http://oceanusmag.whoi.edu/v43n1/raubenheimer.html Shaping the Beach, One Wave at a Time New research is deciphering how currents, waves, and sands change our shorelines By Britt Raubenheimer, Associate Scientist nearshore region—the stretch of sand, for a beach to erode or build up. Applied Ocean Physics & Engineering Dept. rock, and water between the dry land be- Understanding beaches and the adja- Woods Hole Oceanographic Institution hind the beach and the beginning of deep cent nearshore ocean is critical because or years, scientists who study the water far from shore. 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 coast. Shoreline ent fickleness of storms, 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 sea, while a nearby beach weathers a storm • 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 forces intersect and interact in the surf and swash 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. ���������� �������� Breaking Jayne Doucette, WHOI Graphic Services waves cause turbulence in the water column. 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 sediments. dertow. Variations in the strength flows “downhill” (green arrows) from high of the waves and undertow drive the to low-water areas. transport of sediment, 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 rivers and from shore Currents flow estuaries alongshore Sand is exchanged between the ocean and dunes Kimball Diego McDonald, University of California-San The coastal waters off Haleiwa, Hawaii (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 river swash zones: the region that begins where As storms and winds churn the ocean, of sand moves laterally up and down the waves crest and ends where the foamy waves roll across the continental shelf 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 foam storms. waves, currents, and sand movement in and running up onto the beach. As the Waves and currents affect this move- the nearshore. Given weather conditions waves break, they drive currents that flow ment of sediment, but changes in sedi- (winds, offshore waves), a map (islands, both offshore and along the coast. ment levels, in turn, affect the waves and canyons, shoals, 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 erosion. 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 submarine ridges, 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 shores of the Outer Banks of the evolution of coastlines, the safety of cused by mountains 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 Current 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 United States, 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 surf zone. 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 bar changes in wave energy over distances of tants deployed instruments to measure the Several hypotheses explain the diver- 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 Canyon Experiment derlying geology 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 radar 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.