Waves

Understanding waves 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 durations. Fetch Wind Wave height Fetch Wind Wave height (Duration) speed m ft (Duration) speed m ft Fetch 10 (2-3 hrs) 10 kn 15 20 25 0.3 0.5 0.8 1.1 1.0 1.8 2.7 3.7 Fetch 30 (5-7 hours) 10 kn 15 20 25 0.5 0.9 1.3 1.7 1.5 2.8 4.2 5.5 Fetch 20 (4-5 1/2 hrs) 10 15 20 25 0.4 0.8 1.1 1.4 1.2 2.5 3.7 4.7 Fetch 40 (6-8 1/2 hours) 10 15 20 25 0.5 0.9 1.4 1.9 1.8 3.1 4.7 6.1 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.   Wave height and period as a function of wind and duration for unlimited fetch. 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 speed m ft sec m ft sec m ft sec m ft sec 10 kn 15 20 25 30 40 50 0.4 0.6 0.9 1.3 1.8 2.7 3.7 1.2 2.0 3.1 4.2 5.8 8.7 12.0 2.3 3.1 3.9 4.5 5.2 6.3 7.4 0.5 0.9 1.3 1.9 2.4 3.8 5.4 1.6 2.8 4.3 6.1 8.0 12.5 17.7 2.6 3.7 4.6 5.4 6.2 7.7 9.0 0.6 1.1 1.9 2.5 3.4 5.3 1.9 3.7 6.0 8.3 11.0 17.5 3.0 4.2 5.3 6.4 7.3 9.2 0.7 1.4 2.3 3.4 4.5 2.2 4.5 7.5 11.0 14.6 3.4 4.8 6.2 7.4 8.5

Swell waves 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.

Tides and tidal currents Tides are produced by the gravitational attraction of the moon (and to a lesser extent the sun). Tides move around the earth as it revolves each day and the height of the tide varies because the sun, moon and earth are in constant motion relative to each other. The tidal rise and fall in sea level in a partially enclosed area forces water to flow in and out of that area. Hence quite strong currents may be generated by the tides at entrances to inlets, bays, harbours, river mouths and around reefs. The predicted tide heights published in the Australian National Tide Tables assume average weather conditions. Hence if the weather conditions, especially wind and pressure, are unusual, the actual tides can be different. A wind blowing strongly onshore will pile up the water and cause the high tide to be higher than predicted, while winds blowing offshore will have the reverse effect. In addition, a difference in barometric pressure of 10 hPa from the average can cause a difference in tide height of 0.1 m. Low pressure will tend to raise sea level and high pressure will tend to depress it. Along the NSW coast there are normally four tides (two high and two low) each day. The tidal variation is moderate and fairly uniform; the difference between high and low tide is around 1 to 1.5 m, though the range can reach 2 m.

Ocean currents Ocean currents are large-scale movements of water in the oceans and result from a combination of the rotation of the earth, the distribution of land masses and the saltiness and temperature of the water.

The east Australian current The major current off the NSW coast is the east Australian current, which brings warm water from the Coral Sea into the cooler Tasman Sea. The surface of the Coral Sea is about 50 cm above that of the Tasman Sea (at the latitude of Eden) and the water flows down this slope, close to the coast at first. Often, near Sugarloaf Point, the current separates from the coast and forms slow-moving eddies of warm water, which rotate anticlockwise. These large eddies, which can be more than 300 km in diameter, wander along quite complex paths during lifetimes that can exceed a year. The current in the eddies can reach a speed of 4 knots. Between the eddies and the coast generally lies water which has upwelled from a depth of 200 m or more and can be 5°C cooler than the water of the eddies. Across this zone there is frequently a colour change, from a deep, clear blue in the east Australian current to a more cloudy, phyto-plankton-rich green in the upwelled waters. Between the eddies and the coast the direction and strength of the current depend on several factors and are highly variable. Sometimes small clockwise-rotating eddies form which cause 30-40 km sections of the continental shelf to experience north-ward-flowing currents. Such a situation was recognised in 1985 by the CSIRO's Division of Oceanography, from satellite infrared images, in time to advise the southbound participants in the Sydney to Hobart Yacht Race. Since 1983 the Naval Weather Centre, Nowra, has been producing weekly oceanographic charts of the sea-surface temperature in the Tasman Sea by pooling data collected by the navy, merchant shipping, satellite monitoring and research activities. These charts clearly show the location of the warm eddies and of strong changes, or 'gradients', in sea-surface temperatures. Meteorologists studying east coast lows believe that these regions of strong temperature gradient are important both for intensification and movement of these storm systems. In addition, the water temperature affects the distribution of many types of fish. It appears that tuna, for example, tend to gather in regions of strong temperature gradient. Fishermen find the sea-surface temperature data very useful and the CSIRO Division of Atmospheric Physics at Aspendale, Victoria, and Marine Laboratories in Hobart, both provide an operational commercial product. Further information on the east Australian current may be obtained from Dr George Creswell at the CSIRO Division of Oceanography, Marine Laboratories, GPO Box 1538, Hobart 7001.

The forecast description of sea state The Bureau of Meteorology forecasts the wave height of sea and swell in metres. The figure given is an average for deep water in the particular area covered by the forecast. Some local knowledge of how different wind directions and speeds affect the sea where you are heading is very important because of the large variability that can occur along the coastline. This variability is a result of many effects such as coastal topography, local winds, shapes of bays, sea bottom topography, and tides. It is not possible, therefore, to cater for all these variations in the coastal waters forecasts. Another difficulty is that where a strong wind passes over an opposing current, steep breaking seas can develop very quickly. When there is an east coast low, heavy rainfall over the land can cause the coastal rivers to flood. Hence the rough conditions generated by the strong easterly winds can be amplified near river estuaries by the strong opposing flow of the flooding river. Broken Bay, the estuary of the Hawkesbury River, can be especially dangerous. As sea and swell are independent, it is important to realise that even though the weather conditions may indicate light winds, with consequently smooth or slight seas, there may in fact be a moderate or heavy swell which has been generated further out over the ocean, often from a weather system no longer shown on the latest weather maps.

 

Observing the wind with the Beaufort scale The Beaufort scale is a simple scale that can be used to estimate wind speed accurately without the need for instruments. It is based on observations of the effects of wind on waves, trees and a small fishing boat. (The term 'smack' used in the Beaufort scale pertains to a small one-masted yacht with mainsail and jib.)

Beaufort Wind speed * At sea At sea Probable number kn km/h Description On land near the coast far from land wave height** 0 0 0 Calm Calm; smoke rises vertically. Calm Sea like a mirror. 0 m 1 1-3 1-5 Light air Wind direction shown by smoke-drift but not wind vanes. Fishing smack just has steerage way. Ripples with the appearance of scales are formed, but without foam crests. 0.1 m (0.1) 2 4-6 6-11 Light breeze Wind felt on face; leaves rustle; ordinary vanes moved by win Wind fills the sails of smacks which travel at about 1-2 mph. Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break. 0.2 m (0.3) 3 7-10 12-19 Gentle breeze Leaves, small twigs in constant motion; wind extends light flag. Smacks begin to careen, travel about 3-4 mph. Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses. 0.6 m (1) 4 11-16 20-28 Moderate breeze Raises dust and loose paper; small branches are moved. Good working breeze, smacks carry all canvas with good list. Small waves, becoming longer; fairly frequent white horses. 1 m (1.5) 5 17-21 29-38 Fresh breeze Small trees in leaf begin to sway, crested wavelets form on inland waters. Smacks shorten sail. Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray). 2 m (2.5) 6 22-27 39-49 Strong breeze Large branches in motion; whistling heard in telegraph wires; umbrellas hard to use. Smacks have double reef in mainsail; care required when fishing. Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray). 3 m (4) 7 28-33 50-61 Near gale Whole trees in motion; inconvenience felt when walking against the wind. Smacks remain in harbour and those at sea lie-to. Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. 4 m (5.5) 8 34-40 62-74 Gale Breaks twigs off trees; generally impedes progress All smacks make for harbour, if near. Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks along the direction of the wind. 5.5 m (7.5) 9 41-47 75-88 Strong gale Slight structural damage occurs (chimney-pots and slates removed). High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility. 7 m (10) 10 48-55 89-103 Storm Seldom experienced inland; trees uprooted; considerable structural damage occurs. Very high waves with long overhang crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; the surface of the sea takes a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected. 9 m (12.5) 11 56-63 104-117 Violent storm Very rarely experienced; accompanied by widespread damage. Exceptionally high waves (small and medium-sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected. 11.5m (16) 12 over 63 over 117 Hurricane The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected. 14 m * Units are not exact conversions because of established 'Number' conventions. ** Figures in brackets indicate the probable maximum height of waves in metres.