Winds blowing across the sea surface form a series of irregular hills and valleys. Light winds create small wavelets, but when strong winds blow, huge storm seas can develop. (See Beaufort Wind Force Scale)
Waves travel quickly across the surface of the sea, deceiving our senses into thinking the water is moving. In fact, the water itself does not move very rapidly—it is mainly traveling up and down in a circular, or rotary, motion. This is actually a fortunate illusion of nature, since water moving at the speed of most storm waves would make ocean navigation in ships nearly impossible.
How big the waves get depends on three things—the speed of the wind, the duration, or how long the wind blows, and the fetch, or the distance of open water over which the wind is blowing. Light winds, or those that only blow hard for a short time, cannot generate large waves. Ships seek out the safety of a harbor in a storm because they can block a portion of the wind with a body of land, thereby reducing the fetch of open water. Storms of equal size generate much larger waves in the open Pacific than in the Adriatic Sea where fetch is much more limited.
It takes a while after the wind begins to blow before large waves agitate the sea surface. As the wind continues to blow, the waves get higher from trough to crest, and both the wave length and period become longer. Wave length is measured from crest to crest, while the period of a wave is the time it takes for one full wave from crest to crest to pass the same point. Wave height is actually less important than the steepness of the seas and this is a critical point for sailors since steepness is a product of the height and the length, being the angle between crest and trough. Sailboats will glide over very high, long ocean rollers and barely notice them but when the wave length is shortened, the waves become steep.
It is never just the height or the length of the waves that matter, but the relationship between the two.
It is this wave gradient that creates dangerous conditions for small boats. Waves are generally steeper at the beginning of a storm and are at their worst near its center. As the wind continues or strengthens, the water first forms whitecaps and eventually the waves start to break. Waves as high as 30 meters (nearly 100 feet, or as high as a nine-story building) have been recorded during severe storms. This is referred to as a fully developed sea, and in one way this is good because it tends to limit the waves future growth. Good, that is, if you’re not out among them.
These waves radiate out great distances from the storm that generated them, eventually lengthening and becoming reduced in height. We call these swells, and mariners have known for centuries that an increasing swell means bad weather is coming. Beach lovers are often warned of high swells and surf on beautiful sunny days because of storms that may be far over the horizon.
The longer the wave, the faster it travels. As waves leave a storm area, they tend to sort themselves out with the long ones ahead of the short ones, and the energy is simultaneously spread out over an increasingly larger area. As the waves close in on the coast, they begin to feel the bottom and their direction of travel might change due to the contour of the land. Eventually, the waves run ashore, increasing in height up to 1.5 times their height in deep water, finally breaking up as surf.
A dramatic change occurs when the energy in a wave meets the land. In the area of the surf, the waves become steep and the water begins to move forward at the speed of the waves. Waves can also be amplified when traveling into an opposing current, forming dangerous riptides and undertows. These two effects together make the surf area a potentially dangerous place for both people and boats. Many cases of rogue waves actually have their origins in high swells from a distant storm moving over an area of reduced water depth, around a headland, or into an opposite current. Areas with these land features, like the Hourglass Shoals, between Hispaniola and Puerto Rico in the notoriously rough Mona Passage, have developed nasty reputations among sailors.
The most common explanation for rogue waves involves the chance meeting of two smaller waves from different wave trains. It is not uncommon to have a swell from one, or even several directions, at the same time as seas are being driven by a local wind. If a large local wave happens to coincide and be reinforced by an unusually large swell from a more distant area, a wave much bigger than those surrounding it occurs. Statistically, this is not uncommon if we take the number of waves on the ocean into account.
Rogue waves are believed to have been the cause of numerous marine accidents and fatalities. Sailors' logs show that these uncharacteristically large waves occur with no warning, especially along exposed coastlines. Annually, at least one person is swept off the rocks by rogue waves on the southwest coast of Vancouver Island when the seas are reasonably calm. In April 1991, a man was swept from the rocks and drowned by large waves at South Beach near Tofino, BC, during a time when there was no unusual wave activity.
These are obviously very unusual events; however, rogue or freak waves are regularly reported during all sorts of weather including calm days. The unusual confluence of wave trains usually last only several moments, but they are totally unpredictable. The fact that they sometimes come in groups probably contributes to the popular belief that every seventh wave is the highest. Research has shown that this is not the case, but that large waves come in groups of various lengths. In fact, groups of waves of random lengths would be more likely to cause one single freak wave larger than all its peers than if every seventh wave was the largest.
from Australian Bureau of Meterology
The effect of wind on water varies from the tiny ripples on a pond to the mighty rollers of the Southern Ocean. All ocean waves, other than those caused by movements of the sea floor and tidal effects, owe their origin to the generating action of the wind.
As waves move across the ocean, only the shape and energy of the wave moves forward; the water particles remain behind.
When you observe the sea surface you will, in general, notice a complicated pattern of crests and troughs, with waves of different shapes moving in different directions. There is considerable interaction between individual waves - faster moving waves overtake slower waves and they often combine to either reinforce or cancel each other. On occasion, when two or more crests interact, an abnormally high wave can develop (a king or rogue wave) which can be very dangerous.
Some terms to understand
Windwaves(localseas) are waves produced by the local prevailing wind.
Swell waves are waves that have moved well away from the area where they were generated, and have settled into a regular travelling pattern.
Wavelength (L) expressed in metres, is the horizontal distance between successive crests.
Wave period (T), expressed in seconds, is the time between successive crests.
Wave height (H), expressed in metres, is the vertical distance between the top of a crest and the bottom of a trough.
Significant wave height is the average height of the highest one-third of the waves. It is about equal to the average height of the waves as estimated by an experienced observer.
Wind duration is the time over which the wind has been blowing.
Wind fetch is the distance upstream from the point of observation over which the wind blows with constant speed and direction.
A simple wave of wavelength L, wave height H, and velocity V.
Wind waves (local seas)
Wind waves are produced by the local prevailing wind. They travel in the direction of the prevailing wind, i.e. a northerly wind will produce southerly moving waves.
The height of wind waves depends on :
- the strength of the wind
- the time the wind has been blowing
- the fetch.
The higher the wind speed, and the longer the duration and fetch, the higher the wave and the longer the period. Wind waves are steeper than swell waves, with shorter periods and wavelengths. The sea appears more confused than for swell waves alone.
The tables below show the significant wave height for various wind speeds, durations and fetches. For example, with a fetch of 40 nautical miles, a wind of 25 knots and a duration of about 6 hours, a significant wave height of 1.9 metres is expected. For longer fetches, a 40 knot wind blowing for 6 hours will give waves averaging 3.8 metres.
It is important to note that waves higher and lower than the average can occur. Generally, in open water, a wave of 1.86 times the significant wave height can be expected in every thousand waves. If the significant wave height is 3.8 metres, with a period of 7.7 seconds, then a wave of 7 metres can be expected every two hours or so.
Wave height as a function of wind speed and fetch distance (in nautical miles) for differing
Wave height and period as a function of wind and duration for unlimited fetch.
|Fetch||Wind||Wave height||Fetch||Wind||Wave height|
(4-5 1/2 hrs)
(6-8 1/2 hours)
|Note: A range of wind duration for wave height development is given. The lower the wind speed, the longer the duration required to develop the wave height. The longer duration applies to the lower wind speeds and the shorter duration applies to the higher wind speeds.|
Duration 3 hours
Duration 6 hours
Duration 12 hours
Duration 24 hours
|Wind||Wave height||Period||Wave height||Period||Wave height||Period||Wave height||Period|
Swell waves are wind-generated waves that have moved away from their area of formation. They may originate in the heavy seas created by a deep low pressure system offshore. As they move away, they become more rounded and regular in height and period and are often detected thousands of kilometres from their source area. As the swell travels, its height decreases and its period and wave length increase, because short waves have too little energy to enable them to travel long distances against the action of friction. Swell waves are long waves in comparison with the wind waves and may have wavelengths from 30 to 500 times their wave height.
The characteristics of swell waves depend on their size and shape at the outset and the distance travelled. These factors, however, are seldom able to be determined with any degree of confidence.
The most common swell direction along the NSW coast is southerly and these swells are produced by the low pressure systems which pass to the south of the continent. In summer and autumn, lows and tropical cyclones in the Coral Sea can generate large northeast swells.
Waves approaching the coast
Sea waves and swell approaching the coast are progressively modified by the decreasing water depth. They slow down, their direction of motion may change and their shape steepens. As water depth may vary along the wave, different sections of the wave may travel at different speeds. Waves with longer wavelengths (such as swell) sense the sea bottom first and slow down and steepen fur- ther from shore. Hence the cry from the surfer, 'out the back', when a bigger set of waves appears.
A line of waves approaching the shore at an angle will be slowed at the end closest to shore, and the line will wheel around towards the shallower water and become parallel to the shore before breaking. This is also why waves bend around headlands and travel into sheltered bays.
As a wave moves toward the shore, the depth of water becomes so shallow that the wave steepens until it collapses or breaks. The critical depth is about 1.3 times the wave height. Therefore a 1 metre wave will break at a water depth of about 1.3 metres. On a gradually shelving beach the bigger waves will break further out.
In contrast to an unbroken wave, where the water does not move with the wave, a broken wave is a moving turbulent wall of water. Its energy is dissipated by turbulence, frothing water up onto the beach.
For more about winds and waves see Beaufort Wind Force Scale.