WAVES

 

A wave is a disturbance caused by the movement of energy from the source of disturbance through a solid, liquid or gas medium. However, electromagnetic waves can be transmitted without a medium. In the ocean, most waves are formed by wind blowing on the ocean surface. It may be a local storm wind or a prevailing wind. In a typical ocean wave, energy is moving at the speed of the wave, but the water particles have an up-down/to-fro motion, the outline of which traces a circle near the water surface. In other words, the water particles that cause a buoy to bob up/down also have a forward and a backward motion. These circular or elliptical paths that trace the motion of the water particles are called orbitals. Ocean water waves can be reflected off a vertical obstacle such as seawalls or a ship, or they can be refracted (or bent) when the approach shallow waters.


PROPERTIES OF THE WAVE FORM
When water particles are moving upward along an orbital, a wave crest forms, which represents the higher part of the waveform. When water particles are descending along the wave orbit path, a wave trough forms.

The distance between 2 wave crest = wave length (L), which is measured in meters or feet.

The vertical distance between the wave crest and the trough = wave height (H). H also equals the diameter of the uppermost wave orbital and it is commonly used to estimate the quantity of energy in the wave. H is controlled by:

* Wind speed,
* Wind duration, and
* Fetch or the total distance in one direction over which the wind blows.

The observed height of a wave is a combination of all three factors. The faster the wind, the longer it blows and the farther it blows, the higher the waves produced.

The time it takes for 2 successive wave crests (or troughs) to pass a fixed point is the wave period (T), measured in seconds. T is a constant property for each wave set or wave train, and it is determined by the generating force (wind).

Wave frequency is the number of wave crests passing a fixed point every second. Of course, the shorter the L, the greater the number of wave crests passing a fixed point per second (or the higher the wave frequency) and vice versa.

The longer the L the greater the wave speed because (V) = L/T, and T does not change. This relationship is only useful for "deep" and "transitional" waves (see below).

When waves are formed in a stormy area of the ocean, the waves sort themselves out into groups or wave trains with similar T and V. The wave trains move at a speed that is half the speed of the individual waves. This process is called wave dispersion and produces the rounded-crest waves, called swells, some distance from the storm center, whwre the waves awere formed. Swells often travel thousands of kilometers from their origin to shore with the longest waves arriving first and the shortest waves last.

The wave steepness (S) is the ratio of H/L. In the open ocean, moderate wind waves have a maximum steepness of 1:7. If a wind wave gets any steeper than 1:7, it will break and the excess energy in the wave will be dissipated as turbulence known as white caps.


 

NEARSHORE WAVE MOTION
If the wave is traveling in water that is deeper than 1/2 of the wavelength, then it is typically said that the wave does not "touch bottom". Such waves are known as deep water waves. The circular orbitals of waves at the water surface extend down into the water column but with increasingly smaller- -size orbitals that disappear at a depth of 1/2 of the wavelength. In general, there is no net shoreward translation of water molecules in deep water waves. These are the wave types that produces the typical up-and -down motion displayed by buoys.

Shallow water waves travel in water depth less than 1/20 of the wavelength; they not only "touch bottom" but no longer have circular orbits but flat ellipses with a net to - and - fro motion instead of the up-down to-fro motion we noted for the deep water waves. Between 1/2 and 1/20 of the wavelength, the waves are known as transitional or intermediate water waves whose orbits are fatter ellipses.

In general, L and V decrease as waves progress from deep to shallow water, H and S increase, but T remains the same. When the wave orbitals change from circles to ellipses, they begin to slow down. In shallow water, when the bottom orbits flattens, the entire wave becomes even slower due to friction with the bottom, as the L shortens. As a result, the S increases because L is now smaller, whereas the H increases. The bottom orbits become horizontal back-forth motions instead and the entire water particles travels toward the shore at the speed of the wave. In this case, there is a net shoreward translation of the water molecules affected by the wave motion.

Wave refraction also occurs in shallow water by waves bending around to become parallel or quasi- parallel to the shoreline. The wave crests bend because waves commonly approach the shore at an angle rather than head on. As a result, part of the wave closer to shoreline encounters shallow water ahead of other parts. This forces that part of the wave "touching bottom" to slow down and the part still in intermediate waters that is traveling relatively faster, to bend or refract toward the shore.

 

 

Waves "break" or collapse and lose their form when they approach too close to the shore as the S exceeds the stable ratio of 1:7. The breaking waves partly disappear in turbulence and foam, if they break away from the beach. This area is called the surf zone. If they break at the beach toe, the water in the wave travels up the beach face as the swash and the remaining water that did not infiltrate into the beach flows back to the surf zone as the backwash.


WAVE CLASSIFICATION
Waves are classified by one of the following criteria:


* The disturbing force that forms them (Wind waves):


Seiche
Tsunami
Tides

* The wavelength:

Long waves
Short waves

* The restoring force that flattens them:

Capillary waves
Gravity waves


LONG PERIOD WAVES ---- Seiches, Tsunami, and Tides.

These are very long waves that sometimes qualify as shallow water waves because no body of water is deep enough based on the definition of shallow-water waves.

They are typically caused by disturbances from various sources:

· Tsunamis or seismic sea waves are typically caused by ocean floor/coastal earthquakes, landslides or volcanic eruptions.
· Sieches typically develop in bays, coves, lakes or harbors. They can be initiated by sudden pressure changes, or the arrival of a storm surge.
· Tides are caused by the gravitational attractions between Earth, sun, and moon.

SEICHES
Seiches are standing waves in closed (or quasi-closed) basins such as lakes, bays, harbors, etc. The water in a standing wave sloshes up and down against the sides of the basin. However, there is a point in the wave called the node that provides no vertical motion, instead there is a horizontal water movement back and forth across the node.

This movement creates a wave with the node marking the location of the trough and the sloshing edges representing the wave crests. Typically the wave oscillates at a specific frequency, with a specific period (T) that can range from a few minutes to a day. Seiche wave heights (H) are typically less than 3 m, however, the wavelength (L) may be very long (scores of km at least).

 


TSUNAMI
The sudden displacement of ocean water, more often by submarine earthquakes, can generate a long wave called a tsunami (Japanese for harbor wave). These earthquake-generated waves are also called seismic sea waves.

They are particularly dangerous because they can travel very fast and far toward land thousands of km away from the origin of the disturbance. The L can easily reach 100 km and hence are, by definition, always a shallow water wave. With speeds up to 400 mph, such waves can cross the Pacific Ocean in 10 or more hours.

However, tsunami's have low wave steepness (ratio of H:L) at the point of wave generation. This low steepness and long T (5 to 20 minutes) will allow it to pass beneath ships at sea unnoticed. But as a Tsunami approach a shore, bottom friction begins to slow it down and the H begins to increase as the L decreases. Because of the high wave speed, the entire set of changes noted above can be magnified within a very short time. In bays, the tsunami crest height can reach 30 m (100 feet). Other than the monstrous wave height, when the trough reaches shore, it drains water off the land, drying up harbors and grounding ships. Moreover, seismic sea waves make landfall in trains - several large waves arriving at 5 - 20 minute intervals - can wreak tremendous havoc on coastal settlements.

An international tsunami warning system has been in operation in the Pacific Rim region since 1948.


TIDES
Ocean tides are very long waves generated by the gravitational pull of the moon and sun on the Earth. The L is half of the circumference of the Earth making tides very shallow waves.

The relationship for generating tides involves:

T = G(m1m2/r^3)

the masses of the two objects (m1 , m2 ) tugging on each other, and the distance between them (r). Note that r has a greater impact on tidal force because it is cubed. The greater the distance between the two objects, the smaller the tidal force. Hence the sun contributes only 46% of what the moon contributes; the moon being much closer to the Earth. The tide generated by the moon is called the lunar tide and that attributed to the sun is the solar tide.

The Equilibrium Theory of Tides
There are two theories used to explain the behavior of tides. But, by far the equilibrium theory is used more often than the dynamic theory of tides because it is simpler.

The equilibrium theory simply applies the effect of the gravitational pull of the moon and the sun on the Earth's oceans. As such, the moon exerts the greater pull on the oceans along the plane of the moon's orbit with the Earth. The result is that ocean water will be pulled towards the moon as a bulge. But directly opposite this bulge on the far side of the Earth another bulge appears (Newton's 3rd Law of Motion). This "opposite" bulge of water is caused by centrifugal force, which is a force generated by orbiting objects trying to travel in a straight path. In other words, ocean water sloshes up, on the other side of the maximum gravitational pull as if trying to break free and spill outward in space.

The two bulges will pull water away from areas at right angles to the bulges. These depressed ocean surfaces represent areas of low tide and the bulging surfaces are the areas of high tide. As the Earth rotates under the deformed ocean water, some places will experience 2 high and 2 low tides each day. Such a tidal pattern is known as semidiurnal ttides with 6 hours interval between consecutive high and low tides. Other areas on the rotating Earth experiences one high and one low tide daily (diurnal tides). There are some coastlines that experience successive high and low tides with significant differences in tidal height. Such tidal patterns are known as mixed tides. The difference between high and low tides is known as the tidal range.

The phases of the moon as it rotates around the Earth give rise to special tides: a spring tide occurs when the earth, moon, and sun are positioned in a straight line. This happens twice a month during new and full moon phases. Spring tides produce the maximum tidal range when the high tide is very high and the low tide is very low.


Between the new and full moon phase are the 1st and 3rd quarter phases when the position of the Earth is at right angles to the moon and sun. A fraction of the full effect of the lunar tide is canceled by the solar tide. Hence, during neap tides, high tides are not very high and low tides are not very low. This leads to the smallest monthly tidal range. Neap and spring tides alternate at weekly intervals
.


Tidal Currents
The rise and fall of the ocean surface as a tidal bulge or crest approaches the coast generates a current that flows in and out of bays, lagoons, inlets, etc. This current is known as a tidal current. When a tidal current flows up onto land, it is known as the flood current. When the tidal current drains away from the land, it is known as the ebb current. Midway between the high and low tide period is slack water, a time when the flood and ebb change direction and net water flow is nonexistent.


A tidal bore forms in some inlets and river mouths exposed to large tidal ranges. It behaves as a steep wave, up to 3 m high, moving upstream at moderate speeds of 11 - 17 mph.