Where El Niño May Impact California Coastal Areas This Winter

Driving across Highway 37 in the San Francisco Bay Area on the best of days already feels like driving on top of water. Buffeted on both sides by marshlands hosting egrets, herons, ducks and a handful of endangered species, the sea-level highway is a major connector linking Marin, Sonoma, Napa and Solano Counties.

It is also the sinking flagship for California’s vulnerability to sea level rise, according to the “Coastal Flooding Risks for California” report released this week from the University of California, Davis.

Recent shoreline flooding in Southern California may be a precursor of what’s facing California this winter, said report author Fraser Shilling, director of the UC Davis Road Ecology Center, at the Institute of Transportation Studies. Highway 37 and other roads and highways along California’s coastline are extremely vulnerable to floods and failure during this winter’s coming El Niño, the report said.

“We face the risk — though not the certainty — of catastrophic flooding in a variety of coastal towns this winter,” Shilling said. “It’s important to be informed of risk so you can make decisions.” 

Other high-risk areas in the state include:
•    Avila Beach in San Luis Obispo County
•    Moss Landing and Highway 1 in Monterey County
•    Santa Cruz’s beachfront areas
•    Highway 101 within Corte Madera and San Rafael
•    San Rafael’s Canal District
•    The eastern approach to the San Francisco Bay Bridge 
•    San Francisco and Oakland International Airports

New report, decade-long warning

The Coastal Flooding Risks report comes 10 years after the center’s 2016 report, which warned of major flood risk along Highway 37 from future sea-level rise. That report examined potential solutions, such as moving the highway inland, or elevating it onto a causeway — a multibillion-dollar endeavor. Caltrans is currently conducting emergency storm preparations at multiple closure points along the highway.

“Ten years ago, our report said, ‘This is what we expect to start happening within the next 10 years,’” Shilling said. “The problems haven’t been fixed, and some of those things are going to start happening on 37 and other coastal roadways.”

What Long Beach-like flooding could look like in the Bay Area

Earlier this month in Long Beach and Newport Beach, an El Niño-induced Kelvin wave combined with a high tide and wave action to produce an additional 2 to 3 feet of sea level rise. It extensively damaged and flooded coastal properties and environments there.

Shilling wondered how a similar amount of sea level rise would affect Central and Northern California’s towns and highways, as El Niño is expected to deliver a historically wet winter. 

Shilling used the publicly available Surging Seas – Risk Zone Map, a tool created by Climate Central, to understand at what amount of sea level rise coastal California’s towns and highways would flood. High-risk areas lit up the screen in a flood of blue. 

At 2 feet of sea-level rise, airports and lowland beaches and marshes at the Bay Bridge begin to flood. At 2 to 3 feet, portions of San Francisco and Oakland International airports, Corte Madera, San Rafael and West Oakland start to flood, at least temporarily.

But the most obvious areas are along Highway 37, which has stretches that are both protected and unprotected by local levees. Shilling said a blowout of a state-owned levee along the Petaluma River west of Lakeville Highway could close Highway 37 and leave a 6 square-mile agricultural area between Lakeville Highway and the Lakeville marina up to 3 feet underwater.

Commuter costs and risks to endangered species

Major flooding or levee failures along stretches of Highway 37 could also close the road for months. About 40,000 cars commute along Highway 37 every day. Delays cost an estimated $40 per hour per car, making the potential economic impact of closing Highway 37 alone more than $1.5 million per day.

El Niño-related flooding also threatens area marshes and wildlife. The state and federally listed endangered salt marsh harvest mouse is endemic to Bay Area marshes. Arrowhead Marsh in Oakland is also home to one of the largest remaining populations of Ridgway’s rail, a federally endangered bird that rarely flies and spends its days in the cordgrass. Both the bird and the mouse have to scramble when high tides and flooding arrive, leaving them particularly vulnerable this winter when some of their last-remaining habitat is expected to flood completely. 

“Whether you are an endangered species living in a marsh, a commuter on a Bay Area highway, or own a home on the shore, this winter has the risk of being the most catastrophic in recent history,” Shilling said. “My center and others have been sounding the alarm on this issue for over a decade. Catastrophes will start unrolling without significant state and federal government response.”

The study was funded by the UC Davis Road Ecology Center.