The millions of Southern Californians who call the coast home just got some bad news. A study published Monday by the US Geological Survey predicts that, with “limited human intervention ... 31 to 67 percent of Southern California beaches may become completely eroded” by 2100.

That won’t just spell trouble for surfers and sunbathers. “The beaches are an economic driver for many of these communities from tourism, and also the first line of defense against storm impacts and damage," says one of the study’s co-authors, coastal geologist Patrick Barnard.

“It's important, when you have such an urbanized shoreline, to have a nice wide beach which is that first line of defense against these coastal storms,” he explains in a phone interview with The Christian Science Monitor. Taking into account tourism revenues, storm defense, and other “ecosystem services,” the EPA places a value of $31,500 to $72,900 on each acre of California beach.

The USGS’s latest study, which forecasts a one-to-two-meter (about 3.3 to 6.6 feet) rise in sea level, makes clear that these benefits are under threat. But it also indicates a path to preserving them: more sand, and a sustained, coordinated commitment to distributing it along the shore.

“This is going to be a pervasive problem, and not just something [where] you can put a little sand here and there,” Dr. Barnard predicts. “It's going to require a much broader effort to get more sand in the system if we want to maintain the beaches where they are today.”

Waves, tides, and storms all naturally eat away at beaches. The sand that washes out to sea can get replenished by rivers and other inland sources – if they’re not blocked by human activities.

Dams, for instance, can trap sediment behind them and starve beaches downstream. According to the nonprofit International Rivers, “Since the 1920s, dams have reduced by four-fifths the sediment reaching the coast of Southern California.”

Derek Brockbank, executive director of the American Shore & Beach Preservation Association (ASBPA), says that many of the area’s coastal bluffs have been reinforced for safety, but at the cost of the beaches they once sustained. “In California, we’ve got a lot of homes right on top of the cliff,” he tells the Monitor. ”Those cliffs are [artificially] hardened, so that the cliffs don't erode, so that the homes don't fall into the water, but those eroding cliffs are the source of sand for beaches.”

Actions like these have made themselves felt for decades in Southern California. Cesar Espinosa, a beach planner with the Los Angeles County Department of Beaches and Harbors, tells the Monitor over the phone that, “We actually have been, every year, build[ing] beach berms” – large mounds of sand that get seasonally destroyed by winter waves, leaving the beach below intact – “to protect our facilities. And that's been going on for over thirty years.”

Recent stormy weather is making this task harder, and a changing climate could make it even worse.

“This year, we've seen pretty much unprecedented erosion,” says Nicole Mooradian, the beaches and harbors public information officer for L.A. County, which does not have jurisdiction over any of the beaches analyzed in the study. According to the county’s website, massive, El Niño-spawned waves “swept untold volumes of sand from the shoreline into the sea.”

A warming climate could bring more storms like these in coming years, and the two-meter sea level rise that the USGS projects – which Barnard says is “based on ... fairly optimistic future emissions scenarios” – will make preserving the beaches harder still.

But in places like Long Beach and San Diego, where homes, businesses, and critical infrastructure have massed along the coast, there may be little choice.

“The problem,” says Barnard, is that "we've drawn a line in the sand, and said, ‘Here's where we're living, we're not going to let [the beach] go past this point.’ ”

But “if sea level rises, these beaches naturally want to move landward.... You can't maintain a beach in this location, if the sea level's rising, without lots more sediment put into the system.”

The Netherlands, he points out, has long since learned this lesson. The Dutch government is required by law to maintain its entire coastline at its 1990 position. Since 2001, the effort has required some 12 million cubic meters of sand each year.

A similar effort likely wouldn’t work in California, a state with a coastline more than three times longer than Holland’s. The ASBPA’s Mr. Brockbank suggests California – and, eventually, other states – will need to find a “balance of structures, living shorelines, nourishment, and moving development.” Striking this balance, he continues, will require greater coordination among state, local, and federal policy makers.

The Golden State may already be moving in this direction. Its government, working with the US Army Corps of Engineers and other federal agencies, is currently preparing a “Sediment Master Plan” to guide beach replenishment statewide. “That's an example of how better policy can help support beaches and the natural dynamic” of the shorelines, Brockbank explains. “This would be a policy that would help the rebuilding process.”

But it won't be cheap. Mr. Espinosa explained that the state will seek an amendment to the biennial Water Resources Development Act, up for renewal next year, that will create a framework – and free up federal funds – for use in beach replenishment. He predicts that it could serve as a “national model” for other regions to follow.

Like with other environmental initiatives, the Trump Administration’s proposed budget – in particular, a $5 billion cut to the Army Corps of Engineers – bodes ill for this project. But putting sand on beaches doesn’t seem to be a partisan issue. Last November, the Water Infrastructure Improvements for the Nation (WINN) Act passed Congress under the sponsorship of Texas Republican Sen. John Cornyn and was signed into law by then-President Barack Obama. In a hopeful sign for coastal dwellers, it included an Army Corps pilot program to recycle dredged sand for use on beaches.

That program, as part of a statewide sediment management plan, could help plug the missing beaches that the USGS study foresees under a “limited human intervention” scenario.

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“I think it's clear that this is not an outcome that managers and planners and policy people are going to let happen,” Barnard predicts, “because of the economic implications and public safety implications of not having beaches in front of these communities.”

But at the same time, he cautions, “we have to plan for this [1- to 2-meter] range of sea level rise ... Even if it may not happen by 2100, we're definitely going to get there sooner or later.“