Coastal Bluffs Provide More Sand
California Beaches Than Previously Believed
By Rex Graham
Coastal geologists have
assumed for years that sediment-laden rivers that enter the
Pacific Ocean along the Central and Southern California coast
supply up to 90 percent of the sand on the region's beaches.
However, new research by two independent groups of UCSD scientists
indicates that what had been thought to be a minor source of
sand – erosion from coastal bluffs and cliffs –
could account for about half of the region's beach sand.
School of Engineering professor Scott Ashford and Ph.D.
candidate Adam Young used a highly accurate laser scanning
technology to measure the contribution of coastal bluffs
to the supply of beach sand in a 50-mile stretch of Southern
Various types of concrete surfacing
and reinforcement of bluffs as well as layering large boulders
as rip-rap along the base of bluffs tend to "armor"
them, slowing or preventing such erosion. Determining the source
of sand, according to the researchers, is the logical first
step in any effort to preserve Southern Californian beaches.
In a paper to be presented October
12 during the annual meeting of the American Shore and Beach
Preservation Association in San Francisco, Adam Young, a Ph.D.
candidate in UCSD's Jacobs School of Engineering, will report
the unexpectedly high contribution of coastal bluffs and cliffs
to the supply of beach sand. Young, who has also submitted his
results to the Journal of Coastal Research, used laserscanning
technology to generate a series of 3-D topographical maps that
quantified coastal bluff erosion with a high degree of accuracy
during the past six years.
group of researchers at UCSD's Scripps Institution of Oceanography,
which has worked independently of the engineering group
at UCSD, also identified coastal bluffs as a much larger
source of beach sand than have been previously believed.
The study area for both studies was the Oceanside Littoral
Based on the volume of material
that has fallen from the bluffs during the study period, Young
concluded that half of the beach sand in the Oceanside Littoral
Cell, a 50-mile stretch of California coast from La Jolla north
to Dana Point, was likely derived from the bluffs.
In recognition of his research,
which was funded by California Sea Grant, the University of
California's Coastal Environmental Quality Initiative, and UCSD's
Center for Earth Observations and Applications, the American
Shore and Beach Preservation Association on Oct. 11 will present
its 2005 Education Award to Young. In addition, the California
chapter of the association will present its 2005 Robert L. Wiegel
Coastal Studies Scholarship to Young, also in recognition of
his bluff erosion research.
"While keeping in mind that
six years is only a brief snapshot in the life of the Southern
California coastline, our results call into question the conventional
wisdom that coastal bluffs don't contribute much to the beaches,"
said Scott Ashford, a professor of structural engineering at
UCSD and Young's faculty advisor. "Adam's results should
alert all groups interested in the preservation and development
of Southern California's beaches that the assumptions they have
been using to identify the supply of beach sand should now be
Ashford said decades-old photographs
of the Southern California coast taken from the ground and the
air also have documented the steady pace of erosion. However,
he said the photographs lack the precision and accuracy of the
laser scanning technique called LIDAR, an acronym for light
detection and ranging. Ashford said the 3-D maps generated by
LIDAR permitted Young to calculate the unexpectedly high volume
of bluff material that has fallen onto beaches during the study
amounts of sand in Southern California move offshore in
winter and return to the beaches in summer, as this pair
of photographs taken at the same location document. UCSD
researchers have identified coastal bluffs as a large source
of beach sand, but they are also seeking to understand the
complex dynamics of sand redistribution along the coastline.
“A new question we're interested
in now is 'What if we stopped armoring the bluffs and cliffs
and allow them to erode naturally?' " Ashford said. "Would
such a moratorium be enough to replenish the beaches? We need
to do more work to address a range of questions like that."
At the wave washed western edge
of the campus, Neal Driscoll, a geology professor at UCSD's
Scripps Institution of Oceanography, and graduate student Jennifer
Haas have studied the same 50-mile stretch of beach north, but
with a completely different technique. The Scripps team used
a mineralogical fingerprinting technique. They compared sand
grains collected from beaches in the study area to grains taken
from coastal bluffs, rivers, and from dredged material that
the San Diego Regional Beach Sand Project used to replenish
the region's disappearing beaches.
After examining the population
of sand grains on beaches in the La Jolla area, the Scripps
team determined that sea cliffs must be an important source
of sand to those beaches. Based on their observations, Haas
and Driscoll concluded that 50 percent of the sand came from
erosion of the bluffs and cliffs. Haas successfully defended
her master's thesis in spring 2005.
"What is exciting to me is
that both our engineering group at the Jacobs School and the
geology group at Scripps took completely different approaches,
but arrived at the same conclusion, which is that bluffs and
cliffs appear to be a much more important source of sand in
the Oceanside Littoral Cell than had been previously believed,"
The Scripps team found a type
of clear-quartz grains in the coastal cliffs, but collected
predominantly frosted quartz sand grains in the rivers and offshore
borrow sites from which sand has been dredged for placement
on erosion-prone beaches. "In La Jolla, the beaches have
a large proportion of clear quartz, which indicates that the
cliffs are a significant source of beach sand," Driscoll
said. "There's just no other way around it."
Driscoll and Ashford agree that
Central and Southern California rivers carry a huge amount of
sandy sediment to the Pacific Ocean during seasonal downpours.
"When the rains come, the majority of the sediment discharge
occurs during an extremely small percent of the time,"
Driscoll said. "Often, the sediment-laden river water is
denser than seawater, so when this slurry reaches the coast,
it sinks and follows the bottom, escaping the shallow water
region near the shore where it could replenish sand to the beaches."
In dry years there is very little
sediment in Southern California rivers flowing into the Pacific.
"In wet years," Driscoll said, "the rivers flow
like fire hoses, with most of the sediment ending up offshore
in deeper water."
California Sea Grant, the largest
of the 30 Sea Grant programs nationwide and administered by
the University of California, recently awarded $200,000 to Ashford
and Driscoll to collaborate and expand their investigation of
the relationship between bluff erosion and beach sand supply
in the Oceanside Littoral Cell.
The Center for Earth Observations
and Applications at UCSD, which partially funded Young's bluff-scanning
project, was formed in 2005 with a grant from UCSD Chancellor
Marye Anne Fox. John Orcutt, deputy director of scientific affairs
at Scripps Institution of Oceanography, directs the center.
Media Contact: Rex
Graham, Jacobs School of Engineering(858) 822-3075