nwaves

Waves are of practical significance to us...they may swamp a small boat, smash supertankers, damage offshore structures, force commercial vessels to slow their speed, damage shore structures, make students skip school when they want to surf and determine what adaptations organisms along the shoreline must have in order to exist there.

Exposure to waves, tides, currents and pressure presents significant obstacles to the survival of animal and plants living in the ocean. Waves pound the shore, tides inundate and expose marine organisms severally restricting the distribution of seashore life.

Waves are mechanical energy that has been transferred from wind, earthquakes, landslides, or other waves to the ocean water.

Most of the transfer is by wind and waves travel outward from the energy source. As more energy is supplied, waves become larger.

Particles in the ocean are set into an elliptical motion as wind energy acts on water. The energy of the particles move (is transferred) through the ocean, not the particles. Their movement makes the waves shape.

The highest part of the wave is the crest, lowest is the trough, the distance between successive crests or troughs is the wave length, and the vertical height-from the top of the crest to bottom of trough is the wave height, the time between successive crests/troughs passing a fixed point is the period of a wave. Sharp peaks are called seas and as waves move out of their area, the crests become rounded forming a swell, a long, low wave that can travel thousands of miles.

As the wave approaches the shallow water, it changes shape. -the wave length decreases. -the height increases as particles encounter resistance from the bottom. -pathway of the particles become more elliptical as it gets closer to the coastline. -bottom resistance slows the waves. -shortens wave length when depth is 1/2 wavelength. When depth decreases less than 1/2 wavelength (or 1.3x height) the frictional drag along the bottom and forward motion of the wave and steepness of the crest causes the wave to break or collapse against the shore. Stored energy is released as the water falls against the shore.

Marine organisms along the ocean are affected by wave actions. Sandy and rocky shores, exposed to the direct assault of strong waves are known as a high energy environment, as opposed to beaches in protected estuaries, bays and lagoons which is a low energy environment.

Winter usually has higher crests and shorter wave lengths than summer thus release more energy on the shore.

Tsunami- a wave caused by undersea earthquakes near island arcs and trenches that make enormous waves. Energy is transferred to water as the coastal plates shift. Travel through the sea at 100+ mph with a wave length of 100miles when reaching the coast, wave lengths shorten and heights can increase to 100'. Krakatoa 1883 the wave moved around the world.

To create a wave, a generating force is required. This is a result of a pulse of energy which produces waves (throw a stone etc.) The waves produced by the generating force moves away from the point of disturbance.

When the rock hits the surface, it disturbs and displaces the water surface. As the rock sinks, the displaced water flows back into the space from all sides and its momentum forces it upward resulting in a higher surface. The elevated water falls back causing a depression below the surface, which is filled and in turn sets up a series of waves that move outward and away from the point of disturbance until they are dissipated through friction among the water molecules.

The force that causes water to return to the level of undisturbed surface is the restoring force. This has to do with the surface tension (elastic quality of the surface due to the cohesive behavior of the water molecules.) This affects smaller waves, but larger waves are pulled back by the force of gravity and are called gravity waves.

As the wind blows across a smooth water surface, the friction or drag between air and water tends to stretch the surface resulting in wrinkles. Surface tension acts to restore a smooth surface. The wind and surface tension create small waves called ripples or capillary waves. You can see these as wind moves over a smooth surface of a pond or lake, and are called cat's paws as they move across the surface keeping pace with the wind.

As the wind blows, energy is transferred to the water over large areas, for varying lengths of time, and at different intensities. As waves form, the surface becomes rougher, and its easier for the wind to grip the roughened water surface and add energy. There is an increased frictional drag between the air and the water. As the wind energy is increased, the oscillations of the water surface becomes larger and the restoring force changes from surface tension to gravity. A wave is a result of the interaction between a generating force and a restoring force. Generating forces include any occurrence that adds energy to the sea surface: wind, landslide, sea-bottom faulting or slipping, moving ships, and even thrown objects.

The portion of the wave that is elevated above the undisturbed sea surface is the crest. The portion that is depressed below the surface is the trough. The distance between two successive crests or two successive troughs is the length of the wave or wavelength, and the height of the wave is the vertical distance from the top of the crest to the bottom of the trough. The amplitude is equal to 1/2 the wave height or the distance from the crest or trough to the still water or equilibrium surface. The period is the time required for two successive crests (or troughs) to pass a point in space.

While the dimensions and characteristics of waves vary greatly, the regularity in the rise and fall of the water's surface and the relationship between wavelength and wave periods allow math approximations to be made giving more insight to the behavior and properties of waves.

The particles of water get set into motion when a wave passes across the water surface. The ocean wave does not represent a flow of water but a flow of motion or energy from its origin to its eventual dissipation at sea or loss against land.

As a wave crest approaches, the surface water particles rise and move forward. Immediately under the crest the particles have stopped rising and are moving forward at the speed of the crest. When the crest passes, the particles begin to fall and to slow their forward motion. It reaches a maximum falling speed and zero forward speed when the midpoint between crest and trough passes. As the trough advances, particles slow in falling rate and start to move backward until at the bottom of the trough they reach the maximum backward speed and neither rise or fall. As the remainder of the trough passes, the water particles begin to slow their backward speed and start to rise again, until the mid-point between the crest and trough passes.

Now they start their forward motion and continue to rise with the advancing crest. The motion creates a circular path or orbit for the water particles. This is the motion that causes a boat to bob!

The surface water particles trace an orbit whose diameter is equal to the wave height. The same motion is transferred to the water particles below but less energy of motion is found at each succeeding depth. The diameter of the orbits decrease and become smaller and smaller as the depth increases. At a depth of 1/2 the wavelength, the orbital motion has decreased to almost zero.

All this is based on sine waves and while in the ocean, there is a bit of difference because due to real waves, the crests are sharper than the troughs so there is minimal transport of water in the direction of the waves, the motion is often ignored when studying waves.

It is possible to relate the wavelength and period of the wave in order to determine the wave's speed. The speed of the wave (C) is equal to the length of the wave (L) divided by the period (T):

While the period of the wave is not hard to measure at sea, the wavelength is because of no fixed reference point. So the oceanographer determines the period (T) and calculates the wavelength (L) by another equation.

In deep water, the wavelength is equal to the acceleration due to gravity (g) divided by 2p times the square of the wave period (T). The value of the earth's gravity (g) is 9.81 m/sec²

Deep water waves must occur in water that is deeper than 1/2 the wave's length so in order for the waves to behave like the description below, it must be a deep water wave.

Most waves at sea are progressive wind waves. They are build up by the wind, restored by gravity and travel in a particular direction. These waves are formed in local active storm centers or by steady winds of the trade wind and westerly wind belts. An active storm may be large, with unsteady winds and varying directions and strength. The winds in the storm flow in a circular pattern around the low-pressure storm center creating waves that move outward and away from the storm in all directions. In the storm center, the sea surface is jumbled with waves of all heights, lengths and periods. There are no regular patterns. Sailors call this a sea. As waves are being generated, they are forced to get larger by the input of energy forced waves. Due to variations in the winds of the storm area, energy at different intensities is transferred to the sea surface at different rates, resulting in waves with a variety of periods and heights. Once a wave is created with its period, the period doesn't change. The speed may change but the period remains the same. The period is a constant property of the wave until the wave is lost by breaking at sea, through friction, or crashing against the shore.

Once energy or generating forces no longer effect the waves, the forced waves become free waves moving at speeds due to their periods and wavelengths. Some waves produced have long wavelengths and long periods and have a greater speed than those with short. They gradually move through and ahead of the slower ones and escape the storm and appear as a regular pattern of undulating crests and troughs moving across the sea surface. Once away from the storm these waves are called swell. They carry considerable energy which they lose very slowly..

The movement of the faster through and ahead of the slower waves is called sorting or dispersion. Groups of these faster waves move as wave trains or packets of similar waves with about the same period and speed (sets).

Waves that escape, or outrun a storm are no longer receiving energy from the storm winds and tend to flatten out slightly and the crests become more rounded. As they move out across the ocean, they are likely to meet other trains of swell moving out and away from other storm centers. When two wave trains meet, they pass through each other and continue on. Wave trains may intersect at any angle, and many possible sea surface patterns may result. If 2 trains intersect sharply a checkerboard pattern will be formed, and in some cases two or more trains may phase together so they suddenly develop large waves unrelated to any storms that may become so high they break losing some of their energy.

While table 8.1 shows typical wind waves in deep water and their sizes compared to the wave length etc., there are several factors that influence the height of a wave.

Any one of these factors can limit the wave height. If the wind speed is low, it doesn't matter how far and how long the wind blows over the water, no large waves will be produced. If the wind speed is great but short, again, no large waves will be formed and strong wind over a short area will also not produce large waves. When no single limiting factor is present, large waves can form at sea. (40-50' S).

A typical fetch for a local storm is about 500 miles and with the storm moving, and the storm winds circulation around the low pressure area, the winds can continue to follow the waves on the side of the storm which increases fetch and duration of time over which the wind can add energy to the waves. Waves up to 49feet are not uncommon and the wave lengths can be between 330-600feet.

Wave heights taken in the North Atlantic over the past 25 years show a continuing increase in wave height of about 25%. There is no known reason for this increase.

Giant waves over 100' are rare but a Navy Tanker (USS Ramapo)in 1933 encountered a Typhoon and riding on the downside to ease the ride, was overtaken by waves that when measured against the ships superstructure by the officer on watch were 112'high. The period was 14.8 seconds and the wave speed was 90'/sec (60 mph). While conditions to produce waves of this size occur, none have been well documented (or have survived).

Large waves that suddenly appear at sea unrelated to local conditions are called epsodic waves. It occurs due to the combination of intersecting wave trains, depths and currents. Not much is known and when they do occur and swamp ships, witnesses are often removed. They occur near the continental shelf in water about 600' deep and in some areas with prevailing wind, wave and current patterns. (Agulhas Current) p220

Waves carry considerable energy per each unit width of their crest. The unit is measured in meters or unit width of 1 meter. The wave represents a flow of energy and its present as potential energy due to the change in elevation of the surface water, and kinetic energy due to the motion of the water particles in their orbits. The higher the wave, the larger the diameter of the water particle orbit and the greater the speed of the orbiting particle so the greater the potential and kinetic energy. The wave energy is calculated in a fun math formula and represented in fig. 8.8 basically showing that it increases greatly.

Energy average over one wavelength per unit width of crest from the sea surface to a depth of L/2 and related to the square of the wave height.

There is a maximum height for any given wavelength. This value is determined by the ratio of the wave's height to the wave's length and is the measure of steepness of the wave:

If the height to length ratio exceeds 1:7, the wave is too steep, the crest angle will be sharper than 120' and the wave is unstable and will break. A wave length of 70m will cause a wave to break when it exceeds 10m (1:7).

Whitecaps have very short wavelengths(about 1m) and break because the wind increases their height rapidly...quicker than the wavelength increases. Also when wave trains pass through each other, he quick increase in height can cause these waves to break (even in the middle of the ocean).

Waves sometimes run into a strong opposing current which forces the waves to slow down. Remember the speed =L/T and once a wave is formed, the period doesn't change. so its wavelength will shorten if its speed is reduced so the wave will increase in height to satisfy the direct height-energy relationship. If the increased height exceeds the max. the wave breaks. (entering harbors etc during outgoing tides).

When the deep water wave begins to approach the shore and shallow water, the reduced depth begins to affect the shape of the orbits. They become flattened circles or ellipses and the wave begins to "feel" the bottom and the resulting friction and compression of the orbits reduce forward speed of the waves. Remember the speed of the wave is equal to the wavelength divided by the period and 2. the period never changes so when the wave slows because it feels the bottom, there is a reduction in wavelength resulting in an increase in wave height and steepness.. When the wave enters water with a depth of less than 1/20 the wavelength the wave becomes a shallow-water wave. The speed is now determined by the square root of the product of the acceleration due to gravity (g) and depth(D): c=`/gD

When the water depth is between L/2 and L/20, the speed of the wave is also slowed. Waves in this depth range are called intermediate waves. (no simple way to determine speed).

Waves are refracted or bent as they move from deep top shallow water and begin to feel the bottom. Waves usually approach the shore at an angle and when one end of the crest comes in and feels the bottom and the other end is still in deeper water, the shallow water end slows and because the deep water part is still traveling the same speed, the wave crests bend, or refract, and tend to become orientated parallel to the shore.

When waves approach an irregular coastline, certain refraction patterns occur. Waves will slow over submerged ridges and speed over submerged canyons.

A steep, vertical barrier in water deep enough to prevent waves from breaking will reflect the waves.. The barrier may be a cliff, steep beach, breakwater, bulkhead or other structure. Waves approaching this barrier at an angle reflect from the barrier at the same angle. These pass through incoming waves to produce an interference pattern and often steep, choppy seas often result.

When a wave passes its energy though a narrow opening, some wave energy will pass through to the other side and once through the energy radiates out and away from the gap. A portion of the energy is transported sideways from the original wave direction it is diffracted. Energy can be transported sideways and around the end of a barrier and energy can be transported behind the barrier but much is lost.

The surf zone is the area along the coast in which the waves slow, steepen, break, and disappear in the turbulence and spray of expended energy. The width of the zone is related to the length of the arriving waves and changing depth pattern.

Breakers are formed in the surf zone because the water particle motion at-depth is affected by the bottom, slowed down, and compressed vertically. The orbit speed of the particles near the crest are not slowed too much so particles move faster toward the shore than the wave itself. The crest can curl and eventually break (fall over). There are two types, plungers and spillers.

Plunging breakers are usually found on a steep beach, the curling crest outruns the rest of the wave curves over the air below it and breaks with a sudden loss of energy and a splash. Spilling waves occur at flatter beaches and consists of turbulent water and bubbles flowing down the collapsing wave face.

Waves transport water toward the beach in the surf zone. There is a drift of water in the direction the waves are traveling and is intensified in the surf zone and with the waves approaching the beach at an angle, the transport of water moves both toward and along the beach. This water must flow seaward again and will in a quieter zone with smaller waves. Because these regions may be some distance apart, and narrow, the water may flow out quickly forming a rip current.

Earthquakes are often responsible for producing seismic sea waves or tsunamis. They are called tidal waves (incorrectly) and formed if in an area the earths crust suddenly raised or lowered. The displacement causes a sudden rise or drop of the sea level and gravity causes the water to quickly fill it in. Waves with long wave lengths are produced (100-200km) and periods of 10-20 min. Since the aveg. depth is 4000m it is less than 1/20 the wavelength so it acts like a shallow water wave. Because of this they may be refracted, diffracted, or reflected in mid-ocean.. p230

Periods of excessive high water due to changes in the atmospheric pressure and the wind's action on the sea surface are called storm surges or storm tides. These are not typical waves but share characteristics of curving sea surfaces and produce like effects to that of tsunamis.

TSUNAMI READING QUESTIONS

1.What does tsunami mean?
2. How is a tsunami (A) the same as (B) different than regular ocean waves?
3. What happens to a tsunami as it travels into shallow water?
4. How would a tsunami appear to an onlooker in (A) the open ocean (B) along the shore?
5. What is meant by the term run-up?