Scientists are zeroing in on being able to predict beach erosion based on what's happening at sea. But there are still mysteries to be solved.
(Photo, 2003 erosion at Ali'i Beach, Ka'anapali. Courtesy Sean Vitousek.)
Most beach erosion analysis is based on aerial photographs that show historical movements of sand. But new work is using physics to try to predict sand erosion and accretion based on what's happening in the weather, tides, oceanic eddies, seasons and other things.
Recent work at Ka'anapali Beach was discussed in a paper in Coastal Sediments '07, entitled “Model Scenarios of Shoreline Change at Kaanapali Beach, Maui, Hawaii: Season and Extreme Events.”
The authors are Sean Vitousek and Chip Fletcher of the University of Hawai'i's Department of Geology and Geophysics, Mark Merrifield of the UH Department of Oceanography, Geno Pawlak of the UH Department of Ocean Resource Engineering and Curt Storlazzi of the U.S. Geological Survey's Pacific Science Center.
Lead author Vitousek said the work is promising, but there are still significant areas for improvement.
“Modeling shoreline change is a very difficult proposition,” he said in an email.
The group studied the area from Black Rock to Hanakao'o Point at Ka'anapali in part because the area suffers extreme sand movement, or, in the words of the paper, “dynamic seasonal shoreline change forced by longshore transport from two dominant swell regimes.”
They noted that summer south swells push sand to the north end of the beach, while winter north swells push it south, and the sand settles on the reef offshore.
In the summer of 2003, the region suffered severe erosion, associated with multiple events at once: high tides, an ocean swell episode and the movement into Hawaiian waters of a massive oceanic eddy. The water in the eddy was six to eight inches higher than normal sea levels, and appears to have had a significant impact on the erosion.
Another feature the scientists studied was the fossil reef off Hanakao'o Beach. The roughness of the reef surface appears to be a factor in trapping outgoing sand, so that it remains available to rebuild the beaches when weather and oceanic conditions change. Without the roughness of the reef, that sand might be lost to the system.
The scientists applied complex computer models to the action of the ocean and sand, and found that they could predict reasonably well what was happening, but that there is still a significant amount of fine tuning needed it is to be accurate in every situation.
They concluded, in part, that oceanic eddies and the complex surface of a reef are important in sand movement.
“Mesoscale eddies and accretion on perched beaches atop rough reef substrates play a potentially significant role in beach morphology of Hawaiian shorelines, and merit continued investigation,” they wrote in the paper.
Vitousek, said, however, that there are still a number of factors that play some role in beach movement that haven't been applied.
“The modeling needs some work, but it should be applicable in the future with improved computer power and improved physics,” he said.
© 2007 Jan W. TenBruggencate
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