Ocean surface waves are surface waves that occur in the upper layer of the ocean. They usually result from wind or geologic effects and may travel thousands of miles before striking land. They range in size from small ripples to huge tsunamis. There is little actual forward motion of individual water particles in a wave, despite the large amount of energy and momentum it may carry forward.
Wave Formation
The great majority of large breakers one sees on an ocean beach result from distant winds. Three factors influence the formation of "wind waves":
Wind speed
Distance of open water that the wind has blown over; called fetch
Length of time the wind has blown over a given area.
All of these factors work together to determine the size and shape of ocean waves. The greater each of the variables, the larger the waves. Waves are measured by:
Height (from trough to crest)
Wavelength (from crest to crest)
Period (time interval between arrival of consecutive crests at a stationary point)
Waves in a given area typically have a range of sizes. For weather reporting and for scientific analysis of wind wave statistics, their size over a period of time is usually expressed as "significant wave height." This figure represents the average height of the highest one-third of the waves in a given time period (usually twelve hours) or in a specific wave or storm system. Given the variability of wave size, the largest individual waves are likely to be twice the reported significant wave height for a particular day or storm.
Types of Wind Waves
Three different types of wind waves develop over time:
Ripples, or capillary waves
Seas
Swells
Ripples appear on smooth water when the wind blows, but will die if the wind stops. The restoring force that allows them to propagate is surface tension. Seas are the larger-scale, often irregular motions that form under sustained winds. They tend to last much longer, even after the wind has died, and the restoring force that allows them to persist is gravity. As seas propagate away from their area of origin, they naturally separate according to their direction and wavelength. The regular wave motions formed in this way are known as swells.
Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave is too great, breaking is inevitable. A 1:24 slope may be a long, shallow swell found in deep waters. A 1:14 and higher slope is a wave that is too steep to remain coherent. Waves can also break if the wind grows strong enough to blow the crest off the base of the wave.
Spilling type of surfThree main types of breaking waves are identified by surfers or surf lifesavers. Their varying characteristics make them more or less suitable for surfing, and present different dangers.
Spilling, or rolling: these are the safest on which to surf; they can be found in relatively sheltered areas.
Plunging, or dumping: these break suddenly and can "dump" swimmers - pushing them to the bottom with great force. Strong winds can cause dumpers; they can also be found where there is a sudden rise in the sea floor.
Surging: these may never actually break as they approach the water's edge, as the water below them is very deep. These waves can knock swimmers over and drag them back into deeper water.
In the context of sediment transport on beaches, ocean surface waves can also be classified as either constructive or destructive:
Constructive waves tend to be low in height (less than 1 metre), and therefore low in energy. As they approach the beach, the wave front steepens only slowly, gently spilling on the beach surface. Swash rapidly loses volume and energy as water percolates through the beach material. This tends to give a weak backwash that has insufficient force to pull sediment off the beach or to impede swash from the next wave. As a consequence, material is slowly, but constantly, moved up the beach, leading to the formation of ridges (or berms).
Destructive waves are tall, toppling waves carrying a lot of energy. As they approach the beach, they rapidly steepen, and when breaking they plunge down and scour the beach. This creates a powerful backwash, as a significant amount of the energy of the wave has not dissipated during breaking and runup. The backwash inhibits the swash from the next wave. Very little material is moved up the beach, leaving the backwash to pull material away. Destructive waves are commonly associated with steeper beach profiles. The force of each wave may project some shingle well towards the rear of the beach where it forms a large ridge known as the storm beaches.
