Erosion equation: More storms mean less sand and more dangerous conditions at La Jolla’s beaches
This winter’s stormier-than-usual weather has led to “big-league erosion” on La Jolla’s beaches and elsewhere, leading to undesirable and even dangerous conditions both on land and at sea, local experts say.
The increase in beach erosion — the withering away of sand and sediment levels along the coast — has made rip currents “very hazardous right now,” San Diego lifeguard Lt. Lonnie Stephens told the La Jolla Shores Association on March 8.
The elevated rip current risk comes from more severe slopes into deeper waters, according to Bob Guza, a professor emeritus at UC San Diego’s Scripps Institution of Oceanography in La Jolla. “I think our beaches are relatively steep right now,” he said.
Scripps Oceanography has been studying beach erosion since the 1950s, Guza said, and SIO scientists have been watching changes in sand levels — both visible at the beach and in the amount of sand under the water — for decades. But this year’s conditions took them by surprise, Guza said.
“Sand does not just sit on a beach; it goes and it comes” as gravity pulls sand offshore into the water and natural events such as cliff collapses and floods deposit new sediment, Guza said.
Erosion increases in El Niño years, when climate patterns bring very rainy conditions and “monstrous waves,” he said.
This is not an El Niño year, yet San Diego beaches sustained “very significant erosion” in January, comparable to El Niño levels, as a multitude of storms brought a surge in the number of large waves and the amount of rainfall, he said.
This level of erosion is not unprecedented — San Diego experienced similar levels during 2016’s El Niño year — but it’s unusual and the wintry weather isn’t over yet, Guza said.
Scientists had expected “this would be a mild year for erosion,” Guza said. “So much for our prognostications.”
Oceanographers measure beach erosion with the use of GPS devices attached to personal watercraft that function as “an acoustic pinging device,” measuring the depth of the sand under the water, Guza said.
“Most recently, we’ve been using modern tools like LiDAR [and] drones,” he said.
SIO’s studies also try to find out where the sand goes when it leaves the coastline. Some of it will irretrievably end up in underwater canyons.
The rest? “We don’t really know yet where it goes,” Guza said.
When sand disappears from the beaches, the natural method of replacement is flooding via riverbeds.
But those flood channels in San Diego — through Fashion Valley, Mission Valley and others — are dammed to allow for residential and commercial development.
“Massive amounts of infrastructure are built in flood plains,” Guza said. “So the sand that used to get to the beach in floods is now trapped behind dams inland and disposed of” as officials dredge dammed water of sand to reduce the buildup.
Another source of sand accumulation at the beach — cliff collapse — also is partially stemmed by “beach armoring,” or walls built to prevent cliffside homes from falling.
Forty percent of cliffs in North County are now armored, Guza said, often preventing sand from refilling beaches.
“You created a sand shortage where maybe one didn’t exist before,” he said.
On the other hand, increased erosion at unarmored cliffs increases the risk of collapse, which can bring its own danger. Bluff erosion has been threatening coastal railroad tracks in Del Mar and south Orange County.
To help remedy beach erosion, some coastal communities, such as Solana Beach, Cardiff and Encinitas, perform “beach nourishment,” placing a million cubic feet of sand from lagoons or other locations onto an eroded beach, Guza said.
That process, however, can have unintended consequences, he said, such as at Imperial Beach, where “major amounts of sand” were added and eventually migrated south, clogging the Tijuana River.
He pointed to other methods, like construction of sand retention structures. The Mission Bay Entrance Channel Jetty at Mission Beach, a rock wall that extends a long distance offshore, is one example of that, Guza said. It interrupts “the natural transport of sand at Mission Beach [from] north to south.”
But building jetties can be contentious, he said, as different interests, such as surfers, the tourism industry and environmental advocates, have “different visions of how you might want to manage things.”
And, Guza cautioned, beaches need regular maintenance like highways, and the evolution of erosion and its complicated contributing factors mean officials will soon face “hard decisions.”
As sand wears away from a beach, rocks are left, restricting access, Guza said.
“We will not be able to maintain access to all [the beaches] that we can now access,” he said. “It’s not going to happen.”
Just as scientists are trying to understand where sand goes during erosion, they also are looking at the accumulation, or accretion, process, which is similarly mysterious.
“We don’t understand the opposite of erosion,” Guza said.
Being able to manipulate the accretion process might be another method of erosion abatement, he added. ◆
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