Magazine article Oceanus

Shaping the Beach, One Wave at a Time: New Research Is Deciphering How Currents, Waves, and Sands Change Our Shorelines

Magazine article Oceanus

Shaping the Beach, One Wave at a Time: New Research Is Deciphering How Currents, Waves, and Sands Change Our Shorelines

Article excerpt

For years, scientists who study the shoreline have wondered at the apparent fickleness of storms, which can devastate one part of a coastline, yet leave an adjacent part untouched. One beach may wash away, with houses tumbling into the sea, while a nearby beach weathers a storm without a scratch. How can this be?

The answers lie in the physics of the nearshore region--the stretch of sand, rock, and water between the dry land behind the beach and the beginning of deep water far from shore. To comprehend and predict how shorelines will change from day to day and year to year, we have to:

* decipher how waves evolve;

* determine where currents form and why;

* learn where sand comes from and where it goes;

* understand when conditions are right for a beach to erode or build up.

Understanding beaches and the adjacent nearshore ocean is critical because nearly half of the U.S. population lives within a day's drive of a coast. Shoreline recreation is also a significant part of the economy of many states. (See "Rising Sea Levels and Moving Shorelines," page 6.)

For more than a decade, I have been working with WHOI Senior Scientist Steve Elgar and colleagues across the country to decipher patterns and processes in this environment. Most of our work takes place in the breaking waves of the surf and swash zones: the region that begins where waves crest and ends where the foamy white water barely covers our feet.

Our goal is to understand and model waves, currents, and sand movement in the nearshore. Given weather conditions (winds, offshore waves), a map (islands, canyons, shoals, sandbars, the slope of the beach face), and sediment characteristics, we want to be able to model and predict how waves might change, and how those changes might affect currents and the erosion or accretion of sand on the beaches. To do this, we have to get into the water, making observations in the middle of the breaking surf.

If we are successful, we can help coastal policymakers and managers understand how the movement of water affects the evolution of coastlines, the safety of beachgoers, and the dispersal of runoff and pollutants.

What lies beneath

As storms and winds churn the ocean, waves roll across the continental shelf and into shallow water near the shore. They pitch forward and break, spraying foam and running up onto the beach. As the waves break, they drive currents that flow both offshore and along the coast.

Such is the view that most of us get when we stand on the shore. But what lies beneath the waves can make all the difference between 20-foot breakers and gently lapping rollers.

As waves move from deep water toward the shoreline, the ocean bottom alters their direction and strength, just as a lens bends and reflects rays of light. Features such as submarine ridges, canyons, and sandbars influence the propagation of waves, just as winds are directed and focused by mountains and valleys.

The breaking waves and resulting currents pick up and move san& making beaches dynamic, perpetually in motion.

This subtle but steadily flowing river of sand moves laterally up and down the shoreline, as well as offshore during storms and back toward land between storms.

Waves and currents affect this movement of sediment, but changes in sediment levels, in turn, affect the waves and currents. For example, sand eroded from the beach during winter storms may move offshore to form a sandbar. That causes waves to break farther offshore, protecting the beach from further erosion.

To avoid the complicated physics associated with along-coast changes in wave height and direction, most scientific studies of the nearshore have historically been conducted on smoothly sloping beaches with long, straight shorelines. The relatively simple shores of the Outer Banks of North Carolina have been a frequent focus for nearshore research. …

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