Megastorms could flood portions of California’s Central Coast

November 30, 2012

Editor’s note: The article will appear in the January 2013 issue of Scientific American. They are making it freely available now because of the flooding underway in California.

By Michael D. Dettinger and B. Lynn Ingram

Huge flows of vapor in the atmosphere, dubbed “atmospheric rivers,” have unleashed massive floods every 200 years, and climate change could bring more of them.

The intense rainstorms sweeping in from the Pacific Ocean began to pound Central California on Christmas Eve in 1861 and continued virtually unabated for 43 days. The deluges quickly transformed rivers running down from the Sierra Nevada mountains along the state’s eastern border into raging torrents that swept away entire communities and mining settlements. The rivers and rains poured into the state’s vast Central Valley, turning it into an inland sea 300 miles long and 20 miles wide. Thousands of people died, and one quarter of the state’s estimated 800,000 cattle drowned. Downtown Sacramento was submerged under 10 feet of brown water filled with debris from countless mudslides on the region’s steep slopes. California’s legislature, unable to function, moved to San Francisco until Sacramento dried out—six months later. By then, the state was bankrupt.

A comparable episode today would be incredibly more devastating. The Central Valley is home to more than six million people, 1.4 million of them in Sacramento. The land produces about $20 billion in crops annually, including 70 percent of the world’s almonds—and portions of it have dropped 30 feet in elevation because of extensive groundwater pumping, making those areas even more prone to flooding. Scientists who recently modeled a similarly relentless storm that lasted only 23 days concluded that this smaller visitation would cause $400 billion in property damage and agricultural losses. Thousands of people could die unless preparations and evacuations worked very well indeed.

Was the 1861–62 flood a freak event? It appears not. New studies of sediment deposits in widespread locations indicate that cataclysmic floods of this magnitude have inundated California every two centuries or so for at least the past two millennia. The 1861–62 storms also pummeled the coastline from northern Mexico and southern California up to British Columbia, creating the worst floods in recorded history. Climate scientists now hypothesize that these floods, and others like them in several regions of the world, were caused by atmospheric rivers, a phenomenon you may have never heard of. And they think California, at least, is overdue for another one.

A megaflood every century?

Despite greater scientific understanding, the 1861–62 floods are all but forgotten today. Communities, industries and agricultural operations in California and the West have spent the past century spreading out onto many of the same floodplains that were submerged 150 years ago. Residents everywhere are unaware or unwary of the obvious risks to life and property. Meanwhile, though, anxious climatologists worry about the accumulating evidence that a megastorm could happen again and soon.

The concern grows out of research that is looking 2,000 years back in time to piece together evidence revealing the occurrence and frequency of past floods, like detectives returning to a crime scene of long ago. They are sifting through evidence archived in sediments from lake beds, floodplains, marshes and submarine basins.

Geologists have found more evidence in southern California, where two thirds of the state’s nearly 38 million people live today, along the coast of Santa Barbara. Sediments there settle to the seafloor every spring (forming a light-colored layer of algae known as diatoms) and again in winter (forming a dark-colored silt layer). Because the oxygen concentrations in the deep waters there are inhospitably low for bottom-dwelling organisms that would usually churn and burrow, the annual sediment layers have remained remarkably undisturbed for thousands of years. Sediment cores there reveal six distinct megafloods that appear as thick gray silt layers in A.D. 212, 440, 603, 1029, 1418 and 1605. The three most recent dates correlate well with the approximate 1100, 1400 and 1650 dates indicated by the marsh deposits around San Francisco Bay—confirming that truly widespread floods have occurred every few hundred years. (In October, Ingrid Hendy of the University of Michigan and her colleagues published a paper based on a different dating method; it found a set of Santa Barbara dates that were offset from the six specific dates by 100 to 300 years, but the same basic pattern of megafloods every 200 years or so holds.)

The thickest flood layer in the Santa Barbara Basin was deposited in 1605. The sediment was two inches thick, a few miles offshore. The 440 and 1418 floods each left layers more than an inch thick. These compare with layers of 0.24 and 0.08 inch near the top of the core that were left by large storms in 1958 and 1964, respectively, which were among the biggest of the past century. The three earlier floods must have been far worse than any we have witnessed.

Time to Prepare

With atmospheric rivers likely to become more frequent and larger and with so many people now living in their paths, society would be wise to prepare. To provide an example that California emergency managers could use to test their current plans and methods, scientists at the U.S. Geological Survey recently developed the scenario mentioned at the start of this article: a megastorm that rivaled the 1861–62 storm in size but lasted 23 days instead of 43 (so no one could claim that the scenario was unrealistic). To further ensure that the scenario, which was eventually dubbed ARkStorm (Atmospheric River 1000 Storm), was as realistic as possible, scientists constructed it by stitching together data from two of the largest storm sequences in California from the past 50 years: January 1969 and February 1986.

When project leaders ran the events of ARkStorm through a variety of weather, runoff, engineering and economic models, the results suggested that sustained flooding could occur over most lowland areas of northern and southern California. Such flooding could lead to the evacuation of 1.5 million people. Damages and disruptions from high water, hundreds of landslides and hurricane-force winds in certain spots could cause $400 billion in property damages and agricultural losses. Long-term business and employment interruptions could bring the eventual total costs to more than $700 billion. Based on disasters elsewhere in recent years, we believe a calamity this extensive could kill thousands of people (the ARkStorm simulation did not predict deaths).

The costs are about three times those estimated by many of the same USGS project members who had worked on another disaster scenario known as ShakeOut: a hypothetical magnitude 7.8 earthquake in southern California. It appears that an atmospheric-river megastorm—California’s “Other Big One”—may pose even greater risks to the Golden State than a large-magnitude earthquake. An ARkStorm event is plausible for California, perhaps even inevitable. And the state’s flood protection systems are not designed to handle it. The only upside is that today, with improved science and technology, the megastorms could likely be forecasted anywhere from a few days to more than a week in advance. Proper planning and continuing efforts to improve forecasts could reduce the damage and loss of life.

Californians, as well as people all along the West Coast, should be aware of the threats posed by atmospheric rivers and should take forecasts of storms and floods very seriously. Planners and city and state leaders should also take note as they decide on investments for the future. He who forgets the past is likely to repeat it.  (read the full article here)

This article will be published in print as “The Coming Megafloods.”

Michael D. Dettinger is a research hydrologist for the U.S. Geological Survey and a research associate at the Climate, Atmospheric Sciences and Physical Oceanography Division at the Scripps Institution of Oceanography in La Jolla, Calif.

Lynn Ingram is a professor of earth and planetary science at the University of California, Berkeley, and co-author of The West without Water (University of California Press, Spring 2013).