Exposing Failures

Researchers from UC San Diego’s Scripps Institution of Oceanography and the Jacob School of Engineering are studying California’s coastline to understand how sand from sea cliffs contributes to San Diego’s world-famous beaches.

“This is the one we have been waiting for,” said Scripps researcher Liz Johnstone after arriving at a bluff section at Flat Rock, a popular area for beachgoers at Torrey Pines State Park, that collapsed in September 2008. 

A 75-foot section of unstable bluff dumped 1,000 tons of sand onto the beach below in the collapse. The section that came tumbling down was from a 45- to 48-million-year old sandstone formation and was one of several collapses along that same stretch of beach in the state park.

Immediately following the bluff collapse, Johnstone and fellow Scripps graduate student Jessica Raymond headed to the site to map the cliff failure before waves washed it away. 

Using a light detecting and ranging (LIDAR) instrument, the researchers produced a snapshot of the newly exposed cliff face. By comparing the post-collapse imaging data with the previously intact sand, researchers are able to quantify the amount of sand removed from the cliffs.

For the last two years, Johnstone and fellow graduate student Michael Olsen from UCSD’s Structural Engineering Department have mapped 11 bluff collapses along the Southern California coast, from La Jolla to Encinitas, where substantial failures have occurred to create a 3-D snapshot of the cliffs over time in an effort to better understand the erosion process and to quantify the amount of sand removed from the cliffs.

The research team also frequently visits sites where prominent wave notches and sea caves create unstable sea cliffs where future collapses are imminent.

A report by Scripps professor Neal Driscoll concluded that coastal bluffs account for more than half of the sand on San Diego’s local beaches. As waves pound away at the cliffs and water percolates through the sandstone cliffs, researchers hope to better understand this destabilization process and how it disperses to replenish San Diego’s sandy coastline.

“If we didn¹t have these cliff failures, there would be less sand supplied to the beach, enhancing local coastal erosion,” Johnstone said.  “The talus, or sediment failure debris pile, actually acts as a buffer against wave-cut erosion at the base of the cliffs.”

San Diego’s cliffs contribute to the overall sand budget but fears over damage to shoreline structures from erosion has many communities choosing to armor their cliffs with seawalls to keep the sand from being swept away by tides. 

“Although the project is still in its infancy, we have already noticed a substantial amount of change in the sea cliffs,” said Olsen.  “We hope this project will provide valuable information to help local governments make the best-informed decisions for utilization of San Diego’s coastline.”

-- Annie Reisewitz