Coastal geologists have assumed for years that sediment-ladenrivers that enter the Pacific Ocean along the Central and SouthernCalifornia coast supply up to 90 percent of the sand on the region'sbeaches. However, new research by two independent groups of UCSDscientists indicates that what had been thought to be a minor source ofsand – erosion from coastal bluffs and cliffs – could account for abouthalf of the region's beach sand.
Various types of concretesurfacing and reinforcement of bluffs as well as layering largeboulders 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 effortto preserve Southern Californian beaches.
In a paper to bepresented October 12 during the annual meeting of the American Shoreand Beach Preservation Association in San Francisco, Adam Young, aPh.D. candidate in UCSD's Jacobs School of Engineering, will report theunexpectedly high contribution of coastal bluffs and cliffs to thesupply of beach sand. Young, who has also submitted his results to theJournal of Coastal Research, used laserscanning technology to generatea series of 3-D topographical maps that quantified coastal blufferosion with a high degree of accuracy during the past six years.
Basedon the volume of material that has fallen from the bluffs during thestudy period, Young concluded that half of the beach sand in theOceanside Littoral Cell, a 50-mile stretch of California coast from LaJolla north to Dana Point, was likely derived from the bluffs.
Inrecognition of his research, which was funded by California Sea Grant,the University of California's Coastal Environmental QualityInitiative, and UCSD's Center for Earth Observations and Applications,the American Shore and Beach Preservation Association on Oct. 11 willpresent its 2005 Education Award to Young. In addition, the Californiachapter of the association will present its 2005 Robert L. WiegelCoastal Studies Scholarship to Young, also in recognition of his blufferosion research.
"While keeping in mind that six years is only abrief snapshot in the life of the Southern California coastline, ourresults call into question the conventional wisdom that coastal bluffsdon't contribute much to the beaches," said Scott Ashford, a professorof structural engineering at UCSD and Young's faculty advisor. "Adam'sresults should alert all groups interested in the preservation anddevelopment of Southern California's beaches that the assumptions theyhave been using to identify the supply of beach sand should now bere-examined."
Ashford said decades-old photographs of theSouthern California coast taken from the ground and the air also havedocumented the steady pace of erosion. However, he said the photographslack the precision and accuracy of the laser scanning technique calledLIDAR, an acronym for light detection and ranging. Ashford said the 3-Dmaps generated by LIDAR permitted Young to calculate the unexpectedlyhigh volume of bluff material that has fallen onto beaches during thestudy period.
“A new question we're interested in now is 'What ifwe stopped armoring the bluffs and cliffs and allow them to erodenaturally?' " Ashford said. "Would such a moratorium be enough toreplenish the beaches? We need to do more work to address a range ofquestions like that."
At the wave washed western edge of thecampus, Neal Driscoll, a geology professor at UCSD's ScrippsInstitution of Oceanography, and graduate student Jennifer Haas havestudied the same 50-mile stretch of beach north, but with a completelydifferent technique. The Scripps team used a mineralogicalfingerprinting technique. They compared sand grains collected frombeaches in the study area to grains taken from coastal bluffs, rivers,and from dredged material that the San Diego Regional Beach SandProject used to replenish the region's disappearing beaches.
Afterexamining the population of sand grains on beaches in the La Jollaarea, the Scripps team determined that sea cliffs must be an importantsource of sand to those beaches. Based on their observations, Haas andDriscoll concluded that 50 percent of the sand came from erosion of thebluffs and cliffs. Haas successfully defended her master's thesis inspring 2005.
"What is exciting to me is that both our engineeringgroup at the Jacobs School and the geology group at Scripps tookcompletely different approaches, but arrived at the same conclusion,which is that bluffs and cliffs appear to be a much more importantsource of sand in the Oceanside Littoral Cell than had been previouslybelieved," Ashford said.
The Scripps team found a type ofclear-quartz grains in the coastal cliffs, but collected predominantlyfrosted quartz sand grains in the rivers and offshore borrow sites fromwhich sand has been dredged for placement on erosion-prone beaches. "InLa Jolla, the beaches have a large proportion of clear quartz, whichindicates that the cliffs are a significant source of beach sand,"Driscoll said. "There's just no other way around it."
Driscolland Ashford agree that Central and Southern California rivers carry ahuge amount of sandy sediment to the Pacific Ocean during seasonaldownpours. "When the rains come, the majority of the sediment dischargeoccurs during an extremely small percent of the time," Driscoll said."Often, the sediment-laden river water is denser than seawater, so whenthis slurry reaches the coast, it sinks and follows the bottom,escaping the shallow water region near the shore where it couldreplenish sand to the beaches."
In dry years there is very littlesediment in Southern California rivers flowing into the Pacific. "Inwet years," Driscoll said, "the rivers flow like fire hoses, with mostof the sediment ending up offshore in deeper water."
CaliforniaSea Grant, the largest of the 30 Sea Grant programs nationwide andadministered by the University of California, recently awarded $200,000to Ashford and Driscoll to collaborate and expand their investigationof the relationship between bluff erosion and beach sand supply in theOceanside Littoral Cell.
The Center for Earth Observations andApplications at UCSD, which partially funded Young's bluff-scanningproject, was formed in 2005 with a grant from UCSD Chancellor MaryeAnne Fox. John Orcutt, deputy director of scientific affairs at ScrippsInstitution of Oceanography, directs the center.
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