Oceans Ch. 8

Giant Waves

Giant waves over 30.5 m high are rare In 1993 the USS Ramapo, a Navy Tanker en route from Manila to San Diego, encountered a severe storm or typhoon As the ship was running down wind to ease the ride it was overtaken by waves that, as measured against the ships superstructure by the officer on watch were 112 ft high The period of the waves was measured at 14.8 seconds, the wave speed was calculated at 27 m per second and the wavelength at 329 m 1100 ft Other ström waves in this size category have been reported but none has been as well documented

Wave trains

Groups of these faster waves move as this or packets of similar waves with approximately the same period and speed Series of similar waves from the same direction

relationship between wavelength and speed

Increase in wavelength increase height

Tides

The larges waves in the ocean are these These are generated by the combined influence of lunar and solar gravitational attraction and the rotation of Earth

Trough

This is the part of the wave that is depressed the lowest below the undisturbed sea surface This has a flatter shape than the crest

Wave frequency

This is the reciprocal of the wave period This is a measure of how many cycles of the wave pass a stationary point in space in a unit length of time So, if the period of the wave is equal to 10 seconds/cycle then this is 0.1 cycles/second

Dispersion

Waves with long periods and long wavelengths have a greater speed than waves with short periods and short wavelengths The faster, longer waves gradually move through and ahead of the shorter, slower waves this process is called sorting or this Sorting of waves as they move out from a strong center; occurs because long period waves travel faster than short period waves Because of this the distribution of observed waves from any single storm changes with time Near the storm center, the waves are not yet sorted, while farther away the faster longer period waves are out ahead of the slower, shorter period waves

Free waves

When the waves move away from the storm, they are no longer wind-driven forced waves but become these moving at speeds controlled by their periods and wavelengths Wave that continues to move at its natural speed after its generation by a force

Factors determining the size of wind waves

Wind strength Wind duration Fetch

Deep-ocean Assessment and Reporting of Tsunamis (DART)

• Operational, real-time tsunami measurements • The information collected by a network of DART systems positioned at strategic locations throughout the ocean plays a critical role in tsunami forecasting. (

Detecting a tsunami

• Pressure recorder on bottom of ocean • Buoy to communicate readings via satellite • Tsunami Warning Centers issue warning

wind-generated Gravity Waves

• Restoring force is gravity • Most common waves

Tsunami run-up

• Run-up = measurement of height of water onshore observed above a reference sea level • Generally don't get big gigantic wave • Water comes as a fast moving rise in tide that rapidly moves inland • NOT JUST ONE WAVE...multiple waves coming in about 1⁄2 hour or so apart

Tsunamis

• Seismic sea waves - Generated by earthquakes that produce vertical sea floor displacement • Shallow water waves - Long wavelength (100 - 200 km) - Long period (10 - 20 min) - Speed ~200 m/s (depends on water depth) • Wave height - Open ocean (may be as much as several meters but this is spread over a long wavelength) - Close to shore (may be over 30 m)

Rip Currents

• Surf zone transport of water is both onto the beach and along the beach • Wateraccumulates in the surf zone until it can flow seaward • Returnflowinafast, concentrated current • Can stir up sand particles

Tsunami wave speed

• Travel at high speeds : 400 to 500 mph (~200 yards/sec)

Damage due to tsunami

• Waves often full of debris (trees, cars, pieces of wood etc.) • As the wave recedes, the debris drags more stuff with it • Can recede as much as a km out to sea, leaving shoreline empty with flopping fish, boats, etc. on the bottom

Reflection

A straight smooth vertical barrier in water deep enough to prevent waves from breaking reflects the waves The barrier may be a cliff, steep beach, breakwater, bulkhead, or other structure The reflected waves pass through the incoming waves to produce an interference pattern and steep choppy seas often result If the waves reflect directly back on themselves, the resulting waves appear to stand still, rising and falling in place The behavior of waves reflected from a curved vertical surface depends on the type of curvature If the curvature is convex, the reflected wave rays spread and disperse the wave energy, but if the curved surface is concave, the reflected wave rays converge and the energy is focused This situation is similar to the reflection of light from curved mirrors Great care must be taken in designing walls and barriers to protect an area from waves to be sure that the energy of reflected waves is not focused in a way that will rust in another area being damaged

Typical maximum fetch

A typical maximum fetch for a local storm over the ocean is approximately 920 Km (500 nautical mi) Because storm winds circulate around a low pressure disturbance, the winds continue to follow the waves on the side of the storm, along which wave direction is the same as the storm direction This increases both the fetch and the duration of time over which the wind adds energy to the waves If the waves move fast enough , their speeds exceed the speed of the moving storm center and do not grow any larger Waves 10-15 m high are not uncommon under severe storm conditions, such waves are typically between 100 and 200 m long This length is about the same as the length of some modern ships

Wave Speed

A waves speed across the sea surface is related to its wavelength and wave period The speed of any surface wave (C) is equal to the length of the wave (L) divided by the wave period (T) speed= length of wave/wave period c=L/T Once a wave is created, the speed at which the wave moves may change, but its period remains the same Period is determined by the generating force

Earthquakes as a generating force

Another important germinating force is this that produce vertical displacement of the sea floor If the sea floor is disturbed, the water column above it will be displaced also, creating a series of waves that move outward away from the location of this These waves are called tsunamis While tsunamis are relatively uncommon they can be devastatingly destructive This generating force creates tsunamis

Deep water wave speed

C=1.56 T Where speed is in meters per second and period is in seconds

Two Types of Wind-Generated Waves

Common wind-generated gravity waves can be divided into two types based on the depth of the water they travel in compared to their wavelength 1. Deep water waves 2. Shallow water waves Which category they fall into will influence particle motion in the water column, the speed at which they travel, their shape, and how they may change direction as they travel Generally speaking most waves in the open ocean are deep water waves These deep water waves convert into shallow water waves when they approach a coastline and eventually break on the shore

Group Speed

Consider again the waves formed by a stone thrown into the water The wave group, or train, is seen as a ring of waves moving outward from the point of disturbance Careful observation shows that waves constantly form on the inside of ring as it moves across the water As each new wave joins the train on the inside of the ring, a wave is lost from the leading edge, or outside, of the ring, and the number of waves remains the same The outside waves energy is lost in advancing the wave form into undisturbed water Therefore the speed of each individual wave in the group is greater than the speed of the leading edge or the wave train and the wave ring moves outward at a speed one half that of the individual waves This speed is known as this ,the speed at which wave energy is transported away from its source under deep water conditions Group speed= 1/2 wave speed= speed of energy transport V=C/2

Controls on wind wave height

Controls on wind wave height: - Windspeed - Windduration - Fetch

Standing waves

Deep water waves, shallow water waves and internal waves are all progressive waves, they have a speed and move in a direction These waves do no progress, they are progressive waves reflected back on themselves and appear as an alternation between a trough and a crest at a fixed position They occur in ocean basins partly enclosed bays and seas and estuaries A standing wave can be demonstrated by slowly lifting one end for a container partially filled with water and then rapidly but gently returning it to a level position If this is done the surface alternatively rises at one end and falls at the other end The surface oscillates about a point at the center of the container the node the alternations of low and high water at each end are the antinodes A standing wave is a progressive wave reflected back on itself The reflection cancels out the forward motions of the initial and reflected waves If different sized containers are treated the same way the period of oscillation increases as the length of the container or its depth decreases Notice that the single node standing wave contains one half of a wave from The crest is at one end of the container and the trough is at the other end As the wave oscillates a trough replaces the crest and a crest replaces the trough In the case of two nodes there is a crest at either end of the container and a trough in the middle It can also be alternated in that there are tow troughs on the sides and a Crest in the center Standing waves in bays or inlets with an open end behave somewhat differently than standing waves in closed basins Standing waves can also occur in internal waves The oscillation of internal standing waves is slower than the oscillation of the sea surface standing waves may be triggered by tectonic movements that suddenly shake a basin A standing wave in a basin is like a water pendulum

Waves

Energy Moving

Kinetic energy

Energy of motion A waves energy is this due to the motions of water particles in orbit

Interference: Checkerboard

If the two wave trains intersect each other sharply, as at a right angle, then a checkerboard pattern is formed

Universal Sea State Code

In 1806 Admiral Sir Francis Beaufort of the British navy adapted a wind estimation system from land to sea use On land the clues to wind speed included smoke drift, the rustle of leaves, the flapping of flags, slates blowing form the roofs, and uprooting trees Admiral Beaufort related observations of the sea surface state to wind speed and designed a 0-12 (calm to hurricane) wind scale with typical wave descriptions for each level of wind speed This Beaufort scale was adopted by the U.S. Navy in 1838 and the scale was extended from 0-17 At present a Universal Seas State Code of 0-9 , based on the Beaufort scale, is in international use for wind speeds and related sea surface conditions

Progressive wind waves

Most deepwater waves observed at sea are these They are generated by the wind, are restored by gravity, and profess in a particular direction These waves are formed in local storm centers or by the steady winds of the trade and westerly wind belts

Ocean waves

Note that the water molecules in the crest of the wave move in the same direction as the wave, but molecules in the trough move in the opposite direction.

Deep Water Waves, Shallow Water Waves

Note the importance of the relationship between wavelength and depth in determining wave type. Deep Water waves. When d (water depth) > or equal to 1/2 L, ocean waves are unaffected by water depth. The diameter of the orbital paths of water particles under these waves decreases as depth increases below the surface, and shrinks to zero at a d = 1/2 L. Shallow Water waves. When d < or equal to 1/20 L, ocean waves are only under the control of water depth, and the orbits are elongated ellipses with a major axis in the horizontal direction and the minor axis in the vertical direction. Only the minor axis decreases as depth below the surface increases, so that near the bottom, the water motion is only horizontal and moves back and forth with the passage of a wave.

Only wind waves can be

Only wind waves can be deep water waves. Maximum wavelength is about 600 meters. In consequence any wind waves traveling on a basin deeper than 300 meters is a deep wave. Tsunami however have wavelengths of 200 kilometers; half of that is 100 km and no part of any ocean is deeper than 11.034 kilometers or 36,201 feet

Cats Paws

Patches of capillary waves can be seen forming, moving, and disappearing as they are driven by pulses of wind These patches darkened the surface of the water and move quickly keeping pace with gusts of wind Sailors call these fast moving patches this They are a patch of ripples son the water surface, driven by wind

Velocity, deep water waves

S= L/T • S= 1.56 T in m/s meters per second • S= 5 T in feet / s feet per second

Velocity, shallow water waves

S= square g *d

Fetch

The distance over water that winds blows in a same direction

The Wave Height

The height of wind waves is controlled by the interaction of several factors The three most important factors are 1. Wind speed- how fast the wind is blowing 2. Wind duration- how long the wind blows 3. Fetch- the distance over water that the wind blows in the same direction The wave height may be limited by an one of these factors If the wind speed is very low, large waves are not produced, no matter how long the wind blows over an unlimited fetch If the winds speed is great but it blows for only a few minutes, no high waves are produced despite unlimited wind strength and fetch Also if very strong winds blow for a long period over a very short fetch, no high waves form When no single one of these factors is limiting, spectacular wind waves are formed at sea

Ocean waves geometry

The ideal ocean surface wave (with crests and the troughs having identical shapes). The main components of wave includes: 1. the horizontal distance between two adjacent crests (or troughs) is defined as the wave length (L) 2. the vertical distance from the top of the crest to the bottom of the adjacent trough is defined as the wave height (H). 3. the time that it takes for two consecutive crests to pass a fixed point is defined as the wave period (T). 4. The inverse of the period is the wave frequency (f), which is a measure of the number of times one complete wave will occur per unit time.

Spiller

The more common type of breaker is found over wider, flatter beaches, where the energy is extracted more gradually as the wave moves over the shallow bottom This action results in the less dramatic wave form. consisting of turbulent water and bibles flowing down the collapsing wave face This last longer than the plungers because they lose energy more gradually Therefore these giver surfers a longer ride but plungers give them a more exciting one

Ripples of Capillary waves

The most common generating force for water waves is wind As wind blows across a water surface, the friction, or drag, between the air and the water tends to stretch the surface, resulting in these These wrinkles are small waves called these The restoring force for these small waves is the surface tension of the water Wave with a wavelength less than 1.5 cm in which the primary restoring force is surface tension

Water transport and rip currents

The small net drift of water in the direction the waves are traveling is intensified in the surf zone as the shoreward motion of the water particles at the crest becomes greater than the return particle motion at the trough Because the crests usually approach the beach at an angle the surf zone transport of water flows. both toward the beach and along the beach The result is that water accumulates against the beach and flows along the beach until it can flow seaward again and return to the area beyond the surf zone This return flow generally occurs in quieter water with smaller wave heights

Shallow water waves speed

The speed of shallow water waves pends only on water depth All shallow water waves travel at the same speed for a given depth of water L= 3.13 TD The group speed V, of shallow water waves is equal to the speed, C, of each wave in the group In other words shallow water waves are not dispersive waves

The surf zone

The surf zone is the shallow area along the coast in which the waves slow rapidly, steepen, break and disappear in the turbulence and sappy of expend energy The width of this zone is variable and is related to both the wavelength and height of the arriving waves and the changing depth pattern Longer higher waves which feel the bottom before shorter waves become unstable and break farther offshore in deeper water If shallow depths extend offshore for some distance the surf zone is wider than it is over a sharply sloping shore

Significant wave height

This is defined as the average wave height of the highest one-third of the waves in a long record of measured wave heights For example if a wave height record of 1200 successive storm waves is made and the individual wave highest are arranged in order of height, the average height of the 400 highest waves defines this

Crest

This is the part of the wave that is elevated the highest above the undisturbed sea surface Also known as the equilibrium surface

Orbit

This motion rising, moving forward, falling, reversing direction, and rising again creates this for the water particles as the wave passes in deepwater It is the orbital motion of the water particles that causes a floating object to bob or move up and down, forward and backward, as the waves pass This motion affects a fishing boat, swimmer, seagull, or any other floating object on the surface seaward of the surf zone The surface water particles trace an orbit with a diameter equal to the height of the wave This same type of motion is transferred to the water particles below the surface, but less energy of motion is found at each succeeding depth The diameter of the orbits becomes smaller and smaller as depth increases At a depth equal to one-half the wavelength the orbital motion has decreased to almost zero Submarines dive during rough weather for a quiet ride because the wave motion does not extend far below the surface A deep water wave sets water particles in circular motion At the surface the diameter of the particle orbit is equal to the wave height Orbit diameter decreases with increasing depth until there is essentially no motion at a depth of L/2 The water particles of actual seas waves move in orbits whose forward speed at the top of the orbit is slightly greater than the reverse speed at the bottom of the orbit Therefore, each orbit made by a water particle does move the water slightly forward int eh direction the waves travel This comment is due to the shape of the real waves whose crests are sharper than their troughs

Wavelength

This of a wave is the shortest part of the wave form that, if repeated multiple times, will reproduce the wave shape In practical terms this is typically measured as the distance between two successive wave crests or two successive wave troughs

Wave period

This of a wave is the time required for two successive crests or troughs, or one cycle of the wave, to pass a stationary point in space If you stand on the end of a dock and start a stopwatch as the crest of one wave passes by and then stop the stopwatch as the crest of the next wave passes, the measured time is this of a wave Unlike other properties of waves, this of a wave will not change once the wave has formed

Wave height

This of a wave is the vertical distance from the elevation of the crest to the depth of the trough

Amplitude

This of the wave is equal to one-half the wave height, or the vertical distance from either the height of the crest of the depth of the trough to the undisturbed water level

Wave steepness

This of the wave is equal to the wave height divided by the wavelength

Speed of Deep water waves

This type of wind-generated gravity waves are defined as waves that travel in water that is deeper than one-half of their wave length In deep water, the speed of a wave depends primarily on the wavelength of the wave, not on the depth of the water Speed increases as wavelength increases D>L/2

Speed of Shallow water waves

This type of wind-generated gravity waves are defined as waves that travel in water this is shallower than one-twentieth of their wavelength In shallow water, the speed of a wave depends on water depth, not on wavelength D<L/20

Generating forces

Those responsible for creating a disturbance on the water surface The most common of this force for water waves is wind

Restoring force

Those responsible for returning the water surface to an undisturbed state There are only 2 of these forces 1. Gravity which is by far the most common 2. Surface Tension which has a greater influence than gravity on very small waves

Deep water waves

Travel in water deeper than one-half their wavelength

Shallow water waves

Travel in water shallower than one-twentieth their wavelength

Destructive interference

Two waves that cancel each other out, resulting in reduced or no wave If the crests of a wave train coincide with the troughs of another wave train the waves are cancelled

Energy Release

Watching the heavy surf pounding a beach from a safe vantage point is an exciting and exhilarating experience, the trick is to determine at what point one is safe In a narrow surf zone during a period for very large waves the wave energy must be expended rapidly over a short distance Under these conditions the height of the waves and the forward motion of the water particles combine to send the water hight up on the beach The accompanying release of energy is explosive and can result in rocks and debris from the waters edge being hurled high up on the beach by the force of the water Waves do not always expend their energy on the shore Some break farther seaward on sandbars such as those associated with river mouths and estuaries

Wave Refraction, Diffraction, and Reflection

Wave refraction - the slowing and bending of waves in shallow water. Wave diffraction - propagation of a wave around an obstacle Wave reflection - occurs when waves "bounce back" from an obstacle they encounter. Reflected waves can cause interference with oncoming waves, creating standing waves.

Classifying Waves

Waves are classified by Disturbing force: energy that causes ocean waves 1. Gravity 2. Seismic activity- Landslides 3. Wind Restoring force: the dominant force that returns the water surface to flatness after a wave has formed in it 1. Surface tension (for capillary waves) 2. Gravity Wavelength - Most important and the one used on the textbook Wind waves, seiches, tsunami and Tides Arranged from short to long wavelengths ocean waves are generated by very small disturbances (capillary waves), wind (wind waves), rocking of water in enclosed spaces (seiches), seismic or volcanic activities or other sudden displacements (tsunami) and gravitational attraction (tides).

Refraction

Waves are refracted, or bent, as they move from deep to shallow water, begin to feel the bottom, and change wavelength and wave speed When waves from a distant storm center approach the shore, they are likely to approach the beach at an angle One end of the wave crest comes into shallow water and begins to feel bottom, while the other end is in deeper water The shallow water end moves slowly than the portion of the wave in deeper water The result is that the wave crests bend or refract and tend to become oriented parallel to the shore The wave rays drawn perpendicular to the crests show the direction of motion of the wave crests Note that the refraction of water waves is similar to the refraction of light and sound waves

Wave Interaction

Waves that escape a storm and are no longer receiving energy from the storm winds tend to flatten out slightly and their crests become more rounded These waves moving across the ocean surface as swell are likely to meet other trains of swell moving away from other storm centers When the two wave trains meet, they pass through each other and continue Wave trains may intersect at any angle, and many possible interference patters may result If the two wave trains intersect each other sharply, as at a right angle, then a checkerboard pattern is formed

Shallow submerged ridge focused Headland Refraction

When approaching an irregular coastline with headlands jutting into the ocean and bays set back into the land, waves may encounter a submerged ridge seaward from a headland or a depression in from of a bay As waves approach such a coastline and feel the bottom, the portion of the wave crest over the ridge slows down more than the wave crests on either side Therefore the crests wrap around the headland This refraction pattern causes wave energy on the headland Waves must gain in height as their wavelength is shortened and the Total wave energy is crowded into smaller volume of water, this increases the energy per unit of surface area Therefore more energy is expended on a unit length of shore at the point of the headland than on a unit length of shore else where The energy of waves refracted over a shallow, submerged ridge is focused on the headland The converging wave rays show the wave energy being crowded into a small volume of water, increasing the energy per unit length of wave crest as the height of the wave increases

Water displaced by a landslide or ice breaking off a glacier

When water is displaced by this it can produce significant waves The process is something like throwing a stone into water If a stone is thrown into water it will displace, or push aside, the water As the stone sinks, the displaced water flows from all sides into the space left behind, and as the water rushes back, the water at the center is forced upward The elevated water falls back, causing a depression of the surface that is refilled, starting another cycle This process sets up a series of waves, or oscillations, that move outward away from the point of the disturbance

Forced waves

When waves are being generated, they are forced to increase in size and speed by the continuing input of energy, these are known as this Waves generated by storm winds They are faster than free waves

Waves Moving inshore Refraction

When waves from a distant storm center approach the shore, they are likely to approach the beach at an angle One end of the wave crest comes into shallow water and begins to feel bottom, while the other end is in deeper water The shallow water end moves slowly than the portion of the wave in deeper water The result is that the wave crests bend or refract and tend to become oriented parallel to the shore The wave rays drawn perpendicular to the crests show the direction of motion of the wave crests Waves moving inshore at an oblique angel to the depth contours are refracted One end of the wave reaches a depth of L/2 or less and slows, while the other end of the wave maintains its speed in deeper water

Factors Affecting Wind Wave Development

Wind strength - wind must be moving faster than the wave crests for energy transfer to continue Wind duration - winds that blow for a short time will not generate large waves Fetch - the uninterrupted distance over which the wind blows without changing direction

Constructive Interference

additive interference that results in waves larger than the original waves If the waves have similar lengths and heights and approach from opposite directions and if the crests of one wave train coincide with the crests of another train the wave trains reinforce each other

Diffraction

Another phenomenon that is associated with waves as they approach the shore or other obstacles is this This is caused by the spread of wave energy sideways to the direction of wave travel If waves move toward a barrier with a small opening some wave energy passes though the small opening to the other side Once the waves have passed though the opening, their crests decrease in height and radiate out and away from the gap A portion of the wave energy is diffracted, transported sideways from its original direction If more than one gap is open to the waves the patters produced by the spreading waves from the openings may intersect and from interference patters as the waves move through each other If the waves approach a barrier without an opening, diffraction can still occur Energy will be transported at right angles to the wave crests as the waves pass the end of the barrier Energy is transported behind the sheltered or lee side of the barrier This effect is an important consideration in planning the construction of breakwaters and other coastal barriers intended to protect vessels in harbors from wave action and possible damage

Swell

Away from the storm, the faster, longer period waves appear as a regular pattern of crests and troughs moving across the sea surface. These uniform free waves are called this They carry considerable energy, which they lose very slowly The distribution of the waves from a given storm and the energy associated with particular wave periods change predictably with time, allowing the oceanographer to follow waves trains from a single storm over long distances Groups of large long period waves created by storms between 40s and 50s in the Pacific Ocean have been traced across the entire length of that ocean, until they die on the shores and beaches of Alaska Long and relatively uniform wind-generated ocean waves that have traveled out of their generating area

Potential Energy

stored energy A waves energy is present at this due to the change in elevation on the water surface

More Episodic waves

• Abnormally high wave • Relatedtoa combination of - Constructive wave interference - Changing water depth - Currents

Forces Influencing waves

• Generating force - Wind • Wind-generated waves - Earthquakes • tsunami • Restoring force - Surface tension • Capillary waves - Earth's gravity • Gravity waves

Tsunami wavelength

• Long wavelengths (over 100 km) • Periods longer than 1 hour

Energy in tsunami

• Loss of energy in a wave is inversely proportional to λ • Since λ very long, little energy lost • Waves can travel great distances and still be very distructive

What happens when tsunami gets near shore?

• Tsunami slows down (shallower water) - Example: d = 100 m, v = 113 km/hr • Wave gets taller • λ gets shorter, T gets • As wave gets into shallow water bottom of wave drags along ocean floor • Top of wave still moving fast: can cause cresting of wave, and breaking onto shore

Dispersion Graph

1. Area of wave generation is in the storm center where wind moves in a circular pattern 2. Combined long-intermediate and short wave period waves 3. Combined long-and intermediate period waves 4. Long period waves only (swell) The longer waves travel faster than the shorter waves. Waves are shown here moving in only one direction

Wave Energy

A Waves energy is present as potential energy, due to the change in elevation of the water surface, and as 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 Therefore the greater the kinetic and potential energy The energy in a deep water wave is nearly equally divided between kinetic and potential energy The energy in a deep water wave can be described as the total energy distributed over one wave-length of the wave from the sea surface to a depth of L/2 per 1 meter of distance measured along the crest of the wave Wave energy is a function of the square of the wave high and can be expressed as E=1/8 p g H^2 Where p is water density, g is the acceleration of gravity, and H is wave height Cleary, Wave energy increases rapidly with increasing height Wave energy increases rapidly with the square of the wave height

Changes in waves as they reach shallow water

As a deep water wave approaches the shore and moves into shallow water, motion within the wave will eventuality reach the sea floor and the wave will go through a transitional, or intermediate phase before becoming a true shallow water wave Interaction between the wave and the bottom will begin to affect the shape of the orbits made by the water particles The orbits gradually become flattened circles or eclipses The wave beings to feel the bottom and the resulting friction and compression of the orbits reduce the forward speed of the wave Remember that 1. The speed of all waves is equal to the wavelength divided by the period 2. The period of a wave does not change Therefore when the wave feels bottom it slows and the accompanying reduction in the wavelength and speed results in increased height and steepness as the waves energy is condensed in a smaller water volume When the conditions for a shallow water wave is met, the orbits of the water particles are elliptical, they become flatter with depth until as the sea floor only a back and forth oscillatory motion remains Note that the horizontal dimension of the orbit remains unchanged in shallow water waves 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 Deep water waves become intermediate waves and then shallow water waves as depth decreases and wave motion interacts with the sea floor A shallow water wave sets water particles in elliptical motion The height of the ellipse decreases with depth

Shallow-water wave motion

As a deep water wave approaches the shore and moves into shallow water, motion within the wave will eventuality reach the sea floor and the wave will go through a transitional, or intermediate phase before becoming a true shallow water wave Interaction between the wave and the bottom will begin to affect the shape of the orbits made by the water particles The orbits gradually become flattened circles or eclipses The wave beings to feel the bottom and the resulting friction and compression of the orbits reduce the forward speed of the wave Remember that 1. The speed of all waves is equal to the wavelength divided by the period 2. The period of a wave does not change Therefore when the wave feels bottom it slows and the accompanying reduction in the wavelength and speed results in increased height and steepness as the waves energy is condensed in a smaller water volume When the conditions for a shallow water wave is met, the orbits of the water particles are elliptical, they become flatter with depth until as the sea floor only a back and forth oscillatory motion remains Note that the horizontal dimension of the orbit remains unchanged in shallow water waves 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 Deep water waves become intermediate waves and then shallow water waves as depth decreases and wave motion interacts with the sea floor A shallow water wave sets water particles in elliptical motion The height of the ellipse decreases with depth until at the bottom the water simply moves back and forth perpendicular to the direction the wave is traveling

Deep water wave motion

As a wave moves across the water surface, particles of water are set in motion Seaward beyond the surf and breaker zone where the surfaces undulates quietly int water deeper than half the wavelength of waves, the water is not moving toward the shore Such an ocean wave does not represent a flow of water but instead represents a flow of motion or energy from its origin to its eventual dissipation at sea or against the land To understand what is happening during the passing of a deep water wave let us follow the motion of the water particles as the wave moves through the water As the wave crests 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 in there forward motion, reaching a maximum falling speed and a zero forward speed when the midpoint between the crest and trough passes As the trough advances, the particles slow their falling rate and start to move backward, until at the bottom of the trough they have attainted their 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 midpoint between trough and crest passes At this point, the water particles start their forward motion again and continue to rise with the advancing crest This motion rising, moving forward, falling, reversing direction, and rising again creates a circular path or orbit, for the water particles as the wave passes

Wave rays

Line indicating the direction waves travel; drawn at right angles to the wave crest

Rip currents

Because regions of seaward return flow may be narrow and some disgrace apart, the flow in these areas must be swift in order to carry enough water beyond the surf zone to balance the slower but more extensive flow toward the beach These regions of rapid seaward flow are called rip currents They can be a major hazard to surf swimmers Swimmers who unknowingly venture into a rip current will find themselves carried seaward and unable to swim back to shore against the flow, they must swim parallel to the beach or across the rip current and then return to shore Because of the danger associated with rip currents swimmers should be on the lookout for indicators of their presence including 1. Turbid water and floating debris moving seaward thought the surf zone 2. Areas of reduced wave heights in the surf zone 3. Depression sin the beach running perpendicular to the shore Wave action on the beach stirs up the sand particles and temporality suspends them in the water The sand is carried along the beach parallel to the shore until the rip current is reached and the sane is transported seaward Seen from the top rip currents are seen as streaks of discolored turbid water extending seaward through the clearer water of the outer surf zone

Strom Centers

In an active storm area covering thousands of square kilometers, the winds are not steady but vary in strength and direction Storm area winds flow in a circular pattern about the low pressure storm center, creating waves that move outward and away from the storm in all directions When a storm center moves across the sea surface in a direction following the waves wave heights are increased because the winds supply energy for a longer time and over a longer distance In a storm area, the sea surface appears as a jumble and confusion of waves of all heights, lengths, and periods There are no regular patters Capillary waves ride the backs of small gravity waves, which in turn are superimposed on still higher and longer gravity waves This turmoil of mixed waves is called a "sea" and sailors use the expression, "There is a sea building" to refer to the growth of these waves under storm conditions When waves are being generated, they are forced to increase in size and speed by the continuing input of every, these are known as forced waves Because of variations in the winds of the storm area, energy at different intensities is transferred to the sea surface at different pulse rates resulting in waves with a variety of periods and heights Remember, wave periods are a function of the generating force, the speed at which a wave moves away from a storm may change but its period remains the same Area of origin for surface waves generated by the wind; and intense atmospheric low-pressure system

Speed of Deep water waves and shallow water waves

It is clear that the speed of a wave behaves diffenrley in deep water compared to shallow water This is one reason why wind-generated gravity waves are divided into two groups 1. deep water waves 2.Shallow water waves At depths between one-half and one-twentieth the wavelength, waves are in a transitional stage where their characteristics progressively change from deep water waves to shallow water waves In deep water, the speed of a wave depends primarily on the wavelength of the wave, not on the depth of the water In shallow water, the opposite is true- wave speed depends on water depth, not on wavelength

Episodic Waves

Large waves that can suddenly appear unrelated to local sea conditions They are an abnormally high wave that occurs because of a combination of intersecting wave trains changing depths and currents We do not know a great deal about these waves, as they do not exist for long and they can and do swamp ships often eliminating witnesses They occur most frequently near the edge of the continental shelf in water about 200m deep and in certain geographic areas with particular prevailing wind wave and current patters High wave unrelated to local storm conditions

Whitecaps and long waves

Small unstable breaking waves are quite common When wind speeds reach 8-9m/S waves know as this can be observed The waves have short wavelengths and the wind increases their height rapidly As each wave reaches the critical steepness and Crest angle it breaks and is replaced by another wave produced by the rising wind Long waves at sea usually have a height well below their maximum value Sufficient wind energy to force them to their maximum height rarely occurs If a long wave does attain maximum height and breaks in deep water tons of water are sent crashing to the surface The energy of the wave is lost in turbulence and in the production smaller waves Rather than breaking under such conditions it is more likely that the top of a large wave will be torn off by the wind and cascade down the wave face This action ours not completely destroy the wave and is not considered to be the true collapse or breaking of such a wave In addition waves breaks and dissipate if intersecting wave trains pass through each other in proper phase to from a combined wave with sufficient height to exceed the critical steepness Waves sometimes run into a strong opposing current, forcing the waves to slow down Remember that the speed of all waves equals the wavelength divided by the period and that a waves period does not change If the speed of a wave is reduced by an opposing current its wavelength must shorten In such a case the waves energy is confined to a shorter length so the wave increases in height to satisfy the height-energy relationship If the increase in height exceeds the maximum allowable height to length ratio for the shorter wavelength, the wave breaks Crossing a sandbar into a harbor or river mouth during an outgoing or falling tide is dangerous because the waves moving over the bar and against the tidal current steepen and break Entering a harbor or river should be done at the3 change of the tide or on the rising tide, when the tidal current moves with the waves stretching their wavelengths and decreasing their heights

Seiche

Standing waves that occur in natural basins are called this The oscillation of the surface is called seiching In natural basins the length dimension usually greatly exceeds the depth Therefore a standing wave of one node in such a basin behaves as a reflecting shallow water wave

Tsunami

Sudden movements of Earths crust may produce a seismic sea wave or this These waves are often incorrectly called tidal waves Because a seismic sea wave has nothing to do with tides, oceanographers have adopted the Japanese word tsunami meaning harbor wave to replace the misleading term tidal wave Tsunami is the same as a seismic sea wave If a large area maybe several hundred kilometers of earths crust below the sea surface is displaced it may cause a sudden rise or fall in the overlying sea surface In the case of a rise, gravity causes the suddenly elevated water to return to the equilibrium surface level, if a depression is produced gravity causes the surrounding water to flow into it Both events result in the production of waves with extremely long wavelengths 100-200 km and long periods as will over 1 hour The average depth of the ocean is about 4000m this depth is less than one-twentieth the wavelength of these waves so tsunamis are shallow water waves These waves radiate from the point of the seismic disturbance at a speed determined by the oceans depth and move across at about 200 m/s or (400 mph) Because they are shallow water waves tsunamis may be refracted diffracted or reflected in mid ocean by changes in seafloor topography and by mid ocean islands Wehn a tsunami leaves its point of origin it may have a height of several meters but this height is distributed over its very long wavelength It is not easily seen of felt when superimposed on the other motions of the sea surface The energy of a tsunami is distributed from the ocean surface to the ocean floor and over the length of the wave When the path of the wave is blocked by a coast or an island the wave behaves like an other shall water wave It slows to 50 mph its wavelength decreases and its energy is compressed inso a smaller water volume as the depth rapidly decreases This sudden confinement of energy to a smaller volume increases the energy density and causes the wave height to build rapidly, and the loss of energy is equally rapid when the wave breaks A tremendous surge of moving water races up over the land flooding the coast for a period that lasts 5-10 minutes before the water flows seaward The leading edge of the tsunami wave group may be either a crest or a trough If the initial crustal disturbance is an upward motion a Crest arrives first If the crustal motion is downward a trough precede the crest Tsunamis are most likely to occur in ocean basins that are tectonically active The Pacific Ocean ringed by crustal faults and volcanic activity is the birth place of most tsunamis

40-50 S Latitude

The area of the ocean which is ideal for the production of high waves Here in an area noted for high intensity storms and strong winds of long duration Sailors have called the roaring forties and furious fifties There are no landmasses to interfere and lift the fetch length The westerly winds blow almost continuously around Earth, adding energy to the sea surface for long periods of time and over great distances, resulting in waves that move in the same direction as the wind Although this area is ideal for the production of high waves such waves can occur anywhere in the open sea given the proper storm conditions

Where episodic waves are mostly found

The area water the alguhas current sweeps south along the east coast of South Africa and meets the storm waves arising in the Southern Ocean is noted for Episodic waves Storm waves from more than one storm may combine constructively and run into the current and against the continental shelf, producing occasional episodic waves This area is also one of the worlds busiest sea routes, as supertankers carrying oil from the Middle East ride this current on their trip southward to round the Cap of Good Hope en route to Europe and America There are many disappearances of vessels for which episodic waves are now suspected of being the chief cause Many of the casualties are tankers or bulk carriers

Bay Refraction

The central area at the mouth of the bay is usually deeper than the areas to each side, so the advancing waves slow down more on the sides than in the center Because the wavelengths remain long in the center and shorten on each side, the wave crests bulge toward the center of the bay The waves in the center of the bay do not shorten as much therefore, they have less height and less energy to expend per length of shoreline The result is an environment of low wave energy providing sheltered water in the bay Overall, this unequal distribution of wave energy along the coast results in a wearing down of the headlands and a filling in of the bays as the sand and mud settle out in the quieter water If all coastal materials had the same resistance to wave erosion, this process would lead in tie to a straightening of the coast line but the rock structure of many headlands resists wave erosion there fore the cliffs remain Waves refracted by the shallow depths on each side of the bay delivers lower levels of energy inside the bay The diverging wave rays show the spreading of energy over a larger volume of water decreasing the energy per unit length of wave crest as the wave height decreases

Internal Waves

The waves discussed to this point have all formed at the interface of the atmosphere and the ocean This interface marks the common boundary between two fluids of different densities air and water Another interface between two fluids lies below the ocean surface at the pycnocline that separates the shallow mixed layer from the denser underlying water In this case the boundary is less abrupt and the density difference is not as great as it is at the air-water boundary The waves that form along this boundary are known as these These cause the boundary to oscillate as the wave form progresses between the water layers They are slower than surface waves They typically have wavelengths from hundreds of meters to tens of kilometers and periods from tens of minutes to several hours Their height often exceeds 50 and may b limited by the thickness of the surface layer The radius of the circular motion of the water particles is largest at the density boundary (pycnocline or thermocline) depth and decreases downward as well as upward from this depth When wave heights are large the crests of the internal waves may show at the sea surface as moving bands The water over the crests of these waves often shows ripples Sometime instead of visible bands elevations and depressions of the sea surface occur as the internal waves pass Many processes are responsible for internal waves A low pressure storm system may elevate the sea surface and depress pycnocline When the storm system moves away the displaced pycnocline will oscillate as it returns to its equilibrium level When boat propellors create internal waves they carry energy away from the vessel making a dead water effect because the propeller becomes insufficient

Forces Influencing Waves

There are two families of forces that influence waves 1. Generating forces- those responsible for creating a disturbance on the water surface 2. Restoring forces- Those Responsible for returning the water surface to an undisturbed state

The Wave steepness

There is a maximum possible height for any given wave length This maximum value is determined by the ratio of the Waves' height to the wavelength, and it is the measure of the steepness of the wave Steepness= Height/Length S=H/l If the ratio of the height to the length exceeds 1:7 the wave becomes too steep and the wave breaks For example if the wavelength is 70m the wave will break when the wave reaches 10m The angel formed at the wave crest approaches 120 degrees and the wave becomes unstable

Intense low pressure systems

These can produce large differences in atmospheric pressure that raise the sea surface and generate very long waves This can result in storm surge along coastlines

Breakers

These form in the surf zone because the water particle motion at depth is affected by the bottom Orbital motion is slowed and compressed vertically but the orbit speed of water particles near the crest of the wave is not slowed as much The particles at the wave crest move faster toward the shore than the rest of the wave form, resulting in the curling of the crest and the eventual breaking of the wave The two most common types of breakers are plungers and spillers The slow curling over of the crest observed on some breakers begins at a point on the crest and then moves lengthwise along the wave crest as the wave approaches shore This movement of the curl along the Crest occurs because waves are seldom exactly parallel to a beach The curl begins at the point on the crest that is in the shallowest water or the point at which the crest height is slightly greater and it moves along the crest as the rest of the wave approaches the beach If the waves approaching the beach are uniform in length period and height they are the swells from some far distant storm which have had time and distance to sort into uniform groups For example the long surfing waves of the California beaches in Sumer begin in the winter storms of the South Pacific and Arctic Ocean If the waves are of different height lengths and periods and break at varying distances from the beach then unsorted waves have arrived and are probably the product of a nearby local storm superimposed on the swell Sea surface water wave that has become too steep to be stable and collapses

Rouge waves

These freak waves occur due to interference and result in a wave crest higher than the theoretical maximum In this way, two or more similar wave trains traveling in the same or opposite direction and passing though each other can join together in phase and suddenly develop large amplitude waves

Gravity waves

These mammal waves, Capillary waves, die out rapidly while new ripples form constantly in front of each moving wind gust When the wind blows, energy is transferred to the water over large areas, for varying lengths of time, and at different rates As waves form, the surface becomes rougher, and it is easier for the wind to grip the roughened water surface and add energy As wind speeds increase or winds blow for longer times over the water, the waves become larger, and the restoring force changes fro surface tension to gravity The vast majority of water waves that you see on the ocean are wind generated.... these An increase of speed makes an increase in energy which changes the restoring force from surface tension to gravity This then creates these wind generated waves Water wave form in which gravity acts as the restoring force; a wave with wavelength greater than 2 cm

Plungers

These types of breakers form on narrow, steep beach slopes 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