HomeClimate Change NewsHow a megaflood could swamp California’s Central Valley

How a megaflood could swamp California’s Central Valley

The other ‘big one’: How a megaflood could swamp California’s Central Valley

Posted on 30 January 2023 by Guest Author

This is a re-post from Yale Climate Connections by Jeff Masters

When early settlers came to the confluence of the Sacramento and American Rivers before the California Gold Rush, Indigenous people warned them that the Sacramento Valley could become an inland sea when great winter rains came. The storytellers described water filling the valley from the Coast Range to the Sierra during these rare events.

And their warnings became realized when Great Flood of 1861-62 hit. A six-week onslaught of at least 10 powerful Pacific storms in December and January carried mighty “atmospheric rivers” of subtropical moisture into California, dumping torrential rains in the valleys and prodigious snows in the mountains. When an unusually warm storm struck in January, heavy rains fell on the enormous Sierra snowpack, melting it.

A cataclysmic flood ensued, inundating the Central Valley and transforming it into a lake 300 miles long and over 20 miles wide; much of the now densely populated coastal plain in present-day Los Angeles and Orange counties was also inundated. As summarized in a 2013 Scientific American overview, the flood killed thousands of people, drowned one-quarter of the state’s estimated 800,000 cattle, and submerged downtown Sacramento under more than 10 feet of brown water laden with debris from countless mudslides. With the state’s capital city paralyzed, the California legislature was forced to move to San Francisco until the summer of 1862. By that point, the state was bankrupt, as one-third of its taxable properties had been destroyed.

California’s long history of megastorms

Sediment research has found that six storms similar to or even more severe than the 1861-62 storm hit California in the past 2,000 years, arriving about every 200 to 400 years. One study estimated the arrival dates as 212, 440, 603, 1029, 1418, and 1605 AD; the dates vary by a century or more from study to study, but the data makes clear that such megastorms have recurred regularly. The storm that occurred around 1605 appears to have been the mightiest of them all — and far stronger than storm that brought the Great Flood of 1861-62.

Given this history, it is inevitable that another great flood will hit the state someday, and climate change is thought to boost the odds of such an event. And when the next great flood comes, the damages could well dwarf those of any previous global weather disaster, adding up to more than $1 trillion — an extraordinary catastrophe with triple the cost of the feared great quake on the San Andreas fault.

According to a 2011 government scenario, waters, winds, and landslides from such a megastorm would likely overtop dozens of levees, flood nearly a quarter of the square footage of the state’s buildings, wipe out key roads for weeks to months, and leave some communities without power for months.

In this first of a three-part series on California’s vulnerability to a megaflood, we examine the results of this 2011 study, called the “ARkStorm” scenario, which simulated what a repeat of the Great Flood of 1861-62 might do. Part Two looks at the poor state of the U.S. dam infrastructure in general, and more specifically at the California dams at the highest risk of failure in a megaflood. And since the ARkStorm research is more than a decade old, a new study that presents an “ARkStorm 2.0” scenario will be covered in Part Three, which will discuss the future of California megafloods and how climate change likely increases their odds.

California is highly vulnerable to a great flood

Many of the prosperous cities and fertile farmlands of California are built on the flood plains of the rivers that once inundated the valleys every few hundred years. Now the rivers are dammed and lined with levees and drainage channels, protecting the critically important development on California’s flood plains.

A megaflood would be a catastrophe for two main reasons:

What is the ARkStorm scenario?

Were the Great Flood of 1861-62 to recur, it might resemble the ARkStorm scenario — a plausible hypothetical storm conjured up in a 2011 study by 117 experts led by the United States Geological Survey, or USGS. In the acronym, the “AR” stands for atmospheric river, and the “k” stands for the number 1,000 because the storm could be expected to bring 1-in-1000-year rains to some locations.

The storm modeled by these experts could flood up to 25% of all buildings in the state, breach approximately 50 levees, and force the evacuation of 1.5 million people. The hypothetical ARkStorm would flood about 4,000 square miles, much of it agricultural, with a population of about 6.5 million, including much of Sacramento, Silicon Valley, and Stockton. Along with the mammoth evacuation required in the inland region and delta counties, over 220,000 people would need short-term shelter.

Since there is little data on the meteorological conditions during 1861-62, the USGS ARkStorm scenario used a computer-modeled hybrid storm that combined two actual storms that hit California: a Southern California storm from Jan. 19-27, 1969, which killed 115 people and caused inflation-adjusted damages of $3.3 billion, followed by a northern California storm from Feb. 8-20, 1986, which killed 13. An additional tweak was applied to produce a sufficient amount of precipitation to approximately match the limited observations of 1861-62.

The simulated USGS ARkStorm did $725 billion (in 2007 dollars) in damage. Approximately 55% of the damage was to buildings, infrastructure, and agriculture (of which just 5-8% would be covered by insurance), while 45% of the damage resulted from business interruption. Adjusted for inflation, the ARkStorm would cost about $1.1 trillion in 2023 dollars, and additional costs would occur because of increases in wealth and population. In recent decades, wealth on the West Coast has increased by about 3.3% per year (using GDP growth stats); the population of California has increased by about 6% since 2007. This suggests that the ARkStorm would cost approximately $1.7 trillion today (about 7% of U.S. GDP). Complicating this estimate is the fact that many billions of dollars in levee improvements have occurred in California since the ARkStorm report was issued, which would likely lead to a modest decrease in damages.

The authors cautioned that the impacts modeled were not exhaustive, since they did not consider tourism and recreation or loss of cultural value as a result of damage to historic artifacts and sites. And a repeat of one of the stronger storms documented in the past 2,000 years might cause damages much higher.

Much of the rest of this post covers the specific sectors considered in the ARkStorm scenario, demonstrating the extraordinary danger such a flood would likely pose to people, infrastructure, agricultural lands, and property. All costs are in 2007 dollars; adjusting for inflation would make these costs about 50% higher in 2023 dollars.

A megastorm would breach levees and flood buildings

The ARkStorm scenario generated flooding with an estimated return period of 100 to 500 years over many critical California watersheds, in addition to some 1-in-1000-year flooding. A 100-year return period means that a flood has a 1% probability in any given year, or a 26% probability over a 30-year time span. Similarly, flooding with a 500-year return period has a 0.2% probability of occurrence in any given year, or 6% over a 30-year period.

Nearly one-quarter of the total building square footage in California was affected by flooding in the ARkStorm scenario. Most flooded buildings were not a total loss, but rather experienced damage requiring repair costs between 10-50% of replacement cost. (Just one inch of water in a 2,500-square-foot home can cause $27,000 in damage, and 12 inches can cause $72,000 in damage, according to FEMA.) Residential buildings dominated the flood-related building repair costs. Total flood damage to buildings was estimated at $195 billion, with another $103 billion in damage to building contents. Business interruption from building downtimes of one to three years would cost an additional hundreds of billions of dollars.

The scenario hypothesized that urban levees might be threatened or overtopped at 60 to 75 critical sites and that 15-20 breaches might realistically occur. In addition, 30 breaches of levees protecting the islands of the Sacramento-San Joaquin Delta were considered realistic, with two to three breaches occurring per island. The Delta is about 73% farmland, with much of the land lying 10-25 feet below sea level; these fields are some of the most productive agricultural land in the nation. The Delta’s levees have experienced one major breach in the past 30 years, a June 2004 event that flooded the entire island of Jones Tract.

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