Mitigating the Hazards of Coastal Bluff Instability in San Diego County
May 24, 2006
Contact: Marsha Gear, Communications Director, mgear@ucsd.edu, 858-534-0581
The week of May 10, 2004, engineers funded by California Sea Grant surveyed eroding sea cliffs in and Encinitas in northern San Diego County with an infrared laser. The fieldwork is part of an ongoing Sea Grant project to develop methods for mitigating the hazards of bluff retreat in Southern California.
Data collected by the laser are converted into a color-coded contour plot showing gradients in the cliff’s slope. By comparing images taken in spring and fall, engineers can measure the volume of bluff material lost during summer and winter. Image Credit: I-Site 3D Laser Imaging.
The laser instrument that the engineers used emits a beam of infrared light and records "echoes" off cliff faces. A sophisticated software program converts echoes into "point clouds," 3D high-resolution digital images of the scanned area. The laser is capable of recording 9,000 "points" per second, according to the company that manufactures the instrument. The location of each point is accurate to within 25 millimeters, making it possible to monitor relatively small changes in the shape of the bluff associated with seasonal erosion caused by surf and storms.
UC San Diego engineers involved in the project believe that seasonal laser scans of the shoreline might be a relatively inexpensive, high-tech way to monitor bluff retreat and to alert authorities to potentially dangerous situations. The imaging technique, called ground-based LIDAR (an acronym for LIght Detection And Ranging) could
This Star Wars-like device is a laser that emits infrared light and records its “echo.” It was originally designed for use in calculating the volume of material removed from mines. Scott Schiele, a technician for I-Site, the Australian company that manufactures the laser, adjusts the instrument before programming it to take a scan of a section of bluff face in Solana Beach. Photo: Christina S. Johnson, California Sea Grant.
also potentially be used to survey beaches, allowing communities to detect sand loss or gain and thus optimize plans for nourishing beaches with added sand.
Background
Coastal erosion poses serious problems in Southern California. It threatens homes, businesses, roads and railroad lines built close to the edge. It is shrinking the size of popular beach parks and rare coastal wilderness areas.
Although some state parks have lined beaches with rip-rap, most are not engaging in efforts to protect coastal lands with hard structures. Private homeowners and businesses, however, are building sea walls, retaining walls, "soldier piles" and other structures as protection against waves, tides, rising sea level and surface runoff. The visual impacts of these structures draw criticism from some environmentally minded Californians and from businesses and coastal communities dependent on beach-related tourism.
Engineers also debate the consequences of hard structures. Critics say they exacerbate sand loss by deflecting wave energy and are a negative for communities that benefit from the economic and recreational value of sandy beaches.
The laser, mounted to a plank on a lifeguard truck, scans part of the Solana Beach bluffs.
Photo: Christina S. Johnson, California Sea Grant.
Coastal erosion is a problem on both the U.S. East and West coasts, as well as in the Gulf of Mexico. The problem, however, is intensified in Southern California by the region’s high seismic activity, its geology, and its dense coastal development. Erosion-control strategies offering relief in San Diego County will very likely be helpful to other communities on the West Coast with similar erosion problems and high population densities.
Project
California Sea Grant is funding UC San Diego structural engineer Scott Ashford to develop recommendations on how to mitigate the hazards of coastal bluff instability in northern San Diego County. In particular, he is examining whether sea walls, retaining walls and other hard structures
Workers build a towering wall to stabilize a bluff in Solana Beach. Sea Grant engineers are investigating new, less visually dominating, ways of protecting property. Photo: Christina S. Johnson, California Sea Grant.
do indeed slow erosion and if so by how much and at what cost. He is also testing a new environmentally friendly stabilization technique.
The first phase of the project involved estimating erosion rates for the community of Del Mar. To do this, UC San Diego graduate student Adam Young, a Sea Grant Trainee, scanned sets of aerial photos of the coastline collected over a 70-year period. The scans have been converted into a series of GIS-compatible maps, showing the rates of sea bluff retreat since1932. The maps are viewable as 3D topographic images. Young is in the process of mapping erosion rates for Solana Beach and Encinitas.
When completed, he will begin mapping the locations of hard structures in these areas and then will use the GIS maps to measure the pace of erosion (as measured by bluff retreat) since their construction. The idea is to identify the relative effectiveness of different stabilization methods, their relative costs and environmental impacts.
In 2003 engineers began taking a series of laser scans of sections of the coast using LIDAR. These high-resolution images can detect minute changes in the bluffs before and after winter. Beaches usually lose sand in winter, when storms from the north Pacific bring heavy surf.
LIDAR scanning may be able to monitor beach sand loss and gain on a seasonal time scale, offering communities a relatively inexpensive way to monitor the effectiveness of beach nourishment plans. It would also let scientists measure how much bluff material replenishes beaches annually.
Besides evaluating erosion and the effects of hard structures on the coast, the scientists have begun experimenting with the idea of injecting colloidal silica into loosely consolidated sea cliffs, as a way to cement sandstone materials firmly together. Colloidal silica has the viscosity of water and once injected into sandstone would fill spaces between grains, hardening into a stable material. Silica is natural, relatively inexpensive and would have no visual side effects.
Structural engineering professor Scott Ashford of UC San Diego’s Jacobs School of Engineering is the lead investigator of the cliff erosion study. A soils engineer, whose research is usually focused on testing soils, piles and foundations during seismic events, Ashford became interested in sea cliff erosion after a woman in his
community was killed by a sudden bluff collapse. “I knew that something had to be done,” Ashford was quoted as saying in The Coast News, a local newspaper.
Adam Young, a Sea Grant trainee and engineering graduate student at UC San Diego, explains the process by which aerial photos of the coast are converted into 3D digital images. As part of his doctoral thesis and his California Sea Grant project, he is compiling and digitizing a series of historical aerial photos, dating back to the 1930s. Once complete, the images will show in graphic detail the patterns, pace and magnitude of bluff retreat
in northern San Diego County.
Photos: Christina S. Johnson, California Sea Grant
Findings
The engineers’ preliminary findings suggest that erosion rates in northern San Diego County are less than some previously thought. Based on their preliminary analyses, bluffs in Del Mar have been eroding at a rate of about 5 centimeters a year, though some areas are disappearing at a rate of 30 centimeters annually. Some areas with slower rates of erosion, they believe, may be “hot spots” at greater risk of episodic events in the future.
The findings will be presented in a manual and shared with city and state officials, the North County Transit District, U.S. Army Corps of Engineers, California Coastal Commission, land planners and other parties interested in alternative methods for stabilizing sea cliffs. The scientists will also post their results on the web for public downloading.
Collaborating Organizations
California Coastal Commission
City of Del Mar
City of Encinitas
City of Solana Beach
North County Transit District
U.S. Army Corps of Engineers
Trainee
Adam Young
For more information:
Dr. Scott A. Ashford
Department of Structural Engineering
University of California, San Diego
Email: sashford@ucsd.edu
Ocean Engineering
Project No. - R/OE-37, 3.1.2001–2.28.2005
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