Trying To Save Eroding Southern California Coastline

Daily University Science News

Tragically, the number of deaths and destroyed homes continues to climb as the picturesque Southern California coastline crumbles.

The Federal Emergency Management Agency (FEMA) estimates that a quarter of the homes within 500 feet of the U.S. coast could be lost to erosion in the next 50 years.

In response to this dilemma, structural engineers with the UCSD Jacobs School of Engineering are embarking on a new study to determine the best ways to stabilize these fragile cliffs. The results will culminate in a comprehensive, publicly accessible guidebook.

The San Diego communities most affected by bluff instability are Encinitas, Del Mar, Solana Beach and parts of La Jolla and the City of San Diego.

"It's obvious that a major problem exists, but repairs are typically only approved in emergency situations because of concerns over damaging the bluffs or destroying the natural beauty of the coastline," says Scott Ashford, project leader and professor of geotechnical engineering at the Jacobs School. "Therefore, one of our major goals is to pinpoint the most efficient and effective ways to resolve this growing crisis, while maintaining the environmental integrity of the landscape."

By developing a comprehensive guide to assist in bluff stabilization, Ashford hopes to provide the information necessary to act before emergencies arise. His three-year, three-stage research project, titled "Mitigation of Coastal Bluff Instability in San Diego County, California," will evaluate existing stabilization techniques and, he hopes, lead to proactive improvements.

Sponsored by a Sea-Grant from the National Oceanographic and Atmospheric Administration, the quarter-million­dollar endeavor will analyze the overall effectiveness of sea walls, cement slopes, agricultural plantings, and other measures typically used to combat slope erosion and destruction.

The coastal bluffs are formed by sand deposits that build up over time. A natural cementation occurs between the ocean water and sand particles to create a relatively weak substance somewhere between soil and rock.

These steep structures can be found along the Western U.S. coastline, from Southern California to British Columbia. And because they are so steep, a zone of tension exists at the top, which creates cracks and eventual decay.

This is compounded by such natural phenomena as erosion from seawater and irrigation, earthquakes and construction.

Ashford will compare digital aerial photographs to identify, examine, and evaluate existing mitigation methods.

"A powerful computer program will be used to compare photographs over time and identify changes," explains Ashford. "If a sea wall was installed in 1970, for example, we can look at the erosion rate before and after to gauge the structure's overall effectiveness."

Stage two involves extensive computer numerical analysis. Ashford hopes to identify how the different mitigation techniques really affect stress distribution in the slopes. Some do nothing or even compound problems, he says, based on initial observations.

For example, a "sea wall" is a wall built at the base of a bluff to reduce wave erosion. In San Diego County, however, wave erosion is really a secondary factor, according to Ashford.

Agricultural plantings can actually enhance problems. Their root systems often invade existing cracks in the weak bluffs and make the cracks even bigger. Watering the plantings on the sloped surfaces can also add to problems.

In the last stage, Ashford will combine all of the data into a comprehensive guide, complete with information about relative costs. The guidebook will be available on the World Wide Web and accessible to the public.

By working with the California Coastal Commission and the local municipalities, Ashford hopes to compile relevant information that will be invaluable in initiating the proactive improvement of San Diego's coastline.

The overall findings may also be applied to areas outside of San Diego County. Steep slopes composed of other materials can be found in China, Central America, South America, and the central United States (e.g., Missouri, where rivers are prominent. They are subject to the same factors that destabilize slopes in San Diego.