is an ongoing process and will continue as long as the drought continues.

From a letter to science, "Drought threatens California's levees" by Farshid Vahedifard, Amir Aghakouchak, and Joe Robinson,

Prolonged droughts undermine the stability of levee systems by increasing water seepage through soil, soil cracking, soil strength reduction, soil organic carbon (SOC) decomposition, and land subsidence and erosion . The sand-clay mixtures, which form the body of the levees and consequently the entire structure, can lose a substantial amount of strength under dry conditions. Furthermore, levees in California are built on peaty soils, and the extreme drought leads to greater SOC decomposition in these soils. A large amount of the global carbon stock is found in peaty soils, and ~25% of this estimated stock is predicted to diminish under extremely dry conditions. Oxidation of SOC under a prolonged drought can also accelerate land subsidence. In fact, 75% of the land subsidence across California is accredited to oxidation of SOC. Land subsidence can increase the risk of water rising over the top of the levees.

This happened in Australia. Remember the big flooding a couple of years back? Some of that was made worse by levees failing, and those levees had been weakened by prolonged drought. So this is not theoretical.

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"In fact, 75% of the land subsidence across California is accredited to oxidation of SOC."

In fact??

"More than 80 percent of the identified subsidence in the United States is a consequence of human impact on subsurface water, and is an often overlooked environmental consequence of our land and water-use practices."
http://water.usgs.gov/ogw/pubs/fs00165/

And in the San Joaquin Valley of Central California, this is a *serious* problem: "Between 1926 and 1970, groundwater pumping caused widespread aquifer compaction and resultant land subsidence in the valley. Subsidence in some areas exceeded 28 feet.

"At the center of the subsidence bowl near El Nido, more than 21 inches of subsidence was recorded between the two years. The rate of subsidence in this area – nearly 1 foot a year – is among the highest ever measured in the San Joaquin Valley."
http://www.usgs.gov/newsroom/article.asp?ID=3731

Where did Vahedifard, Aghakouchak, and Robinson get their comparative data on SOC subsidence to make that claim?

By Brainstorms (not verified) on 24 Aug 2015 #permalink

And every step of the way....what did the state authorities do? Nothing. This isn't something that just occurred in 2015. There's been indicators and signals going on for at least two decades....maybe longer. The state chose to look the other way and invest in other things (rapid-rail for example). The sad fact is that if they did something....they would have been sued by various environmental groups and a simplified three-year repair plan would gone on for two decades in some court. The state, in all ways, is screwed up.

By Pepsi_dude (not verified) on 24 Aug 2015 #permalink

Because their antiquated & insane "water rights" laws are screwed up. And no one in the legislature has the balls to stand up to the property holders and rewrite them (to be more like every other states' laws).

Once we pump all the water out and the pore space goes flat, that's it: it's permanent; no possibility of recharging our groundwater. They think things are bad now?

By Brainstorms (not verified) on 25 Aug 2015 #permalink

From http://www.reuters.com/article/2015/08/19/us-usa-drought-california-idU…

"Parts of the San Joaquin Valley in Central California are sinking faster than ever due to excessive groundwater pumping as the state deals with a devastating drought, a NASA report released on Wednesday said.

"Some areas are experiencing nearly 2 inches (5 cm) of sinking per month, a trend that could damage infrastructure such as bridges, roads and aqueducts, the California Department of Water Resources said in a statement about the report.

"The department said long-term sinking has already destroyed thousands of private and public groundwater well casings in the agriculture-dependent valley, adding that over time more sinking could permanently reduce how much water can be stored in the underground aquifer."

By Brainstorms (not verified) on 26 Aug 2015 #permalink