Breaking Waves and Shifting Sands
The Science of Surf
Black's Beach, near San Diego, is renowned to surfers for its tasty waves. Blue-green waters swirl in a jumble of rip currents, whirling eddies, and powerful crossing swells. It is not a beach for novice surfer dudes and dudettes.
But less than three kilometers down the coast, toddlers, scuba divers, and sea kayakers frolic in the gentle waves of La Jolla Shores, where the surf seldom rises more than a half meter.
It's the same Pacific coastline, the same weather and wind patterns. So how can a surfer's paradise lie so close to a parent's dream beach? WHOI scientists Britt Raubenheimer, Steve Elgar, and two dozen colleagues from the Nearshore Canyon Experiment spent the autumn of 2003 trying to find out.
Steve Elgar, a senior scientist in the Applied Ocean Physics and Engineering (AOPE) Department, studies the surf zone, where the water covers his head and the waves start to rise and break toward the shore. Britt Raubenheimer, an associate scientist in AOPE, works in the swash zone, the part of the beach alternately covered and uncovered by the foamy white water that wets your ankles and wipes out sand castles.
They both work in the waves, with a knack for building experiments and collecting observations that few others have the patience or moxie to attempt. They share an interest in using their measurements to improve the mathematical models of coastal processes. And they love getting their feet wet in the surf. Their science is better off for the partnership.
A mess of physics
Nearly all theoretical models that explain the physics of the deep ocean break down and go haywire near the shore. The coast is a puzzle with a thousand moving pieces.
Even when the ocean looks still, everything is in motion. Waves shape beaches, and beaches shape the waves. Winds blow from ever-shifting directions. Waves come in at different periods and angles. Waters rush toward the beach and back out, while sloshing sideways too. Some waves are born in storms thousands of kilometers away; others are whipped up by nearby winds.
The slope of the beach, the shape and size of its sand grains, the position of offshore islands, canyons, shoals, and sandbars-even the level of the local groundwater table-all play a role in coastline dynamics. All told, the beach is a mess, a mathematical nightmare.
"Our goal is to understand and model waves, currents, and sand movement to better predict conditions along the shoreline," Elgar said. "We would love to reach a level of understanding where you could give us a map, some information about the sediments, tell us the direction and strength of the winds, and we could tell you what the waves will be like along the coast."
From a science perspective, understanding the nearshore region is critical because it is where the greatest quantity of sediment and suspended material moves through the ocean. Waves and currents deliver nourishing sediments to the beaches and nutrients to clams, crabs, and other shoreline organisms. Similarly, land-based sediments, chemicals, plants and animals, and fresh water must cross this region as they wash out to the deep ocean.
From a societal perspective, understanding the nearshore is vital to living wisely along the coast. "Studies of the nearshore will directly benefit beachside communities concerned with erosion and beach nourishment," said Tom Drake, program manager for the Coastal Geosciences Program at the Office of Naval Research. "They will give coastal planners an improved ability to predict how pollution from storm runoff moves through nearshore waters. And they will aid biologists attempting to trace the complex paths of larvae and other marine organisms in the surf zone."
The nearshore is growing in importance, as the U.S. coastal population is estimated to grow by 3,600 people per day. Nearly 53 percent of Americans live in coastal counties.
Yet the surf and swash zones have typically been neglected. "There's a lot of tedious, physical work to be done, and it is labor-intensive, which makes it tough to fund," Raubenheimer said. Scientists and equipment are constantly pounded by waves and sandblasted by strong, sediment- and debris-laden currents. "But the difficulty," she said, "is part of what makes it interesting."
For much of the past 15 years, Elgar and Raubenheimer and other surf zone researchers have focused their surf and swash experiments on the East Coast of the United States, where the gently sloping, broad continental shelf allows waves and sediments to behave somewhat predictably. When researchers can make the right measurements in the Atlantic-they have been developing and refining their tools for a generation-then the physics problems are at least manageable.
In the wake of several research campaigns at an Army Corps of Engineers facility in Duck, North Carolina, scientists including Elgar developed a nearshore processes model, a testable mathematical hypothesis of how the beach works. That model has been built, tested, and refined repeatedly with field observations from Duck and other facilities where the seafloor geometry is relatively simple and the variables are few.
But researchers have known all along that most coasts are not as simple as North Carolina. Many continental shelves have abrupt, irregular seafloors that cause large gradients in the waves and currents. Scripps Canyon and La Jolla Canyon in Southern California provide one of the more dramatic extremes, with wild changes in wave energy over distances of just a few hundred meters.
"Our present theories for how waves move over an irregular seafloor are best suited to smooth or gently rolling underwater hills," Drake said. Elgar, Raubenheimer, and longtime collaborator Robert Guza of the Scripps Institution of Oceanography (SIO) decided it was time to try modeling a more complex environment.
Situated just a few hundred meters off Torrey Pines State Beach near San Diego-and close to a research pier operated by SIO-the Scripps and La Jolla canyons are the poster children for unusual seafloor geometry. The two canyons start as one, several kilometers offshore, and cut as much as 300 meters into the seafloor. The chasm forks, with branches running perpendicular to the coast and coming as close as 100 meters to the shoreline.
"As waves pass over the canyons, the steep topography may act like a magnifying glass and concentrate ocean wave energy in hot spots, creating large waves," Raubenheimer said. The canyon acts like a circus mirror as well, distorting, reflecting and trapping ocean waves along the shore to make legendary rip currents.
But just how the canyon affects the direction and speed of nearshore currents is poorly understood. "The currents there are the most complex I've seen anywhere," said Guza.
Like surfers itching for a challenge, the trio took on the waves of Scripps Canyon from September through November 2003. In the Nearshore Canyon Experiment (NCEX), they collaborated with more than 20 scientists from 10 institutions to measure how the underwater topography of submarine canyons affects wave propagation and nearshore currents.
Attacking the shore
Before NCEX, little real-life data had been collected to support theories and models of how the surf and swash zones behave on complicated coasts. The focus of the 2003 experiment was to collect field measurements that could stretch and validate the numerical models.
In the oceanographic equivalent of the Normandy invasion, they attacked the shore from water, land, and air. They set up tripods and frames of instruments with sensors to measure wave heights, water speed and direction, and the movement of sediment on the bottom. They launched jellyfish-like drifters with global positioning system (GPS) sensors to follow the currents up, down, and along the coast. From airplanes and from cliffs above the beach, they collected video and radar images of the surf zone. They drove Waverunner personal watercraft headlong into the surf, with GPS receivers and bottom-finding sonar strapped on the back.
A crew of 25 people-many of them outstanding athletes-struggled to keep the equipment free of kelp and sand, not to mention keeping it upright. "It was the hardest place we've ever worked, and there were few easy days," Elgar said. "We've never run an experiment in a place where the circulation was so crazy."
Divers dropped off from boats 100 meters north of the equipment would often drift so fast that they could not reach the instruments. Other times they would walk or swim out from the beach into strong shoreward currents, get themselves into two or three meters of water, and then find there were no currents at all...or currents pulling out to sea.
The effort was worth it. "We gathered a spectacular data set," Elgar said after they achieved a 98 percent data return. Critical to their success were many practice runs and equipment trials leading up to the experiment. They also monitored the performance of their instruments in real time, connecting their equipment in the surf zone with their battery-powered computers on the beach via cables.
Awash in data
With the field experiment completed, Elgar and Raubenheimer have months of data processing and sorting to do. They are probably years from presenting conclusions to scientific colleagues. But they already have some tantalizing preliminary insights.
As predicted, the direction and strength of waves are definitely altered by the shape of the Scripps and La Jolla canyons, just as winds are focused and directed by mountains and valleys. In some places, the offshore ridges reflect waves in divergent directions. In other places, waves are focused, causing hotspots for erosion along the beach.
Breaking waves cause an increase in the mean water level, piling up water along the beach as you might pile up water at one end of a bathtub. They found evidence that changes in the water levels caused by changes in wave heights might be important for driving the wildly divergent surf along the coast.
"Breaking waves transfer their energy into the water column, and that momentum lifts the water up," Raubenheimer said. The funhouse bouncing and focusing of waves near La Jolla create extreme gradients between water levels-similar to high- and low-pressure systems in the atmosphere. "With all the waves breaking at different angles, the water column is lifted unevenly, which might be what is driving the strong currents."
So now it is time to go back to the computers and models, new observations in hand. "This data set comes from an environment completely different from anywhere the models have been tested," said Elgar. "The question now is: can our models work in a complicated situation? It is an iterative, evolutionary process, and it will take two to three years for the modelers to integrate these data and adjust. But ultimately, we have to let the observations guide us and figure out the important physics that is missing."