LOS ANGELES (DPA) - Scientists call it California's "other big one", and they say it could cause three times as much damage as a major earthquake ripping along the San Andreas fault.

Although it might sound absurd to those who still recall five years of withering drought and mandatory water restrictions, researchers and engineers warn that California may be due for rain of biblical proportions - or what experts call an ARkStorm.

This rare mega storm - which some say is rendered all the more inevitable because of climate change - would last for weeks and send more than 1.5 million people fleeing as floodwaters inundated cities and formed lakes in the Central Valley and Mojave Desert, according to the US Geological Survey. Officials estimate the structural and economic damage from an ARkStorm (for Atmospheric River 1,000) would amount to more than US$725 billion (S$983 billion) statewide.

In heavily populated areas of the Los Angeles Basin, epic runoff from the San Gabriel Mountains could rapidly overwhelm a flood control dam on the San Gabriel River and unleash floodwaters from Pico Rivera to Long Beach, says a recent analysis by the US Army Corps of Engineers.

In a series of recent public hearings, corps officials told residents that the 60-year-old Whittier Narrows Dam no longer met the agency's tolerable-risk guidelines and could fail in the event of a very large, very rare storm, such as the one that devastated California more than 150 years ago.

Specifically, federal engineers found that the Whittier Narrows structure could fail if water were to flow over its crest or if seepage eroded the sandy soil underneath. In addition, unusually heavy rains could trigger a premature opening of the dam's massive spillway on the San Gabriel River, releasing more than 20 times what the downstream channel could safely contain within its levees.

The corps is seeking up to US$600 million in federal funding to upgrade the 5km-long dam, and says the project has been classified as the agency's highest priority nationally because of the risk of "very significant loss of life and economic impacts." The funding will require congressional approval, said Doug Chitwood, lead engineer on the project.

Standing atop the 17m-tall dam recently, Chitwood surveyed the sprawl of working-class homes, schools and commercial centres about 22km south of Los Angeles and said, "All you see could be underwater."

The dam - which stretches from Montebello to Pico Rivera and crosses both the San Gabriel and Rio Hondo rivers - is one of a number of flood-control facilities overseen by the corps. Throughout much of the year, it contains little water.

A government study, however, used computer models to estimate the effects of 900-year, 7,500-year and 18,000-year storm events.

In each case, officials say as many as 1 million people could be affected.

Among the communities hardest hit in a dam failure would be Pico Rivera, a city of about 63,000 people immediately below the dam. In a worst-case scenario, it could be hit with water 6m deep, and evacuation routes would be turned into rivers.

In recent years, officials with the US Department of Interior and the US Geological Survey have sought to raise awareness of the threat of mega storms and promote emergency preparedness. Part of the challenge, however, has been characterising the scale of such storms.

When scientists speak of a 900-year storm, that does not mean the storm will occur every 900 years, or that such a storm cannot happen two years in a row. It means that such a storm has a 1 in 900 - or .001 percent - chance of occurring in any given year.

The estimates used by the US Army Corps of Engineers are intended to protect the region from a storm similar to the one that hit California during the rainy season of 1861-62. That's when a series of intense storms hammered the state for 45 days and dropped 90cm of rain on Los Angeles. So much water fell that it was impossible to cross the Central Valley without a boat, and the state capital was moved temporarily from Sacramento to San Francisco.

Some researchers, however, say climate change has cast doubt on 20th century assumptions. They argue that, in a warming world, regions such as California will experience more whiplashing shifts between extremely dry and extremely wet periods - similar to how California's long drought was quickly followed by the wettest rainy season on record in 2016-17. These intense cycles will seriously challenge California's ability to control flooding as well as store and transport water.

Daniel Swain, a University of California, Los Angeles climate scientist, said hydrological and forecast data used by the corps must be updated.

"The Army Corps' estimates of the impacts of an extremely serious weather event ... are categorically underestimated," he said. "That's because we only have about a century of records to refer to in California. So, they are extrapolating in the dark."

As an example, Swain said, until recently it was thought a flood the magnitude of the 1861-62 event was likely to occur every 1,000 to 10,000 years. New research has changed that view considerably, he said.

"A newer study suggests the chances of seeing another flood of that magnitude over the next 40 years are about 50-50," he said.

Whittier Narrows, Swain added, is therefore just one of "many pieces of water infrastructure that may not be up to the challenge of the brave new climate of the 21st century."

Such was the conclusion of a study by University of California Irvine researchers that was published recently in the scientific journal Geophysical Research Letters. After examining 13 California reservoirs - most of them 50 years or older - the authors argued that the risk of dam failure was likely to increase in a warming climate. The study cited the 2017 crisis at Oroville Dam, when extreme water flows caused its spillway to disintegrate, triggering the evacuation of more than 180,000 people.

In the case of Whittier Narrows Dam, Travis Longcore, a spatial scientist at the University of Southern California, suggested people had grown complacent about the effectiveness of the area's flood control system. "People tend to forget about the power of Southern California's river systems," he said.

The San Gabriel River ranks among the steepest rivers in the United States. Starting high up in the mountains, it then meanders in a gravelly channel before arriving at lush Whittier Narrows - a natural gap in the hills that form the southern boundary of the San Gabriel Valley. From there, its flows are tamed in a concrete-covered channel for most of its final journey to the Pacific Ocean.

Now, based on the corps' findings, LA County and municipal officials are working with the federal government to develop emergency plans that can be implemented if necessary before the repair project at the dam is completed in 2026.