WAVES

WAVES

8.2.2 Waves: Define;Wave, significant wave height, fetch, swellExplain;Importance of Wind force, duration, fetch in formation and growth of wavesThe method of estimating wave heights and wave periodsFactors affecting wave height and directionInformation available from wind roseThe method of estimating wave length of the wave

Influid dynamics,wind wavesor, more precisely, wind-generated wavesaresurface wavesthat occur on thefree surfaceofoceans,seas,lakes,rivers, andcanalsor even on smallpuddlesandponds. They usually result from thewindblowing over a vast enough stretch of fluid surface. Waves in the oceans can travel thousands of miles before reaching land. Wind waves range in size from smallripplesto huge waves over 30 m high.When directly generated and affected by local winds, a wind wave system is called awind sea. After the wind ceases to blow, wind waves are calledswells. More generally, a swell consists of wind-generated waves that are notor are hardlyaffected by the local wind at that time. They have been generated elsewhere or some time ago.Wind waves in the ocean are calledocean surface waves.

Tsunamisare a specific type of wave not caused by wind but by geological effects. In deep water, tsunamis are not visible because they are small in height and very long inwavelength. They may grow to devastating proportions at the coast due to reduced water depth.

Wave formation

The great majority of large breakers one observes on a beach result from distant winds. Five factors influence the formation of wind waves:Wind speedDistance of open water that the wind has blown over (called thefetch)Width of area affected by fetchTimedurationthe wind has blown over a given areaWater depth

All of these factors work together to determine the size of wind waves. The greater each of the variables, the larger the waves. Waves are characterized by:Wave height(fromtroughtocrest)Wave length(from crest to crest)Wave period(time interval between arrival of consecutive crests at a stationary point)Wave propagationdirection

Factors affecting wave height and directionSo in total, wave height is affected by: - Windspeed - Windduration - Fetch - distance of wind travel over open water - Depth of water / roughness of sea bed - Direction and speed of tide First, thewindspeedmust be blowing faster than the transfer of energy from wave crest to wave crest.The second factor is the amount of time the wind blows, or windduration.The third factor is thefetch,the uninterrupted distance over the sea for which the wind blows without a change in direction.The fetch could be limited by the surrounding coast of the mainland by ,say 10 miles to thousands of miles at sea.Fourth the depth, As waves enter shallow water their speed decreases, wavelength decreases, and height increases.Waves therefore tend to break in shallow water, for example over a bar at the entrance to a harbour.If the tide directionis against the wind, this will also increase wave height and decrease wavelength. Shallow estuaries and harbours will experience large waves in an strong onshore wind, particularly with a Spring ebb tide, and must be avoided in such winds.

Types of wind waves

Three different types of wind waves develop over time: Capillary waves, or ripples Seas Swells

Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops. Seas are the larger-scale, often irregular motions that form under sustained winds. These waves tend to last much longer, even after the wind has died. As waves propagate away from their area of origin, they naturally separate into groups of common direction and wavelength. The sets of waves formed in this way are known as swells.

Significant wave heightWaves in a given area typically have a range of heights. For weather reporting and for scientific analysis of wind wave statistics, their characteristic height over a period of time is usually expressed assignificant wave height. This figure represents anaverageheight of the highest one-third of the waves in a given time period (usually chosen somewhere in the range from 20 minutes to twelve hours), or in a specific wave or storm system. The significant wave height is also the value a "trained observer" (e.g. from a ship's crew) would estimate from visual observation of a sea state.

Maximum Wave Height Maximum wave height is defined as the largest of all crests to adjacent trough value in the record.It is possible that the maximum wave height is not the one causing the maximum crest height.The height of wind generated waves is a function of: fetch length, windspeed, wind duration and the depth of water.

STILL-WATER LINE - The level of the ocean if it were flat without any waves.CREST - The highest part of the wave above the still-water line.TROUGH - The lowest part of the wave below the still-water line.WAVE HEIGHT - The vertical distance between the crest and the trough.WAVE LENGHTH - The horizontal distance between each crest or each trough.WAVE PERIOD - The time it takes for two successive waves to pass a particular point. For example, it you are standing on a pier and start a stopwatch as the crest of a wave passes and then stop the stopwatch as the crest of the next wave passes, you have measured the wave period.WAVE FREQUENCY - The number of waves that pass a particular point in a given time period.AMPLITUDE - The amplitude is equal to one-half the wave height or the distance from either the crest or the trough to the