US Flood Maps Outdated Thanks to Climate Change. Also, CA faces a realistic possibility of an extreme statewide megaflood after a month of storms, some winter.

FEMA’s director says the federal government’s flood maps are outdated and don’t take into account recent rainfall trends. (Guardian)

And NYTimes, August 12, 2022, California’s coming megaflood, with modeling and animation

California faces the growing threat of another kind of calamity, one whose fury would be felt across the entire state.

This one will come from the sky.

According to new research, it will very likely take shape one winter in the Pacific, near Hawaii. No one knows exactly when, but from the vast expanse of tropical air around the Equator, atmospheric currents will pluck out a long tendril of water vapor and funnel it toward the West Coast.

This vapor plume will be enormous, hundreds of miles wide and more than 1,200 miles long, and seething with ferocious winds. It will be carrying so much water that if you converted it all to liquid, its flow would be about 26 times what the Mississippi River discharges into the Gulf of Mexico at any given moment. When this torpedo of moisture reaches California, it will crash into the mountains and be forced upward. This will cool its payload of vapor and kick off weeks and waves of rain and snow.

Drenching rains will pummel hills and towns.  LA and suburbs could get 2 inches of rain an hour.  Failing dams threaten farms and people in the Central Valley.  And it happens through repeated storms until the state reaches a statewide average of 16 inches in a month with some areas up to 40 inches, including those hills around LA. Some communities might be damaged beyond repair.

The coming superstorm — really, a rapid procession of what scientists call atmospheric rivers — will be the ultimate test of the dams, levees and bypasses California has built to impound nature’s might.

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But in a state where scarcity of water has long been the central fact of existence, global warming is not only worsening droughts and wildfires. Because warmer air can hold more moisture, atmospheric rivers can carry bigger cargoes of precipitation. The infrastructure design standards, hazard maps and disaster response plans that protected California from flooding in the past might soon be out of date.

As humans burn fossil fuels and heat up the planet, we have already increased the chances each year that California will experience a monthlong, statewide megastorm of this severity to roughly 1 in 50, according to a new study published Friday. (The hypothetical storm visualized here is based on computer modeling from this study.)

In the coming decades, if global average temperatures climb by another 1.8 degrees Fahrenheit, or 1 degree Celsius — and current trends suggest they might — then the likelihood of such storms will go up further, to nearly 1 in 30.

At the same time, the risk of megastorms that are rarer but even stronger, with much fiercer downpours, will rise as well.

2016

This was the peak rainfall rate in downtown Los Angeles during a notably wet winter.0.4 inches per hour

2072

This is how much rain might fall there if greenhouse-gas emissions remain high.

These are alarming possibilities. But geological evidence suggests the West has been struck by cataclysmic floods several times over the past millennium, and the new study provides the most advanced look yet at how this threat is evolving in the age of human-caused global warming.

The researchers specifically considered hypothetical storms that are extreme but realistic, and which would probably strain California’s flood preparations. According to their findings, powerful storms that once would not have been expected to occur in an average human lifetime are fast becoming ones with significant risks of happening during the span of a home mortgage.

“We got kind of lucky to avoid it in the 20th century,” said Daniel L. Swain, a climate scientist at the University of California, Los Angeles, who prepared the new study with Xingying Huang of the National Center for Atmospheric Research in Boulder, Colo. “I would be very surprised to avoid it occurring in the 21st.”

Unlike a giant earthquake, the other “Big One” threatening California, an atmospheric river superstorm will not sneak up on the state. Forecasters can now spot incoming atmospheric rivers five days to a week in advance, though they don’t always know exactly where they’ll hit or how intense they’ll be.

Using Dr. Huang and Dr. Swain’s findings, California hopes to be ready even earlier. Aided by supercomputers, state officials plan to map out how all that precipitation will work its way through rivers and over land. They will hunt for gaps in evacuation plans and emergency services.

The last time government agencies studied a hypothetical California megaflood, more than a decade ago, they estimated it could cause $725 billion in property damage and economic disruption. That was three times the projected fallout from a severe San Andreas Fault earthquake, and five times the economic damage from Hurricane Katrina, which left much of New Orleans underwater for weeks in 2005.

Dr. Swain and Dr. Huang have handed California a new script for what could be one of its most challenging months in history. Now begin the dress rehearsals.

“Mother Nature has no obligation to wait for us,” said Michael Anderson, California’s state climatologist.

In fact, nature has not been wasting any time testing California’s defenses. And when it comes to risks to the water system, carbon dioxide in the atmosphere is hardly the state’s only foe.

THE ULTIMATE CURVEBALL

On Feb. 12, 2017, almost 190,000 people living north of Sacramento received an urgent order: Get out. Now. Part of the tallest dam in America was verging on collapse.

That day, Ronald Stork was in another part of the state, where he was worrying about precisely this kind of disaster — at a different dam.

Standing with binoculars near California’s New Exchequer Dam, he dreaded what might happen if large amounts of water were ever sent through the dam’s spillways. Mr. Stork, a policy expert with the conservation group Friends of the River, had seen on a previous visit to Exchequer that the nearby earth was fractured and could be easily eroded. If enough water rushed through, it might cause major erosion and destabilize the spillways.

He only learned later that his fears were playing out in real time, 150 miles north. At the Oroville Dam, a 770-foot-tall facility built in the 1960s, water from atmospheric rivers was washing away the soil and rock beneath the dam’s emergency spillway, which is essentially a hillside next to the main chute that acts like an overflow drain in a bathtub. The top of the emergency spillway looked like it might buckle, which would send a wall of water cascading toward the cities below.

Mr. Stork had no idea this was happening until he got home to Sacramento and found his neighbor in a panic. The neighbor’s mother lived downriver from Oroville. She didn’t drive anymore. How was he going to get her out?

OROVILLE DAM

Sacramento

NEW EXCHEQUER DAM

San Francisco

Firebaugh

Los Angeles

Mr. Stork had filed motions and written letters to officials, starting in 2001, about vulnerabilities at Oroville. People were now in danger because nobody had listened. “It was nearly soul crushing,” he said.

“With flood hazard, it’s never the fastball that hits you,” said Nicholas Pinter, an earth scientist at the University of California, Davis. “It’s the curveball that comes from a direction you don’t anticipate. And Oroville was one of those.”

A man leans back in an office chair with his hands on head. At left, a wall of books and documents on a bookshelf.

Ronald Stork in his office at Friends of the River in Sacramento.

An aerial view looking down on a dam and its spillway, with lines of boats resting on the lake formed by the dam, with tree-covered mountains in the distance.

The spillway of the New Exchequer Dam.

Such perils had lurked at Oroville for so long because California’s Department of Water Resources had been “overconfident and complacent” about its infrastructure, tending to react to problems rather than pre-empt them, independent investigators later wrote in a report. It is not clear this culture is changing, even as the 21st-century climate threatens to test the state’s aging dams in new ways. One recent study estimated that climate change had boosted precipitation from the 2017 storms at Oroville by up to 15 percent.

A year and a half after the crisis, crews were busy rebuilding Oroville’s emergency spillway when the federal hydropower regulator wrote to the state with some unsettling news: The reconstructed emergency spillway will not be big enough to safely handle the “probable maximum flood,” or the largest amount of water that might ever fall there.

Cumulative

precipitation

Bigger Storms at Oroville

16 in.

New modeling suggests that a 30-day megastorm could deliver about twice the amount of precipitation that preceded the 2017 Oroville Dam spillover.

12 in.

30-DAY MEGASTORM

Modeled estimate

8 in.

2017 PRECIPITATION

Jan. 8 to Feb. 7, 2017

4 in.

0

10

20

30

daysSources: Global Historical Climatology Network, Huang and Swain (2022) Measurements taken from the Oroville weather station and the nearest modeled data point

This is the standard most major hydroelectric projects in the United States have to meet. The idea is that spillways should basically never fail because of excessive rain.

Today, scientists say they believe climate change might be increasing “probable maximum” precipitation levels at many dams. When the Oroville evacuation was ordered in 2017, nowhere near that much water had been flowing through the dam’s emergency spillway.

Yet California officials have downplayed these concerns about the capacity of Oroville’s emergency spillway, which were raised by the Federal Energy Regulatory Commission. Such extreme flows are a “remote” possibility, they argued in a letter last year. Therefore, further upgrades at Oroville aren’t urgently needed.

In a curt reply last month, the commission said this position was “not acceptable.” It gave the state until mid-September to submit a plan for addressing the issue.

The Department of Water Resources told The Times it would continue studying the matter. The Federal Energy Regulatory Commission declined to comment.

“People could die,” Mr. Stork said. “And it bothers the hell out of me.”

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WETTER WET YEARS

Donald G. Sullivan was lying in bed one night, early in his career as a scientist, when he realized his data might hold a startling secret.

For his master’s research at the University of California, Berkeley, he had sampled the sediment beneath a remote lake in the Sacramento Valley and was hoping to study the history of vegetation in the area. But a lot of the pollen in his sediment cores didn’t seem to be from nearby. How had it gotten there?

When he X-rayed the cores, he found layers where the sediment was denser. Maybe, he surmised, these layers were filled with sand and silt that had washed in during floods.

It was only late that night that he tried to estimate the ages of the layers. They lined up neatly with other records of West Coast megafloods.

“That’s when it clicked,” said Dr. Sullivan, who is now at the University of Denver.

His findings, from 1982, showed that major floods hadn’t been exceptionally rare occurrences over the past eight centuries. They took place every 100 to 200 years. And in the decades since, advancements in modeling have helped scientists evaluate how quickly the risks are rising because of climate change.

For their new study, which was published in the journal Science Advances, Dr. Huang and Dr. Swain replayed portions of the 20th and 21st centuries using 40 simulations of the global climate. Extreme weather events, by definition, don’t occur very often. So by using computer models to create realistic alternate histories of the past, present and future climate, scientists can study a longer record of events than the real world offers.

Dr. Swain and Dr. Huang looked at all the monthlong California storms that took place during two time segments in the simulations, one in the recent past and the other in a future with high global warming, and chose one of the most intense events from each period. They then used a weather model to produce detailed play-by-plays of where and when the storms dump their water.

Those details matter. There are “so many different factors” that make an atmospheric river deadly or benign, Dr. Huang said.

A woman stands with her arms behind her against a wall that appears to show a satellite or aerial view of a river system with lush vegetation.

Xingying Huang of the National Center for Atmospheric Research in Boulder, Colo. Rachel Woolf for The New York Times

A ground-level-view of a dam’s imposing and tall spillway, with dry earth and grass in the foreground.

The New Don Pedro Dam spillway.

A man wearing a lanyard bearing his I.D. gestures at a large photograph projected on a screen showing floodwaters and flood damage on a mountainside.

Wes Monier, a hydrologist, with a 1997 photo of water rushing through the New Don Pedro Reservoir spillway.

In the high Sierras, for example, atmospheric rivers today largely bring snow. But higher temperatures are shifting the balance toward rain. Some of this rain can fall on snowpack that accumulated earlier, melting it and sending even more water toward towns and cities below.

Climate change might be affecting atmospheric rivers in other ways, too, said F. Martin Ralph of the Scripps Institution of Oceanography at the University of California, San Diego. How strong their winds are, for instance. Or how long they last: Some storms stall, barraging an area for days on end, while others blow through quickly.

Scientists are also working to improve atmospheric river forecasts, which is no easy task as the West experiences increasingly sharp shifts from very dry conditions to very wet and back again. In October, strong storms broke records in Sacramento and other places. Yet this January through March was the driest in the Sierra Nevada in more than a century.

“My scientific gut says there’s change happening,” Dr. Ralph said. “And we just haven’t quite pinned down how to detect it adequately.”

Better forecasting is already helping California run some of its reservoirs more efficiently, a crucial step toward coping with wetter wet years and drier dry ones.

On the last day of 2016, Wes Monier was looking at forecasts on his iPad and getting a sinking feeling.

Mr. Monier is chief hydrologist for the Turlock Irrigation District, which operates the New Don Pedro Reservoir near Modesto. The Tuolumne River, where the Don Pedro sits, was coming out of its driest four years in a millennium. Now, some terrifying rainfall projections were rolling in.

First, 23.2 inches over the next 16 days. A day later: 28.8 inches. Then 37.1 inches, roughly what the area normally received in a full year.

If Mr. Monier started releasing Don Pedro’s water too quickly, homes and farms downstream would flood. Release too much and he would be accused of squandering water that would be precious come summer.

But the forecasts helped him time his flood releases precisely enough that, after weeks of rain, the water in the dam ended up just shy of capacity. Barely a drop was wasted, although some orchards were flooded, and growers took a financial hit.

The next storm might be even bigger, though. And even the best data and forecasts might not allow Mr. Monier to stop it from causing destruction. “There’s a point there where I can’t do anything,” he said.

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KATRINA 2.0

How do you protect a place as vast as California from a storm as colossal as that? Two ways, said David Peterson, a veteran engineer. Change where the water goes, or change where the people are. Ideally, both. But neither is easy.

Firebaugh is a quiet, mostly Hispanic city of 8,100 people, one of many small communities that power the Central Valley’s prodigious agricultural economy. Many residents work at nearby facilities that process almonds, pistachios, garlic and tomatoes.

Firebaugh also sits right on the San Joaquin River.

For a sleepless stretch of early 2017, Ben Gallegos, Firebaugh’s city manager, did little but watch the river rise and debate whether to evacuate half the town. Water from winter storms had already turned the town’s cherished rodeo grounds into a swamp. Now it was threatening homes, schools, churches and the wastewater treatment plant. If that flooded, people would be unable to flush their toilets. Raw sewage would flow down the San Joaquin.

Luckily, the river stopped rising. Still, the experience led Mr. Gallegos to apply for tens of millions in funding for new and improved levees around Firebaugh.

Levees change where the water goes, giving rivers more room to swell before they inundate the land. Levee failures in New Orleans were what turned Katrina into an epochal catastrophe, and after that storm, California toughened levee standards in urbanized areas of the Sacramento and San Joaquin Valleys, two major river basins of the Central Valley.

The idea is to keep people out of places where the levees don’t protect against 200-year storms, or those with a 0.5 percent chance of occurring in any year. To account for rising seas and the shifting climate, California requires that levees be recertified as providing this level of defense at least every 20 years.

An aerial view looking down on a vegetation-lined river, while further in the distance just next to the river lie rows of houses in a residential neighborhood.

Firebaugh, Calif., on the San Joaquin River, is home to 8,100 people and helps power the Central Valley’s agricultural economy.

A man rests on a metal fencing looking out over a calm river.

Ben Gallegos, the Firebaugh city manager.

An inflatable castle and water slide sits in the driveway of a one-story home, with a couple of adults standing in the driveway watching kids play in the castle. Another kid is on a bicycle, and the driveway glistens from water splashed from the castle.

A 6-year-old’s birthday celebration in Firebaugh.

The problem is that once levees are strengthened, the areas behind them often become particularly attractive for development: fancier homes, bigger buildings, more people. The likelihood of a disaster is reduced, but the consequences, should one strike, are increased.

Federal agencies try to stop this by not funding infrastructure projects that induce growth in flood zones. But “it’s almost impossible to generate the local funds to raise that levee if you don’t facilitate some sort of growth behind the levee,” Mr. Peterson said. “You need that economic activity to pay for the project,” he said. “It puts you in a Catch-22.”

A project to provide 200-year protection to the Mossdale Tract, a large area south of Stockton, one of the San Joaquin Valley’s major cities, has been on pause for years because the Army Corps of Engineers fears it would spur growth, said Chris Elias, executive director of the San Joaquin Area Flood Control Agency, which is leading the project. City planners have agreed to freeze development across thousands of acres, but the Corps still hasn’t given its final blessing.

The Corps and state and local agencies will begin studying how best to protect the area this fall, said Tyler M. Stalker, a spokesman for the Corps’s Sacramento District.

The plodding pace of work in the San Joaquin Valley has set people on edge. At a recent public hearing in Stockton on flood risk, Mr. Elias stood up and highlighted some troubling math.

The Department of Water Resources says up to $30 billion in investment is needed over the next 30 years to keep the Central Valley safe. Yet over the past 15 years, the state managed to spend only $3.5 billion.

“We have to find ways to get ahead of the curve,” Mr. Elias said. “We don’t want to have a Katrina 2.0 play out right here in the heart of Stockton.”

As Mr. Elias waits for projects to be approved and budgets to come through, heat and moisture will continue to churn over the Pacific. Government agencies, battling the forces of inertia, indifference and delay, will make plans and update policies. And Stockton and the Central Valley, which runs through the heart of California, will count down the days and years until the inevitable storm.

An aerial view looking on a river with a small recreational boat motoring through. The river abuts farmland on either side.

T​​he Sacramento-San Joaquin Delta near Stockton, Calif.

Augmented Reality

What will the megastorm’s rain look like?

This augmented-reality experience simulates the extreme rainfall in a future California megastorm. To experience this effect in your space, you will need the Instagram app.

To view on Instagram, open the camera on your device and point to the QR tag below.

Sources

The megastorm simulation is based on the “ARkHist” storm modeled by Huang and Swain, Science Advances (2022), a hypothetical statewide, 30-day atmospheric river storm sequence over California with an approximately 2 percent likelihood of occurring each year in the present climate. Data was generated using the Weather Research and Forecasting model and global climate simulations from the Community Earth System Model Large Ensemble.

The chart of precipitation at Oroville compares cumulative rainfall at the Oroville weather station before the 2017 crisis with cumulative rainfall at the closest data point in ARkHist.

The rainfall visualization compares observed hourly rainfall in December 2016 from the Los Angeles Downtown weather station with rainfall at the closest data point in a hypothetical future megastorm, the ARkFuture scenario in Huang and Swain (2022). This storm would be a rare but plausible event in the second half of the 21st century if nations continue on a path of high greenhouse-gas emissions.

THE ULTIMATE CURVEBALL

On Feb. 12, 2017, almost 190,000 people living north of Sacramento received an urgent order: Get out. Now. Part of the tallest dam in America was verging on collapse.

That day, Ronald Stork was in another part of the state, where he was worrying about precisely this kind of disaster — at a different dam.

Standing with binoculars near California’s New Exchequer Dam, he dreaded what might happen if large amounts of water were ever sent through the dam’s spillways. Mr. Stork, a policy expert with the conservation group Friends of the River, had seen on a previous visit to Exchequer that the nearby earth was fractured and could be easily eroded. If enough water rushed through, it might cause major erosion and destabilize the spillways.

He only learned later that his fears were playing out in real time, 150 miles north. At the Oroville Dam, a 770-foot-tall facility built in the 1960s, water from atmospheric rivers was washing away the soil and rock beneath the dam’s emergency spillway, which is essentially a hillside next to the main chute that acts like an overflow drain in a bathtub. The top of the emergency spillway looked like it might buckle, which would send a wall of water cascading toward the cities below.

Mr. Stork had no idea this was happening until he got home to Sacramento and found his neighbor in a panic. The neighbor’s mother lived downriver from Oroville. She didn’t drive anymore. How was he going to get her out?

OROVILLE DAM

Sacramento

NEW EXCHEQUER DAM

San Francisco

Firebaugh

Los Angeles

Mr. Stork had filed motions and written letters to officials, starting in 2001, about vulnerabilities at Oroville. People were now in danger because nobody had listened. “It was nearly soul crushing,” he said.

“With flood hazard, it’s never the fastball that hits you,” said Nicholas Pinter, an earth scientist at the University of California, Davis. “It’s the curveball that comes from a direction you don’t anticipate. And Oroville was one of those.”

A man leans back in an office chair with his hands on head. At left, a wall of books and documents on a bookshelf.

Ronald Stork in his office at Friends of the River in Sacramento.

An aerial view looking down on a dam and its spillway, with lines of boats resting on the lake formed by the dam, with tree-covered mountains in the distance.

The spillway of the New Exchequer Dam.

Such perils had lurked at Oroville for so long because California’s Department of Water Resources had been “overconfident and complacent” about its infrastructure, tending to react to problems rather than pre-empt them, independent investigators later wrote in a report. It is not clear this culture is changing, even as the 21st-century climate threatens to test the state’s aging dams in new ways. One recent study estimated that climate change had boosted precipitation from the 2017 storms at Oroville by up to 15 percent.

A year and a half after the crisis, crews were busy rebuilding Oroville’s emergency spillway when the federal hydropower regulator wrote to the state with some unsettling news: The reconstructed emergency spillway will not be big enough to safely handle the “probable maximum flood,” or the largest amount of water that might ever fall there.

Cumulative

precipitation

Bigger Storms at Oroville

16 in.

New modeling suggests that a 30-day megastorm could deliver about twice the amount of precipitation that preceded the 2017 Oroville Dam spillover.

12 in.

30-DAY MEGASTORM

Modeled estimate

8 in.

2017 PRECIPITATION

Jan. 8 to Feb. 7, 2017

4 in.

0

10

20

30

daysSources: Global Historical Climatology Network, Huang and Swain (2022) Measurements taken from the Oroville weather station and the nearest modeled data point

This is the standard most major hydroelectric projects in the United States have to meet. The idea is that spillways should basically never fail because of excessive rain.

Today, scientists say they believe climate change might be increasing “probable maximum” precipitation levels at many dams. When the Oroville evacuation was ordered in 2017, nowhere near that much water had been flowing through the dam’s emergency spillway.

Yet California officials have downplayed these concerns about the capacity of Oroville’s emergency spillway, which were raised by the Federal Energy Regulatory Commission. Such extreme flows are a “remote” possibility, they argued in a letter last year. Therefore, further upgrades at Oroville aren’t urgently needed.

In a curt reply last month, the commission said this position was “not acceptable.” It gave the state until mid-September to submit a plan for addressing the issue.

The Department of Water Resources told The Times it would continue studying the matter. The Federal Energy Regulatory Commission declined to comment.

“People could die,” Mr. Stork said. “And it bothers the hell out of me.”

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WETTER WET YEARS

Donald G. Sullivan was lying in bed one night, early in his career as a scientist, when he realized his data might hold a startling secret.

For his master’s research at the University of California, Berkeley, he had sampled the sediment beneath a remote lake in the Sacramento Valley and was hoping to study the history of vegetation in the area. But a lot of the pollen in his sediment cores didn’t seem to be from nearby. How had it gotten there?

When he X-rayed the cores, he found layers where the sediment was denser. Maybe, he surmised, these layers were filled with sand and silt that had washed in during floods.

It was only late that night that he tried to estimate the ages of the layers. They lined up neatly with other records of West Coast megafloods.

“That’s when it clicked,” said Dr. Sullivan, who is now at the University of Denver.

His findings, from 1982, showed that major floods hadn’t been exceptionally rare occurrences over the past eight centuries. They took place every 100 to 200 years. And in the decades since, advancements in modeling have helped scientists evaluate how quickly the risks are rising because of climate change.

For their new study, which was published in the journal Science Advances, Dr. Huang and Dr. Swain replayed portions of the 20th and 21st centuries using 40 simulations of the global climate. Extreme weather events, by definition, don’t occur very often. So by using computer models to create realistic alternate histories of the past, present and future climate, scientists can study a longer record of events than the real world offers.

Dr. Swain and Dr. Huang looked at all the monthlong California storms that took place during two time segments in the simulations, one in the recent past and the other in a future with high global warming, and chose one of the most intense events from each period. They then used a weather model to produce detailed play-by-plays of where and when the storms dump their water.

Those details matter. There are “so many different factors” that make an atmospheric river deadly or benign, Dr. Huang said.

A woman stands with her arms behind her against a wall that appears to show a satellite or aerial view of a river system with lush vegetation.

Xingying Huang of the National Center for Atmospheric Research in Boulder, Colo. Rachel Woolf for The New York Times

A ground-level-view of a dam’s imposing and tall spillway, with dry earth and grass in the foreground.

The New Don Pedro Dam spillway.

A man wearing a lanyard bearing his I.D. gestures at a large photograph projected on a screen showing floodwaters and flood damage on a mountainside.

Wes Monier, a hydrologist, with a 1997 photo of water rushing through the New Don Pedro Reservoir spillway.

In the high Sierras, for example, atmospheric rivers today largely bring snow. But higher temperatures are shifting the balance toward rain. Some of this rain can fall on snowpack that accumulated earlier, melting it and sending even more water toward towns and cities below.

Climate change might be affecting atmospheric rivers in other ways, too, said F. Martin Ralph of the Scripps Institution of Oceanography at the University of California, San Diego. How strong their winds are, for instance. Or how long they last: Some storms stall, barraging an area for days on end, while others blow through quickly.

Scientists are also working to improve atmospheric river forecasts, which is no easy task as the West experiences increasingly sharp shifts from very dry conditions to very wet and back again. In October, strong storms broke records in Sacramento and other places. Yet this January through March was the driest in the Sierra Nevada in more than a century.

“My scientific gut says there’s change happening,” Dr. Ralph said. “And we just haven’t quite pinned down how to detect it adequately.”

Better forecasting is already helping California run some of its reservoirs more efficiently, a crucial step toward coping with wetter wet years and drier dry ones.

On the last day of 2016, Wes Monier was looking at forecasts on his iPad and getting a sinking feeling.

Mr. Monier is chief hydrologist for the Turlock Irrigation District, which operates the New Don Pedro Reservoir near Modesto. The Tuolumne River, where the Don Pedro sits, was coming out of its driest four years in a millennium. Now, some terrifying rainfall projections were rolling in.

First, 23.2 inches over the next 16 days. A day later: 28.8 inches. Then 37.1 inches, roughly what the area normally received in a full year.

If Mr. Monier started releasing Don Pedro’s water too quickly, homes and farms downstream would flood. Release too much and he would be accused of squandering water that would be precious come summer.

But the forecasts helped him time his flood releases precisely enough that, after weeks of rain, the water in the dam ended up just shy of capacity. Barely a drop was wasted, although some orchards were flooded, and growers took a financial hit.

The next storm might be even bigger, though. And even the best data and forecasts might not allow Mr. Monier to stop it from causing destruction. “There’s a point there where I can’t do anything,” he said.

tk

tk

KATRINA 2.0

How do you protect a place as vast as California from a storm as colossal as that? Two ways, said David Peterson, a veteran engineer. Change where the water goes, or change where the people are. Ideally, both. But neither is easy.

Firebaugh is a quiet, mostly Hispanic city of 8,100 people, one of many small communities that power the Central Valley’s prodigious agricultural economy. Many residents work at nearby facilities that process almonds, pistachios, garlic and tomatoes.

Firebaugh also sits right on the San Joaquin River.

For a sleepless stretch of early 2017, Ben Gallegos, Firebaugh’s city manager, did little but watch the river rise and debate whether to evacuate half the town. Water from winter storms had already turned the town’s cherished rodeo grounds into a swamp. Now it was threatening homes, schools, churches and the wastewater treatment plant. If that flooded, people would be unable to flush their toilets. Raw sewage would flow down the San Joaquin.

Luckily, the river stopped rising. Still, the experience led Mr. Gallegos to apply for tens of millions in funding for new and improved levees around Firebaugh.

Levees change where the water goes, giving rivers more room to swell before they inundate the land. Levee failures in New Orleans were what turned Katrina into an epochal catastrophe, and after that storm, California toughened levee standards in urbanized areas of the Sacramento and San Joaquin Valleys, two major river basins of the Central Valley.

The idea is to keep people out of places where the levees don’t protect against 200-year storms, or those with a 0.5 percent chance of occurring in any year. To account for rising seas and the shifting climate, California requires that levees be recertified as providing this level of defense at least every 20 years.

An aerial view looking down on a vegetation-lined river, while further in the distance just next to the river lie rows of houses in a residential neighborhood.

Firebaugh, Calif., on the San Joaquin River, is home to 8,100 people and helps power the Central Valley’s agricultural economy.

A man rests on a metal fencing looking out over a calm river.

Ben Gallegos, the Firebaugh city manager.

An inflatable castle and water slide sits in the driveway of a one-story home, with a couple of adults standing in the driveway watching kids play in the castle. Another kid is on a bicycle, and the driveway glistens from water splashed from the castle.

A 6-year-old’s birthday celebration in Firebaugh.

The problem is that once levees are strengthened, the areas behind them often become particularly attractive for development: fancier homes, bigger buildings, more people. The likelihood of a disaster is reduced, but the consequences, should one strike, are increased.

Federal agencies try to stop this by not funding infrastructure projects that induce growth in flood zones. But “it’s almost impossible to generate the local funds to raise that levee if you don’t facilitate some sort of growth behind the levee,” Mr. Peterson said. “You need that economic activity to pay for the project,” he said. “It puts you in a Catch-22.”

A project to provide 200-year protection to the Mossdale Tract, a large area south of Stockton, one of the San Joaquin Valley’s major cities, has been on pause for years because the Army Corps of Engineers fears it would spur growth, said Chris Elias, executive director of the San Joaquin Area Flood Control Agency, which is leading the project. City planners have agreed to freeze development across thousands of acres, but the Corps still hasn’t given its final blessing.

The Corps and state and local agencies will begin studying how best to protect the area this fall, said Tyler M. Stalker, a spokesman for the Corps’s Sacramento District.

The plodding pace of work in the San Joaquin Valley has set people on edge. At a recent public hearing in Stockton on flood risk, Mr. Elias stood up and highlighted some troubling math.

The Department of Water Resources says up to $30 billion in investment is needed over the next 30 years to keep the Central Valley safe. Yet over the past 15 years, the state managed to spend only $3.5 billion.

“We have to find ways to get ahead of the curve,” Mr. Elias said. “We don’t want to have a Katrina 2.0 play out right here in the heart of Stockton.”

As Mr. Elias waits for projects to be approved and budgets to come through, heat and moisture will continue to churn over the Pacific. Government agencies, battling the forces of inertia, indifference and delay, will make plans and update policies. And Stockton and the Central Valley, which runs through the heart of California, will count down the days and years until the inevitable storm.

An aerial view looking on a river with a small recreational boat motoring through. The river abuts farmland on either side.

T​​he Sacramento-San Joaquin Delta near Stockton, Calif.

Augmented Reality

What will the megastorm’s rain look like?

This augmented-reality experience simulates the extreme rainfall in a future California megastorm. To experience this effect in your space, you will need the Instagram app.

To view on Instagram, open the camera on your device and point to the QR tag below.

Sources

The megastorm simulation is based on the “ARkHist” storm modeled by Huang and Swain, Science Advances (2022), a hypothetical statewide, 30-day atmospheric river storm sequence over California with an approximately 2 percent likelihood of occurring each year in the present climate. Data was generated using the Weather Research and Forecasting model and global climate simulations from the Community Earth System Model Large Ensemble.

The chart of precipitation at Oroville compares cumulative rainfall at the Oroville weather station before the 2017 crisis with cumulative rainfall at the closest data point in ARkHist.

The rainfall visualization compares observed hourly rainfall in December 2016 from the Los Angeles Downtown weather station with rainfall at the closest data point in a hypothetical future megastorm, the ARkFuture scenario in Huang and Swain (2022). This storm would be a rare but plausible event in the second half of the 21st century if nations continue on a path of high greenhouse-gas emissions.

A New Threat Looms over California – A Megaflood

A new study provides a dire warning for how climate change will impact weather patterns in California, with periods of unprecedented rainfall expected to increase in frequency and intensity. There is no telling exactly when the event will occur, but the research has made it clear: the storm is coming.August 18, 2022California's Central Valley from space. | Stuart RankinCalifornia’s Central Valley from space. | Stuart Rankin

Marcus KahnCalifornia Communications and Development Associate@americanrivers 

It has become clear that there’s another threat looming just over the horizon for California – a megaflood. A new study from Daniel Swain and Xingying Huang of UCLA provides dire modeling of how climate change will impact weather patterns throughout the state, with periods of unprecedented rainfall expected to increase in frequency and intensity in the years to come. There is no telling exactly when the event will occur, but the research has made it clear: the storm is coming.

This superstorm, often referred to as atmospheric rivers, will put California’s flood mitigation infrastructure: levees, dams, floodplains and more to the ultimate test. Communities in the floodplains of the Central Valley are especially vulnerable and face the greatest risk from flood events that haven’t been witnessed in our lifetime. American Rivers has put together 3 key priorities to prepare for these drastic changes, inspired by an article published by the New York Times on August 12th.

1. Reconnect rivers to their natural floodplains

Over the past century, California’s rivers have been confined by levee construction and cut off from their historic floodplains. Our rivers have limited capacity to spread and slow flood flows, increasing the risk of levee failure and catastrophic flooding. By connecting rivers to their historic floodplains, we make room for rivers to flow naturally during high flood events while replenishing groundwater aquifers and reconnecting wildlife habitats. These multiple benefits (or “multi-benefit”), nature-based solutions are essential to landscape-level climate resilience and are a comprehensive natural solution to the impacts of anthropogenic climate change.

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2. Invest in multi-benefit, nature-based flood risk reduction solutions in the Central Valley

As noted in the New York Times article, investments in flood risk reduction infrastructure have been inadequate given the rapid pace of climate change. Federal and state agencies have been unable to meet their goals of reducing flood risk reduction benchmarks, leaving poor communities vulnerable to severe flooding. We need diverse coalitions of legislators, flood managers, conservationists, and farmers to advocate for more federal and state funding dollars to support more flood risk reduction projects and better planning for the floods in our future.

3. Limit urban development in high-risk floodplains

Floodplains are areas where rivers would flow and undulate across the landscape if they were not constricted by levees – and they are the most at risk for flooding and levee failure. While levees are often built to protect critical infrastructure—such as highways and airports— this can lead to further development on lands presumed to be protected by the levee. Building entire neighborhoods and commercial facilities in high-risk floodplains next to critical infrastructure places lives, jobs, and entire regional economies at risk. By limiting urbanization in floodplains, we can reduce the amount of damage we expect to be caused by increased flooding.

These priorities guide our work in California’s Central Valley, where American Rivers designs and implements on-the-ground flood resilience projects and advocates for state and federal policies that keep communities safe. If you would like to stay up to date on our work in California, check out our California Region page.TAGS:Climate Change & RiversClimate Change & RiversFloodingFloods & FloodplainsinfrastructureInfrastructure

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New Study Shows a Potential California Megaflood Could Be the Most Costly Natural Disaster In Human History

In California, residents are conditioned to fear the “Big One” — an earthquake capable of leveling or severely damaging an entire city or region. But as the climate crisis worsens, environmental scientists believe that it’s not a tremor Californians should worry about most, but rather a megaflood that could engulf the entire state.

On average, an earthquake of a magnitude 8 or higher occurs once a year, globally. However, that frequency in the Golden State is far lower; tremors of such scale historically strike California about every century or so. Should one hit along the San Andreas fault line, the quake could kill upwards of 1,800 people and leave another 50,000 or more with injuries, experts predict — while leaving hundreds of thousands without shelter or access to running water or electricity.

As dystopian as all of the above sounds, it pales in comparison to what a megaflood could mean for California.

A recently published study in Science Advances delves into how the current scale of the climate crisis has already doubled the chances of a disastrous flood happening in California within the next four decades. Under normal circumstances, such a once-in-a-lifetime flood would be expected to inundate California roughly every 200 or more years. According to Daniel Swain, a climate scientist with UCLA and one of the researchers who led the study, climate change is heightening the frequency of atmospheric rivers like the “bomb cyclone” that hit our region last October. And a megaflood could be spurred by an unrelenting series of those storms over a period of weeks. It’s also happened before — geological evidence points to 300-year flooding events across the state, and one such megaflood in the winter of 1861-62 flooded the entire Central Valley and much of newly built Sacramento, and an estimated 4,000 people were killed by the flood.

“We got kind of lucky to avoid it in the 20th century,” Swain tells the Times.  “I would be very surprised to avoid it occurring in the 21st.”

As the New York Times notes in the California Today newsletter, newly elected governor Leland Stanford “had to take a rowboat to his inaugural events in January 1862,” the state capital was temporarily moved to San Francisco.

Swain describes a megaflood as “a very severe flood event across a broad region that has the potential to bring catastrophic impacts to society in the areas affected.” And in 1862, there were far fewer people living in California — with most residing in the northern parts of the state. In Southern California, after 43 straight days of rain, the Santa Ana river was four miles wide in some spots before it began to recede.d

As CNN notes, Swain compares such a megaflood to the 1,000-year flash flood events seen this summer in the St. Louis area and Kentucky — though the one that’s looming for California would encompass a much wider area. As in… the entire state.

The Times also produced a big visual package over the weekend about a “California megastorm, based on the study.

Swain worked on the study with Xingying Huang of the National Center for Atmospheric Research in Boulder, Colorado.

Experts suggest that California’s lowlands would turn into a “vast inland sea” and displace millions of residents. Hundreds of billions of dollars in the state’s GDP — which is the largest sub-national economy in the world – would be lost. Entire infrastructures and utilities would be lost at both city and county levels. All in all, it’s predicted that if such an event transpires, it would cost a trillion dollars to bounce back from the damages, recovery would take years if not decades, and some municipalities might even be damaged to the point of being irreparable and uninhabitable.

And yes: This would be the most costly natural disaster in human history, by far. As of now, Hurricane Katrina still holds that title after it struck the Gulf Coast in 2005, causing an estimated $172.5B in collective damages to the Louisiana and Alabama coasts.

Unlike a flash flood, the megaflood event that could potentially hit California as early as 2047 wouldn’t be an overnight occurrence. Weather models extrapolated from the data in the study show the flood would come by way of a 30-day cumulative precipitation pattern affecting the entire state of California.

Coupled with a growing amount of burn-scarred areas — which are less able to absorb excess rainfall than healthy soils — and the state’s steep terrain, as well as yearly snowmelt runoffs found in higher elevations, it’s a perfect storm for a disaster unlike any seen in human history.

Graph: Courtesy of Science Advances

The chance of this happening increases exponentially as the “global mean temperature anomaly” is extended by the climate crisis. Warm-phase El Niño years will also play a role in determining when such a flood will strike; the weather models suggest that after the “tipping point” between the average likelihood of a megaflood happening at this scale sits at around the two-degree Celsius mark for observed median global temperature increases.

While the bulk of human-caused global warming sits in the hands of just 90 companies — businesses that have, on average, contributed 63% of the greenhouse gasses emitted globally every year — there’s still something we can do as individuals to thwart the climate crisis and, by proxy, the chance of a megaflood swallowing California within the coming decades.

“If we work together to decrease future emissions, we can also reduce the risk of extreme events,” Xingying Huang, another of the authors of the study, said to CNN. In tandem with decreasing our collective greenhouse gas emissions, and bolstering California’s flood and water management systems, Swain believes “the extent of [megaflood] losses can be significantly reduced.”

Previously: Remembering California’s Great Flood Of 1862

Photo: In an aerial view, floodwaters cover a section of highway 1 on November 20, 2021 in Abbotsford, British Columbia. The Canadian province of British Columbia declared a state of emergency on Wednesday following record rainfall earlier this week that has resulted in widespread flooding of farms, landslides and the evacuation of residents. (Photo by Justin Sullivan/Getty Images)

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Outdated FEMA maps, The Guardian, Sept 2022

Deanne Criswell makes admission as ‘extremely dangerous and life-threatening situation’ hits Georgia

Pet owners rescue two cats and a dog from their home in East St Louis, Illinois in July.
Pet owners rescue two cats and a dog from their home in East St Louis, Illinois, in July. Photograph: Derik Holtmann/AP

Edward Helmore and agenciesSun 4 Sep 2022 17.48 EDT

Flood maps used by the federal government are outdated, the director of the Federal Emergency Management Agency, or Fema, said on Sunday, considering a series of devastating floods caused by excessive rainfall induced by climate change.

Jackson, Mississippi, to go without reliable drinking water indefinitely<br>A volunteer carries bottles of water at a water distribution site as the city of Jackson is to go without reliable drinking water indefinitely after the water treatment plant pumps failed, leading to the emergency distribution of bottled water and tanker trucks for 180,000 people, in Jackson, Mississippi, U.S., September 2, 2022. REUTERS/Carlos Barria

Deanne Criswell told CNN’s State of the Union: “The part that’s really difficult right now is the fact that our flood maps don’t take into account excessive rain that comes in. And we are seeing these record rainfalls that are happening.”

Criswell was questioned about the situation in Jackson, Mississippi, where city water facilities failed in the aftermath of heavy flooding on the Pearl River, leaving residents without drinkable or usable water.

On Sunday, thunderstorms and heavy rain pounded parts of north-west Georgia, sparking flash floods. Local news reports showed roads under water and homeowners struggling.

The Georgia governor, Brian Kemp, declared a state of emergency in Chattooga and Floyd counties, directing all state resources to help with “preparation, response and recovery activities”.

The National Weather Service (NWS) reported rainfall of up to an inch an hour. Up to 12in was estimated to have fallen, according to Kemp’s executive order.

“This is an extremely dangerous and life-threatening situation,” the NWS said. “Do not attempt to travel unless you are fleeing an area subject to flooding or under an evacuation order.”

Such excessive rainfall and flooding will bring back into focus difficulties in predicting and responding to the climate crisis.

Criswell said Fema would work with local jurisdictions “to help them better identify what their needs are and help them create better predictive models, because we have to start thinking about what the threats are going to be in the future as a result of climate change, so they can put the mitigation measures in place”.

Last month, rainfall in Missouri, Kentucky and Illinois broke century-old records, causing devastation to communities, buildings and infrastructure.

“Fema’s maps right now are really focused on riverine flooding and coastal flooding and we work with local jurisdictions to update the maps,” Criswell said.

A photo by the National Park Service shows the scene after a flash flood in Zion national park, Utah.

A 2020 study of flood risks by the non-profit First Street Foundation that analyzed every property in the 48 contiguous US states found federal maps underestimated by 67% the number of homes and businesses in significant danger.

In Georgia on Sunday, the NWS declared a “flash flood emergency” for Summerville, Lyerly and James H Floyd State Park in Chattooga county. Floyd county, just to the south, was also under a flash flood warning. The NWS advised people to avoid non-emergency travel.

The city of Summerville advised residents who use the city’s water services to boil water prior to drinking, cooking or preparing baby food, due to flash flooding at the Raccoon Creek filter plant. FEMA’s director says the federal government’s flood maps are outdated and don’t take into account recent rainfall trends. (Guardian)