Researchers Tracking Sand on Nourished Beaches in San Diego


Robert Guza
Scripps Institution of Oceanography
UC San Diego

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January 4, 2013

Coastal physicist Bob Guza at Cardiff State Beach in northern San Diego County. Photo: C. Johnson

January 4, 2013

Contact: Christina S. Johnson,, 858-822-5334

CARDIFF-BY-THE-SEA – Last month, coastal physical oceanographer Bob Guza finally got a taste of what he’s been waiting for – pounding surf and high tides that together are eating away at, sculpting really, beach sand at Cardiff State Beach in northern San Diego County.

"Hopefully, we can dial up some really big waves this month," says Guza, a professor at UC San Diego's Scripps Institution of Oceanography, who has been studying beach and surf-zone processes for more than 40 years. "Our interest is to understand events – big storms – and to calculate erosion during this time."

Steep sand ledge at Cardiff State Beach

The steep sand ledge speaks to the dramatic changes in beach sand levels that can occur with tides and waves. Photo: C. Johnson

Cardiff State Beach is one of eight county beaches recently rebuilt with sand dredged from offshore, as part of a more than $20-million regional beach sand project led by the San Diego Association of Governments.

With support from California Sea Grant, U.S. Army Corps of Engineers and California Department of Boating and Waterways, Guza, his technicians and graduate students are now collecting data that will help determine the transport and fate of the sand at several of the replenished beaches.

The laser instrument that is gathering data on beach sand

The laser instrument that is gathering data on beach sand is on the right hand side of the top of the scaffold. It scans a beach transect every seven seconds and a 600-meter sector every 30 minutes. Photo: C. Johnson

At Cardiff, the monitoring is especially high-tech and noteworthy, as it is being conducted with an optical laser-scanning instrument, mounted atop a 30-foot scaffold located at the back of the beach along Highway 101. From its perch, the laser scans a 600-meter sector of beach every 30 minutes, recording changes in beach sand level and the wave run-up that caused it.

This information, coupled with wave and tidal data, builds a cause-and-effect understanding of how waves, tides, storms and series of storms act in concert on beach sand. Instruments deployed in the surf-zone document where the sand goes after it is swept off the beach.

Previous field-work in northern San Diego County, some of it supported by Sea Grant, has shown that much of the sand taken off our beaches in winter is deposited in nearshore sand bars and returned during summer's long, gentle swells from the Southern Hemisphere.

Among the many goals of the current project is to add detail to this general knowledge of sand movement patterns.

Such information would be of great value in budgeting for long-term beach sand replenishment that coastal engineers say is required to compensate for the loss of the natural sand supply from rivers and coastal cliff erosion.

"Our beaches are sand-starved," Guza explains. "If we want sandy beaches, we will have to maintain them the same way we maintain our roads and other infrastructure."

Cardiff State Beach

Cardiff State Beach is now covered in a pad of sand but winter swells will sweep away this sand, exposing the cobbles beneath. To protect Highway 101, the back of the beach is lined with riprap. Photo: C. Johnson

Yet another goal of the project is to refine an existing computer model of beach dynamics to include an "erosion-resistant" layer. Anyone familiar with San Diego beaches knows that Cardiff is often a cobble beach, not a sandy one. Guza and his team have found that these potato-sized cobbles resist erosion and thus change how waves and tides interact with the shoreline.

The laser data that is now being collected at Cardiff will be used to validate an improved dynamical model, one that incorporates the erosion-resistant properties of all those so-painful-to-walk-on cobbles.

An ultimate outcome from this effort will be an improved ability to forecast flooding over Highway 101 and the railroad tracks, especially during El Nino episodes, when sea levels can rise 20-plus centimeters and storms are typically more intense.