- The Washington Times - Thursday, August 9, 2007

urfers need more than a surfboard and the ocean. Their ride is better if they know the physical properties of waves and what to look for in the weather.

Waves form in the ocean when wind blows across the surface of the water and transfers energy to the water, says James H. Duncan, professor of mechanical engineering at the University of Maryland’s Clark School of Engineering in College Park.

The surface of the water rises and falls because of the ocean’s restoring force of gravity, says Mr. Duncan, who holds a doctorate in geophysical fluid dynamics.

“A wave is not moving water,” says Jeffrey Hanson, research oceanographer for the U.S. Army Corps of Engineers’ Field Research Facility in Duck, N.C. “The water moves in an up-and-down motion, which is the primary motion.”

The water’s movement is difficult to see because the water is clear, but placing a piece of styrofoam on the water’s surface can help mark the movement, Mr. Duncan says. As the elevation of the water surface increases, the styrofoam can be seen moving forward, and as it decreases, it moves back, he says. The vertical and horizontal movements combine to form a circular motion, he says.

“The particles of water go in a circle when a wave goes by,” Mr. Duncan says.

Surfers ride wind-driven gravity waves, which are generated by the wind and propagate from the disturbance, says Mr. Hanson, who holds a doctorate in ocean-wave physics. He is an amateur surfer of two years.

This type of wave constitutes the majority of the ocean’s waves, Mr. Hanson says. Other types of waves include tsunamis, or seismic waves caused by an underwater earthquake or volcanic eruption, and much smaller waves that result from boats on the water, he says.

“Waves are disorganized as the wind transfers its energy to the ocean,” says Michael Forte, hydrographer with the Army Corps of Engineers. He is a member of Surfrider Foundation USA, a nonprofit environmental organization based in San Clemente, Calif., that protects oceans, waves and beaches. He has been surfing for 15 years.

Waves near a storm range in size and wavelength, which is measured from crest to crest or trough to trough, Mr. Hanson says. Wind speed and wind duration influence the size of the waves, along with wind fetch, or the distance over the ocean that the wind blows, and the speed of the storm’s movement across the ocean, he says.

“As the waves travel away from the generating region or storm, they sort themselves out,” he says.

The longer waves travel faster and move in front of the shorter waves, Mr. Hanson says. The waves become what are called “swells” when they leave the generating area and no longer receive their energy from the wind, he says.

“When they reach the distant shore, they’re quite uniform. They are well organized in these beautiful surfing waves,” Mr. Hanson says.

Swells are similar to icebergs, with most of their energy below the surface, says Kevin Wallis, forecast manager of surfline.com, a Web site in Huntington Beach, Calif., that provides surfing forecasts and reports.

“The energy of swells can extend hundreds of feet below the surface of the water,” says Mr. Wallis, who has been surfing for 14 years.

Waves increase in amplitude when they move into shallow water because their energy is packed into a shorter distance, Mr. Duncan says. The size increase results in a decrease in speed and wavelength, he says.

“If the amplitude gets too big, it rolls over. We say it breaks,” Mr. Hanson says.

Waves that reach shallow water “feel” the bottom, which exerts drag and slows them down, Mr. Hanson says.

“Very little energy is lost in their travel until they start to feel the bottom,” Mr. Forte says. “The bigger the wave, the sooner it’s going to feel the bottom, so a bigger wave is going to break in deeper water.”

Waves can break on the sand of the beach, called beach break, or from underwater structures in a reef break and points of land jutting out from the shoreline in a point break, says Harry Purkey, mid-Atlantic regional director for the Eastern Surfing Association, an amateur surfing association based in Virginia Beach. He has been surfing for 35 years.

Depending on skill level, surfers look for different breaking characteristics and wave heights (larger waves take more experience), says Bruce Richmond, a geologist for the U.S. Geological Survey’s Coastal and Marine Geology Program in Santa Cruz, Calif. He holds a doctorate in earth science.

Experienced surfers favor peeling waves, which form when incoming waves hit a variation in topography, such as a rock or coral reef or a sandbar, Mr. Richmond says. Peeling waves break at one point and peel along slowly rather than breaking all at once, he says.

“It allows you to travel along the face of the wave,” says Mr. Richmond, who has been surfing more than 40 years.

Surfers also like plunging breakers, which plunge forward and create a tube or pipeline of air inside the wave, and spilling breakers, in which water spills or cascades down the front of the wave, Mr. Hanson says. They avoid surging breakers, which surge up a steeply sloping beach, then run back down, he says.

Surfers look for the condition of the wave, which they prefer to be smooth with little chop, and weather conditions of no wind or a light offshore wind blowing from land to sea, Mr. Richmond says.

The light winds groom the surface of the waves and shape them, holding them up until the last second before throwing them over, Mr. Forte says. Waves on a windy day break sooner and crumble, he says.

“When winds blow offshore, they make the waves break later and hold the face of the waves up longer,” Mr. Purkey says. “It will make them steeper and faster.”

Surfers paddle into a wave and push up onto their boards into a standing position to ride it downhill, turning sideways to stay in the same position in the wave, Mr. Duncan says. They find the angle where their downhill speed is the same as that of the wave, he says.

“Gravity pushes you down the front face of the wave,” he says. “If you just went straight, you might go faster than the wave and end up in the trough, and the ride would be over.”

Riding a wave is unlike skiing or biking downhill, Mr. Purkey says.

“You’re riding this thing nature created with no artificial propulsion,” he says. “That hill doesn’t move, and it’s the same hill every time. No two waves are alike.”

LOAD COMMENTS ()

 

Click to Read More

Click to Hide