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http://oceanusmag.whoi.edu/v43n1/raubenheimer.html

Shaping the , One at a Time New research is deciphering how currents, , and change our shorelines

By Britt Raubenheimer, Associate nearshore —the stretch of , for a beach to erode or build up. Applied & Engineering Dept. , and between the dry be- Understanding and the adja- Oceanographic Institution hind the beach and the beginning of deep cent nearshore ocean is critical because or years, who study the water far from . To comprehend and nearly half of the U.S. population lives Fshoreline have wondered at the appar- predict how shorelines will change from within a day’s drive of a . Shoreline ent fickleness of , which can dev- day to day and year to year, we have to: recreation is also a significant part of the astate one part of a coastline, yet leave an • decipher how waves evolve; economy of many states. adjacent part untouched. One beach may • determine where currents will form For more than a decade, I have been wash away, with houses tumbling into the and why; working with WHOI Senior Scientist Steve , while a nearby beach weathers a • learn where sand comes from and Elgar and colleagues across the coun- without a scratch. How can this be? where it goes; try to decipher patterns and processes in The answers lie in the physics of the • understand when conditions are right this environment. Most of our work takes

A Mess of Physics Near the Shore Many intersect and interact in the surf and zones of the coastal ocean, pushing sand and water up, down, and along the coast. Variations in the height and direction of waves, as well as the shape of the seafl oor, drives currents that feed back and rearrange the system.

High water level When waves approach the beach Low at an angle, they drive along- water level shore currents (blue arrows).

High water level ����� When alongshore currents converge, rip ����������� currents can form.

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Breaking Jayne Doucette, WHOI Graphic Services waves cause turbulence in the . In shallow water, this Breaking Different wave heights lead to different turbulence can stir up the sand bed, sus- waves create un- water levels in the nearshore. That drives pending . dertow. Variations in the strength flows “downhill” (green arrows) from high of the waves and drive the to low-water areas. transport of , causing sandbars to move offshore or onshore.

1 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu Sand ripples migrate toward the shore Undertow currents flow offshore

Sand moves in Sandbars and ripples and out from move toward or away and from shore Currents flow alongshore

Sand is exchanged between the ocean and Kimball Diego McDonald, University of California-San The coastal off Haleiwa, (near Honolulu) swirl with currents and suspended sediments. Waves approach the beach in different directions, driving alongshore currents of different strengths. As waves break, they generate turbulence that suspends sand and drives undertows.

place in the breaking waves of the surf and What lies beneath This subtle but steadily flowing swash zones: the region that begins where As storms and churn the ocean, of sand moves laterally up and down the waves crest and ends where the foamy waves roll across the and shoreline, as well as offshore during white water barely covers our feet. into shallow water near the shore. They storms and back toward land between Our goal is to understand and model pitch forward and break, spraying storms. waves, currents, and sand movement in and running up onto the beach. As the Waves and currents affect this move- the nearshore. Given conditions waves break, they drive currents that flow ment of sediment, but changes in sedi- (winds, offshore waves), a map (, both offshore and along the coast. ment levels, in turn, affect the waves and , , sandbars, the slope of the Such is the surficial view that most of currents. For example, sand eroded from beach face), and sediment characteristics, us get when we stand on the shore. But the beach during winter storms may move we want to be able to model and predict what lies beneath the waves can make all offshore to form a sandbar. That causes how waves might change, and how those the difference between 20-foot breakers waves to break farther offshore, protecting changes might affect currents and the ero- and gently lapping rollers. the beach from further . sion or accretion of sand on the beaches. As waves move from deep water to- To avoid the complicated physics as- To do this, we have to get into the water, ward the shoreline, the ocean bottom sociated with along-coast changes in wave making observations in the middle of the alters their direction and strength, just as height and direction, most scientific stud- breaking surf. a lens bends and reflects rays of light. Fea- ies of the nearshore have historically been If we are successful, we can help coastal tures such as , canyons, conducted on smoothly sloping beaches policymakers and managers under- and sandbars influence the propagation of with long, straight shorelines. The rela- stand how the movement of water affects waves, just as winds are directed and fo- tively simple of the of the evolution of coastlines, the safety of cused by and valleys. North Carolina have been a frequent focus beachgoers, and the dispersal of runoff The breaking waves and resulting for nearshore research. It was assumed and pollutants. currents pick up and move sand, making that waves, currents, and sand levels were beaches dynamic, perpetually in motion. uniform up and down these beaches.

2 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu However, recent studies by Jeff List of feedback between sandbars and coastal form. Rip currents are a danger to the U.S. Geological Survey have shown water levels is important during storms. swimmers and have been observed that even on these long, straight coast- to carry huge plumes of sand and pollut- lines, one section of beach may recede events in the surf ants offshore. shoreward by tens of yards during a storm, In contrast to much of the East Coast Scientists from WHOI and 10 other in- while a few miles down the coast the of the , many continental stitutions recently conducted a major field beach may be unaltered. Most of the erod- shelves have abrupt, irregular changes in program in those complicated Southern ed sand eventually returns after the storm, the seafloor that cause large changes in the California waters to determine how abrupt but that is no consolation to the owners waves beyond the . coastal-seafloor topography affects waves, of homes and structures destroyed by the For instance, the steep topography of currents, and changes to sand levels. The shifting shoreline. Scripps and La Jolla submarine canyons team of more than three dozen scientists, in Southern California produces dramatic engineers, students, and research assis- Stepping up to the changes in wave energy over distances of tants deployed instruments to measure the Several hypotheses explain - a few hundred meters. As waves pass over effect of the canyons on waves, and the re- gent erosion rates along the same coast. the canyons, the seafloor acts like a mag- sulting changes in the flows onshore. Perhaps there are differences in the un- nifying glass, concentrating ocean wave In the Nearshore Experiment derlying of the region or in the energy into hot spots. This makes Black’s (NCEX), sponsored by the Office of Naval flow of groundwater to the ocean. Maybe Beach a world-famous surf spot, where- Research and the National Science Foun- something as subtle as the size of sand as La Jolla Shores (just two miles to the dation, my colleagues and I deployed pres- grains makes a difference. Or perhaps south) is well known to novice sea kayak- sure gauges and current meters in the surf these seemingly “simple” and similar sea- ers and scuba divers for its gentle waves. and swash zones to measure wave heights floors produce more changes in offshore These changes in wave heights along and directions, and the resulting move- waves than we think. the coast result in complex flows of water ment of water and sand. “Drifters” de- My research group is intrigued by a dif- and changes to sand levels on the beach. signed to operate in the breaking waves of ferent theory: the location of underwater Water that piles up on the shore near the the surf zone were used to determine the sandbars in the surf may cause variable large breaking waves at Black’s Beach locations and speeds of rip currents. Beach erosion along the coast. Sandbars appear tends to flow south toward La Jolla surveys were conducted frequently to see to protect beaches by causing increased Shores and north toward Del Mar. how the seafloor and sand levels evolved breaking and dissipation of wave energy When these currents intersect with oppos- under changing surf conditions. before the waves can attack the shoreline. ing currents—perhaps between the heads Other scientists bounced sig- To test the sandbar hypothesis we used of the two canyons—strong offshore-di- nals off the water surface to determine its observations of waves and mean water rected flows, called rip currents, can speed, just as police use radar guns levels collected at the Army Corps of En- gineers’ Field Research Facility in Duck, N.C. We found that sandbars affect the coastal water levels and flooding dur- ing storms. When a sandbar is near the beach, waves break in shallow water and drive more water onto the shore. This causes flooding and allows the surf to reach dunes and manmade structures. We believe that shallow sandbars may lead to increased erosion. Sandbar locations can be variable along straight , which may explain Jeff List’s findings. And recent aerial observa-

tions collected by Tom Lippmann of Ohio Corps of Engineers Army U.S. Herman C. Miller, State University show that the numbers The ocean surges onto the streets of South Nags Head, N.C., wiping out dunes and damaging and locations of sandbars influence a houses during the Halloween “perfect storm” of 1991. Ocean waters wash over barrier islands storm’s effect on a beach. But further re- during storms because winds push water against the shore, low atmospheric allows search is needed to determine whether the the water level to bulge upward, and breaking waves water toward the shore.

3 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu Wave energies directly onshore of the canyons are very low.

An extremely rapid drop-off at Scripps Canyon changes the direction of waves arriving from the northwest, redirecting them (curved large arrow) toward Black’s Beach. , Scripps Institution of The exceptionally high waves at Black’s occur

because wave energy also arrives directly Explorations from the northwest (straight arrows). Paul Horn, Courtesy of Paul The bottom topography of the Scripps and La Jolla submarine canyons steers and focuses wave energy near . Red dots show the placement of wave-measuring instruments from the Nearshore Canyon Experiment.

to track moving cars. Video cameras Oceanography. The new NCEX measure- predict whether it would be sunny or rainy chronicled as it was carried along ments will improve the prediction models, . Now we can pre- the coast by the complex currents. helping not only those who play at the dict the weather as much as 10 days in ad- We have only begun to analyze our - beach for the day, but also civic leaders vance. By the middle of the 21st century, servations, but eventually we will be able who must manage our beaches and coasts we ought to be able to predict the weather to update and improve numerical models for the long term. at the beach…both above and below the of the important physical processes, and to One hundred years ago, we could not water line. test the model predictions with real-world measurements. Britt Raubenheimer is an Associate Scientist in the Depart- ment of Applied Ocean Physics and Engineering at Woods Hole Beach forecasts Oceanographic Institution. She decided to study physics during Our long-term goal is to predict coastal the ninth , when she had a choice between taking either physics or (which included cutting up cute little frogs). wave heights and directions, nearshore She fell in love with research while attending Middlebury College, currents, and changes to beach sand lev- as she worked with astronomers at an in the Canary els. We may not achieve this goal until we Islands to collect observations of a supernova remnant. Rauben- can study nearshore processes on a wide heimer spent the vacations of her youth backpacking, handglid- ing, rock climbing, canoe camping, and backcountry skiing, so she range of coasts (for instance, - knew she wanted to take her physics skills outdoors. She became lines, , and large bays), but it is interested in nearshore oceanography while studying coastal achievable within our lifetime. during her first job, at the U.S. Geological Survey’s of- fice in Saint Petersburg, . She completed her training with Surfers, sport-fishermen, and recre- a doctorate in oceanography from the Scripps Institution of Oceanography. Now her job requires her ational boaters frequently use predic- to go to the beach and to scuba dive to deploy instruments. Raubenheimer recently received a Young tions of wave heights along the coast of Investigator Award from the Office of Naval Research and a Career Award from the National Science Foundation. As part of the latter award, she developed a program offering six-month undergraduate Southern California, as generated by our fellowships to expose students to scientific research. colleagues from the Scripps Institution of

4 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu