An underwater survey off San Diego has revealed geological details of how sand builds up along Southern California’s continental shelf and could help resource managers locate deposits to rebuild beaches, according to a recent report by scientists at Scripps Institution of Oceanography at UC San Diego.
The data show the depth of sand levels along 10 kilometers (6.2 miles) of shoreline from La Jolla Cove north to Torrey Pines State Beach and how the sediments are distributed on the shallow, gently sloping seabed adjacent to the shoreline.
The scientists also identified an area of the seafloor uplifted offshore of Torrey Pines State Park that results from a jog in the Rose Canyon fault, similar to the uplift that created Mount Soledad. This uplifted area appears to play a major role in the accumulation of sand in the area, according to Leah Hogarth, a Scripps graduate student and lead author of the article in the journal Geology of the Geological Society of America.
“This study shows how the local tectonic structure controls the long-term accumulation of sediments on this region of the nearshore shelf,” Hogarth said. “There are many locations along the Southern California coastline south of Point Conception that might have similar patterns in sediment distribution.”
The survey found the offshore sand thickness in the area goes from nearly zero offshore Torrey Pines beach to as thick as 20 meters (65 feet) southward toward La Jolla Cove. Adjacent to the uplifted area off Torrey Pines is a pocket of sand some 15-20 meters (49-65 feet) thick that the scientists call a “sediment depocenter.” It is nearly 2 kilometers (1.2 miles) offshore at a depth of about 30-40 meters (95-130 feet), which is likely too deep to be affected by waves and climate conditions, according to Neal Driscoll, a Scripps professor and coauthor of the report.
“The study location is ideal because the geometry of the fault structure lets us separate the tectonic influences on sand accumulation from the effects that relate to sea-level fluctuations,” Driscoll said. “This gives insight into where California’s offshore sand resources and hardgrounds might be located on a long-term basis and could assist coastal resource managers in identifying potentially reliable sources for replenishing sand to beaches.”
The sediment thickness offshore La Jolla shows that tectonics and sea-level fluctuations control long-term sediment accumulation in the region and that waves and long-shore currents control sediment distribution, Driscoll said. This study indicates that interactions between right-lateral fault segments offshore Southern California play a major role in creating pockets of sediments, he added.
During the survey, the scientists towed a specially outfitted underwater device behind Scripps research vessel Robert Gordon Sproul making multiple passes over the 20 square kilometer (7.5 square mile) area. Onboard the device were instruments to produce images of both the seafloor and sub-bottom. The sub-bottom profiles are produced by acoustic signals that penetrate through approximately 25 meters (80 feet) of sand, showing the thickness of the sediments. The scientists estimate some 60 million cubic meters (78.5 million cubic yards) of sand are within the study area.
“Knowing where the sand deposits are stable could help make for better choices of beach replenishment sources,” Hogarth said. “If we are careful to take sands from areas offshore where they will be replenished naturally or where those sands do not already contribute to the beach, we can better ensure we are not further disrupting the balance of sediment supply to beaches.”
A major public works effort to restore sand to beaches along sections of San Diego County was carried out in 2001 by the San Diego Association of Governments (SANDAG). More than 1.5 million cubic meters (2 million cubic yards) of sand was dredged from offshore and pumped onto beaches in Oceanside, Carlsbad, Encinitas, Solana Beach, Del Mar, San Diego, and Imperial Beach. SANDAG officials said it was the first regional beach restoration project undertaken on the West Coast. The City of Imperial Beach is working on a large-scale beach restoration project with a goal of periodic replenishment over the next 50 years.
— Chuck Colgan