San Diego Scientist Gets Closer to Understanding Why the Coast Collapses

Science/Environment

San Diego Scientist Gets Closer to Understanding Why the Coast Collapses

Using lasers, a local scientist has helped researchers everywhere get one step closer to predicting coastal cliff collapse.

Adam Young Coastal Erosion 2
Adam Young and team deploying erosion monitoring instruments at Torrey Pines State Beach. / Photo courtesy of Erik Jepsen

Adam Young spent the last three years firing a laser from the back of his truck at Del Mar’s cliffs which are crumbling into the Pacific Ocean.

Cliff collapses along the California coast killed three Encinitas beachgoers in 2019. That same year, another bluff collapse in Del Mar destabilized a set of train tracks regularly carrying passengers between Los Angeles and San Diego. Policymakers need to make big decisions about how best to reckon with earth that seems to fall at random, but scientists still don’t understand what truly causes them to fall.

That’s what Young, a coastal geomorphologist (the study of how the earth’s surface formed and changes) at Scripps Institution of Oceanography, wants to know: If we know how ocean waves and winter rains eat away at a cliff face, can we eventually predict where and when it will collapse?

“There haven’t been good datasets to investigate that until now,” Young said. “And there’s a lot of important infrastructure along the coast that are affected by coastal processes and erosion. These problems are only going to be worse as the sea level rises.”

That’s especially true for San Diego. Climate scientists know, in the worst case climate change scenario (which means the global economy doesn’t substantially ramp-down the use of planet-warming fossil fuels soon) San Diego could see up to a foot more water by 2030 and possibly over 10 feet by 2100.

But Young’s study doesn’t focus on future climate change. Right now, ocean waves crashing against the cliffs eat away at the bottom layer of rock and rains wash away soil and stability from the top. There’s just not enough detailed data about how these two processes work together to bring down a shoreline.

To get that, Young ventured to the beach in a big white research truck armed with a device called LiDAR (which stands for light detection and ranging) which uses light from a pulsing laser to take a kind of 3D picture of the surface. The LiDAR scans the face of a cliff from top to bottom. Picture one of those barcode readers used at checkout lines in a grocery store, but bigger and with the silvery sheen and shape of a robot used to explore a foreign planet.

Adam Young Coastal Erosion
Adam Young and team deploying erosion monitoring instruments at Torrey Pines State Beach / Photo courtesy of Erik Jepsen

This kind of ground-based LiDAR is expensive equipment. Past data gathered on cliff erosion were typically taken from an airplane-based laser flown across the face of a cliff maybe four times a year with the seasons. The simple change, from plane to truck, was an important one in terms of collecting the level of detailed measurements Young now has.

His research team also buried about 10 long pole-like sensors in the beach to measure how high and fast a wave was when it broke against the face of the cliff. They recorded over 4,000 landslide events in the three years of study and may have created the most detailed dataset on the crumbling of a cliff face to date, Young said.

Young’s Del Mar research, published Monday in a scientific journal, represents a stride in understanding reasons why cliffs fall. But Young can’t pinpoint exactly where a future landslide will occur yet. All of the data he’s collected must now be worked into highly complex models used to make predictions.

“As we start to understand how the (waves and rain) are eroding the cliff, then we can build better models so we can understand how things will evolve in the future,” Young said.

This particular dataset is specific to Del Mar, so other scientists would have to replicate his study on other shorelines to make predictive models for their specific coast.

That’s because precisely how a cliff crumbles will also depend on the kind of rock that makes up the cliff. In Del Mar, the base of the cliff is 45 million-year-old rock that is so compact it’s difficult for groundwater to seep through. On top of that is a layer of much younger sandstone, a porous sedimentary rock and remnant of an ancient coastline. When it rains, water makes its way through that softer upper layer but has a problem getting through that tougher bottom layer, Young said.

It’s an unfortunate marriage as the destabilized sandstone top is prone to slipping off just because the rocks happened to form that way ages ago.

Young’s studying a few other sites in Carlsbad, Encinitas, Torrey Pines and Solana Beach. He hopes all this data will help us understand what factors might trigger a big landslide before it happens all along southern California.

“I just like understanding how the coast is evolving and why for a societal purpose, to try and provide information that can help coastal managers out,” Young said.

California’s Erosion Policy Problem

As scientists pull together a clearer picture of the threats beaches and cliffs face in the future, policymakers are starting to react.

Last year, California Sen. Pat Bates, a Republican representing portions of San Diego and Orange counties, introduced a bill that would give homeowners the right to build seawalls sidestepping oversight by the California Coastal Commission. Bates will likely reintroduce that bill in some form during the 2021 legislative session, her spokesman Ronald Ongtoaboc said Wednesday.

Seawalls are highly controversial in California, viewed as a property defense against sea-level rise and the crumbling of coastal cliffs. At the same time, seawalls prevent the natural replenishment of beach sand from cliff faces and land runoff, or allow more wave energy to thrash back and pull even more beach sand away from the shore, argues Laura Walsh, San Diego County Surfrider Foundation policy coordinator.

Sea walls are just one way to adapt to climate change effects on the ocean. There are other ways to slow down wave energy from rising seas and protect beaches like building artificial reefs further out to sea, for instance.

During a hearing on the bill in May, an Encinitas dentist who lost three members of his family to a cliff collapse on top of heir picnic in 2019 gave emotional testimony in favor of Bates’ bill.

“There’s no plan for … any bluff changes to increase safety on this very popular stretch of beach where my accident occurred,” J. Patrick Davis then said.

The Coastal Commission representative argued the Encinitas incident was a tragic accident, not due to a lack of action by the commission to approve or deny a seawall in the area.

“The bill is designed to make it faster, easier and cheaper to build seawalls primarily to protect private residential development,” Sarah Christie, the commission’s legislative director, testified. “For every seawall that is built, the public loses a beach.”

Correction: An earlier version of this story misstated the age of coastal rock in Del Mar, and the name of state Sen. Pat Bates’ spokesman. It also misstated the regulatory change sought by Bates’ private seawall bill. The coastal rock is 45 million years old, not 15 million. Bates’ spokesman’s name is Ronald, not Robert. And Bates’ bill would allow seawalls to sidestep oversight from the Coastal Commission, rather than allow them to be built without Coastal Commission approval.

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