What are waves? toward the moon. An equal bulge in the surface of Tsunamis The strength of the wind the ocean occurs on the side of the earth opposite Water waves are a manifestation of energy moving of the moon due to the centrifugal force generated Often misnamed tidal waves, Tsunamis are long through the ocean. In their simplest form, waves are by the earth’s rotation. In between the 2 bulges the period waves that originate when a strong The amount of time the wind blows sinusoidal in shape. The high water levels are the ocean surface is depressed due to the movement of earthquake or landslide occurs under the ocean. wave crests and the low water levels are the wave water toward the bulges. The motion of the earth sends a strong impulse of The distance (called fetch) over which the troughs. The vertical distance between a crest and energy into the water generating surface waves wind blows in a straight line across the trough is the wave height H. The distance between with open ocean heights of less than 2 feet but ocean two crests or troughs is the wave length L and the with wave lengths of over 100 miles and wave time it take for two crests or troughs to pass the speeds approaching 500 miles per hour! same point is the wave period T.
The velocity, C, at which a wave moves through the ocean is related to the ratio of its wave length to wave period: C = L/T The wave length is proportional to the square of the wave period, L ~ T2 so longer waves move As the moon rotates around the earth, the bulges faster than move as well resulting in 2 high and 2 low tides per shorter ones. day at most locations on the coast with a tidal period The amount of of approximately 12 ½ hours. energy, E, in the wave is a The exact number of the tides is dependent on the The stronger the wind and the longer it blows function of the shape of the ocean bottom along the coast. This can across the fetch, the larger the sea will become. A wave height and have a significant effect on the height and period of As a tsunami approaches the coast, the wave fully developed sea occurs when the waves is proportional to: the tide along the coast as seen in the tidal curves interacts with the shallow bottom. Because of their reach the maximum size possible for a specific E ~ 1/8 H2. for the east coast. long wave lengths tsunamis rarely break along the wind, duration and fetch. coast. Instead the water runs up along the shore Some coastal geometries have the right size and similar to the tide. In some cases where the coast shape to produce amplified tides that result in very Swell is close to the center of the earthquake the water large tidal ranges. Some locations in the Bay of surge associated with a tsunami can reach What causes waves? Fundy, Nova Scotia experience a tidal range of over When waves travel out of the region of strong elevations over 100 feet high. Damage to winds, they become sorted as the longer waves 40 ft! lighthouses over 150 feet above sea level has A disturbing force is necessary to create waves on travel out from the storm faster than the shorter the ocean surface. The type of disturbing force been documented during tsunami events. waves. This results in waves of similar In shallow coastal regions, the currents generated determines the characteristics of the generated by the incoming tide can produce Tidal Bores, characteristics moving together across the ocean waves. The 3 main wave generation forces in order visible waves that move inland from the coast. Wind Generated Waves in groups. Waves lose very little energy as they of decreasing wave period are: propagate across the deep ocean. Removed from Wind blowing across the surface of the ocean the influence of the wind the wave crests of the transfers energy into the water. Initially, light winds swell become rounded. Gravitational Attraction of Moon and Sun generate small Ripples called capillary waves on producesTides the water surface.
Earthquakes and Underwater Landslides If the wind increases, the added roughness generate Tsunami created by the capillary waves increases the rate of energy transfer and waves begin to form on the Wind Stress over the ocean generates ocean surface. Wind Waves In the the region where wind is blowing across the ocean surface and waves are being generated, Tidal Waves the sea surface is characterized by steep waves with many different lengths moving randomly in Tides are generated by the gravitational attraction of the direction of the wind. This condition is referred the earth, moon and sun. On the side of the earth to as sea. How large the waves get is dependent closest to the moon, the ocean surface bulges out on3factors: Wave Shoaling and Refraction (Cartoons courtesy of Office of Naval Research)
When waves enter shallow water (water depths On beaches with equal to ½ the wave length) friction generated by gentle slopes, the the seabed begins to slow the motion of the waves. wave crest breaks If part of the wave is still in deep water, the slower first and the white portion of the wave will bend toward the coast. water spills down This process is called wave refraction. the face of the wave.
On very steep COASTAL PROCESSES beaches, the does not break at all but COOPERATIVE EXTENSION surges up the beach instead.
Current Wave Conditions
photo by Jon Sullivan For real-time information about ocean wave conditions see the following web pages: This factual brochure was prepared through SSuurrff’’ss UUpp!! the Stevens – New Jersey Sea Grant Coastal Processes Cooperative Extension formed to As the bottom of the wave slows, the waves crest promote the sustainability and wise use of our becomes more peaked and the distance between coastal resources that provide the basis of a wave crests shortens. This process is referred to strong coastal economy. as wave shoaling.
For additional information contact:
Dr. Thomas Herrington Coastal Processes Specialist, Stevens Institute of Technology Castle Point on Hudson, Hoboken, NJ 07030 PH (201) 216-5320 FX (201) 216-8214 email:[email protected] or Photo courtesy of the NOAA Marine Prediction http://attila.stevens- Merchant Vessel in fully developed sea. tech.edu/%7Etherring/SeaGrant.html Wave Breaking
When waves reach a critical wave height to length This publication was supported by the National Sea Grant ratio of 1/7, the wave will begin to break. In the surf College Program of the U.S. Department of Commerce’s photo by Jon Sullivan zone, this critical ratio generally occurs when the National Oceanic and Atmospheric Administration under NOAA Grant #NA16RG1047. The views expressed herein do wave height is ¾ of the water depth. There are 3 not necessarily reflect the views of any of these organizations. types of breakers that occur along the shore: NJSG-03-549. plunging, spilling and surging. The type of breaker depends on the slope of the seabed Compiled By: Dr. Thomas Herrington, Stevens Institute of Technology offshore of the beach. When waves go from deep Designed By: Jacqueline Harris, NJMSC water to shallow water quickly the bottom of the All About wave slows so quickly Photo courtesy of the Pacific Tsunami Museum that the crest of the wave plunges forward Surge associated with the 1946 tsunami that creating a tubing wave struck Hawaii. Notice height of white water along like those found in Waves tree trunks. Hawaii.