California Flooding: Why the State Is Bracing for a Bigger Threat in October

California is preparing for a potentially dangerous rise in coastal water as a large Pacific “Kelvin wave” moves toward the state, with the warm-water system expected to reach Southern California and the San Francisco Bay Area in early October.

Scientists warn that the added water level could make coastal flooding worse when combined with high tides, storms, and heavy rain later this year.

The concern is not that a giant ocean wave will crash into California like a tsunami. Instead, the Kelvin wave is a huge area of warm water moving beneath the ocean surface. As it reaches the coast, it can temporarily raise sea levels and leave less room for storm water to drain into the ocean.

Why California is preparing now

The Kelvin wave is approaching California as the Pacific experiences a strong El Niño pattern. Scientists expect the wave to move north toward Alaska after reaching the West Coast.

Along parts of the California coast, water levels could rise by about a foot, according to scientists cited by The Guardian. That would come on top of the roughly 8 to 12 inches of sea-level rise California has already experienced over the past century because of climate change.

Mike Jacox, a research oceanographer with the National Marine Fisheries Service, said Kelvin waves are not necessarily dangerous by themselves.

“But when there are very high tides or storm surges, their effects can be amplified,” Jacox said.

That combination is what concerns emergency officials. Higher coastal water levels can make it harder for rainfall and swollen creeks to drain into the ocean, increasing the chance of flooding in low-lying areas.

October could be an early warning

The immediate concern is the arrival of the Kelvin wave in early October, but scientists say the larger risk could come later in the winter.

November and December will bring some of California’s highest tides of the year. At the same time, the state typically begins experiencing wetter and more powerful storms.

Jonathan Warrick, a research geologist with the U.S. Geological Survey, said the incoming Kelvin wave should be viewed as an early part of a larger period of concern.

“This coming Kelvin wave is just the beginning,” Warrick said. “The thing to look out for is what will happen in January, February and March; that’s when we get the biggest storms, we get our largest waves, and we have some very large tides.”

If high tides, heavy rain, storm surge and elevated sea levels occur together, their effects could compound. That could mean more coastal erosion, flooding and damage to roads, homes and businesses.

Bay Area faces a particular risk

The San Francisco Bay Area is among the areas receiving close attention because large numbers of people and major infrastructure are located near sea level.

Climate scientist Daniel Swain told Inside Climate News that coastal flooding is becoming increasingly difficult to avoid under these conditions.

“Coastal flooding is at this point practically certain,” Swain said. He also warned that a wetter-than-average winter could increase inland flooding.

“Both of these in some places can combine where you have freshwater outflows and saltwater inflows mingling,” Swain said. “The damage can be devastating.”

The region has already seen the kind of problems that can occur when high tides combine with heavy rainfall. In January, parts of the Bay Area flooded during high tides, even without the additional effect of the current Kelvin wave.

Communities are already taking precautions

Local officials are not waiting for the winter storms to arrive. Emergency managers in parts of the Bay Area have begun distributing sandbags, clearing drains and culverts, checking vulnerable infrastructure and putting temporary flood barriers in place.

Nick Brubaker, chief resilience officer for Marin County’s Office of Emergency Management, said the expected sea-level rise has changed how officials are preparing.

“It certainly changes our baseline assumptions on how we’re going to be planning for impact,” Brubaker said.

He added that higher water levels could prevent creeks and streams from draining normally.

“We could see some significant flooding,” Brubaker said.

The risks are not limited to the Bay Area. California’s coastline could face erosion and flooding as storms arrive on top of already elevated water levels.

California has seen this combination before

Scientists are also looking at the winter of 1997-98, another strong El Niño period. Heavy storms then caused severe coastal erosion, landslides, flooding and damage to roads and homes across California.

Scientists cannot predict the current season with certainty, and they cannot say exactly where or when flooding will occur. But emergency officials are preparing for the possibility that elevated sea levels, high tides and major storms could overlap.

The immediate Kelvin wave is therefore less important as a single event than as an added factor that could raise California’s starting water level before the state’s more hazardous winter weather arrives.