Sea Grant Study: Coastal Erosion

June 21 2002

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

Residents of coastal communities in California are well aware of the dangers of coastal erosion, which can lead to flooding, collapsed bluffs, closed roads and toppled houses. As for the risk nationwide, the Federal Emergency Management Agency in a 2000 report to Congress estimated that as many as 87,000 houses are in danger of crashing into the surf in the next 60 years.

Sea Wall Del Mar, California (February 1984)

In response to these concerns, California Sea Grant is supporting research on coastal erosion issues specific to California, where about 80 percent of the state's 33 million residents live within 50 kilometers of the Pacific Ocean. About 86 percent of the state's shoreline is classified as actively eroding.

Gary Griggs, director of the Institute of Marine Sciences at the University of California at Santa Cruz, is one scientist who has received support from California
Sea Grant to study topics such as the effects of El Niño on bluff erosion and the influence of geology on erosion rates.

In a recent study, Dr. Griggs and graduate student Ben Benumof measured erosion rates during the intense 1997-98 El Niño. In that winter, storm after storm battered the shore, causing bluff failures, landslides, floods and enhanced cliff erosion. Confirming earlier studies by others, Dr. Griggs and Benumof showed that erosion in California occurs episodically, not at a steady pace.

Their study also led to some surprises. Dr. Griggs said: "What was most striking was that erosion rates varied directly with the strength of the bluff material. Rock strength is really the dominating factor in bluff erosion, not wave energy, along the shoreline in San Diego."

Although beach retreat and flooding pose a high risk to the East Coast and Gulf states, bluff collapse is the greatest threat in California, particularly in counties such as San Diego and Santa Cruz, where houses have been built on unstable sea cliffs. As demonstrated during the 1989 Loma Prieta earthquake and the more recent Northridge earthquake, these houses are particularly vulnerable to cliff collapse during earthquakes.

Prior to the El Niño study, Dr. Griggs was funded by FEMA to map erosion hazards for San Diego and Santa Cruz. For these maps, he and his students uses past erosion rates to forecast the position of the shoreline in 60 years. Based on their predictions, entire neighborhoods are at risk of tumbling into the sea, if action is not taken.

Dr. Griggs' concerns about coastal erosion and beach development go back decades, and in the 1980s Dr. Griggs was funded by California Sea Grant to analyze the effectiveness of seawalls. "We looked at the history of every seawall between San Francisco and Carmel," he said. "We saw structures that lasted 40 years and others that lasted only 40 days."

The FEMA report to Congress in 2000, which was commissioned to evaluate the feasibility of insuring coastal properties as part of the federal Flood Insurance Program, noted that seawalls, revetment and other hard structures slow erosion, offering some protection to beach property. The report also cited the aesthetic drawbacks of armoring the coast. Addressing this dilemma, California Sea Grant has recently funded a three-year project to examine the effectiveness of seawalls and other hard structures in preventing bluff failure in San Diego County.

Over the next three years, a group of structural engineers at UC San Diego, led by Dr. Scott Ashford, will digitize and then analyze old and recent aerial photos of the coast, building on some of the photo-imaging techniques developed by Dr. Griggs. These images will be used to estimate erosion rates along stretches of the coast where seawalls, riprap and soil-cement buttresses have been built. In the second and third years of the study, the scientists will investigate why some structures are more effective at slowing erosion than others, and they will then assess costs and environmental consequences of armoring. Seawalls, for instance, may deplete beaches of sand.

The group will ultimately publish a manual providing guidelines for bluff stabilization techniques for property owners, engineers, city planners and regulators. Dr. Ashford said of the study: "I hope to find some alternatives to simply putting a seawall on the beach."