Oceanography Ch.8

Surf beat

Mixed interference explains the varied sequence of higher and lower waves.

Spilling Breaker

A mass of air and water at the front of a breaking wave, caused by a gently sloping shoreline that gradually extracts energy from the wave and produces breakers with low energy.

Wave reflection

A vertical barrier can reflect waves back into the ocean with little loss of energy, similar to how a mirror reflects light. If a wave strikes a barrier at a 90 degree angle, it can interfere with the next incoming wave and create unusual waveforms. More commonly, waves approach a barrier at an angle, causing the water to be reflected back at that angle. Example of this is the wedge, in which waves reflected off a jetty slam back into one another, due to their identical wavelengths, creating constructive interference and plunging breakers that have killed many.

Disturbing force

All waves begin as disturbances; this creates the energy that creates the disturbance.

Transverse waves

Also known as side to side waves. Moves at right angles to the direction of the vibrating particles. Only occurs in solids because solids are the only form of matter strong enough to transmit this type of motion. Called a body wave because it transfers energy through a body of matter.

Capillary waves

As the wind blows over the ocean, it creates stresses and pressure. These factors deform the ocean surface into small, v shaped troughs that have wavelengths less than 1.74 centimeters. Called this because of capillarity, a property of water resulting from surface tension.

Surfing

Balancing the downward motion of the particles due to gravity with the buoyant force of the surfboard.

Tsunami/seismic sea waves

Great amounts of energy can be released through fault slippage, volcanic eruptions, and underwater avalanches caused by turbidity currents.

Splash waves

Caused by mass movement into the oceans, such as when large icebergs fall into the ocean.

Pacific Tsunami Warning Center

Coordinates information about 25 Pacific Rim Countries, and is headquartered in Hawaii. Uses seismic waves to predict destructive tsunami.

Ocean waves

Created along an air water interface, by the air simply moving across the surface of the water. Also simply called waves.

Internal waves

Created by the movement of water with different densities. Associated with a pynocline because they travel along the boundary of water with different densities. Can be caused by tidal movement, turbidity currents, wind stress, or even passing ships. Can even take control of submarines and drag them to depths exceeding their limits. A parallel slick caused by surface debris may indicate internal waves.

Atmospheric waves

Created by the movements of different air masses, and represented by ripple like clouds in the sky. Especially common when a cold front (high density air) moves into the area.

Plunging breaker

Curling breaker with an air pocket. Best waves for surfing. The particles in the crest outrun the wave, leaving nothing to support their movement. Occurs in moderately steep beach slopes.

Wave base

Defined as the 1/2 of the wavelength measured from still water level. The longer the wave, the deeper the wave base.

Decay Distance

Distance over which waves change from a choppy sea to a uniform swell. Can last up to several hundred kilometers.

Idealized waves

Even though they do not exist in nature, they help us understand wave characteristics. An idealistic wave transmits energy from a single source, and is also called a sine wave, because the way it moves resembles an oscillating pattern expressed by a sine curve.

Swell wave train

Group of swells that leave the ocean area. The leading wave keeps disappearing, but the number of waves stays the same because the wave that disappears is replaced by another. Because of the constant replacement, swell wave trains move at a velocity that is half the velocity of a single wave.

Still water level

Halfway between the trough and crest, also called the zero energy level. This is the level of the water if there were no waves.

Destructive interference

Happens when waves with the same wavelengths come together out of phase, meaning that they come together trough to crest, resulting in a canceling effect if the waves are of the same height.

Deep water waves

If the water depth is greater than the wave base. They have no interference with the ocean bottom, and they include all wind generated waves in the open ocean, where water depth far exceeds wave base. Look like small circles that greatly decrease in size as they increase in depth. The longer the wavelength, the faster the wave. Formulas include speed, which is 1.25 square root the length, and 2.26 square root the length. Also can be found only knowing the period, since wave speed is defined as wavelength divided by the period. That formula is 1.56 times the period in meters and 5.12 times the period in feet.

Mixed interference

In most areas, it is most likely that waves of mixed heights and lengths will come together and produce a mixture of constructive and destructive interference, producing this type of interference.

Splash waves

Large objects that splash into the water create this type of wave. An example would be a meteor impact.

Constructive interference

Occurs when wave trains of similar wavelengths come together in phase, meaning trough to trough, crest to crest, creating a wave with the same wavelength as the two wave systems, but with a height equal to the addition of the two heights of the wave systems.

Transitional Waves

Partially composed of shallow water waves and deep water waves, the wavelengths are between two and twenty times water depth. Since deep water wave speed is dependent on wavelength, and shallow water waves are dependent on depth, transitional wave speed is partially dependent on both. They look like circles that get smaller as depth increases, but not as small as deep water waves.

Beaufort Sea Scale

Scale devised by British Admiral Sir Francis Beaufort that describes the appearance of the sea surface from dead calm conditions to hurricane force winds.

Tsunami

Seismic sea waves, caused by underwater events such as fault slippage, underwater volcanoes avalanches caused by turbidity currents. Majority are caused by fault movement. Faults that produce vertical displacement produce tsunami because it affects the whole water column and affects the ocean basin, while horizontal movement does not affect the volume of the ocean basin, therefore, it does not create tusuami. Only shallow water wave, so it's speed is determined by depth. Receding water means that the tsunami will arrive very soon. On the side of a fault that leads upward, the crest leads the trough, while on a fault that leads downward, the trough leads. Most tsunami are formed in the Pacific Ring of Fire, because of the large amount of earthquakes created in the Pacific Ocean.

Shoaling

Shallow water that, as deep water waves approach and encounter depths that are less than one half their wavelength, lose some of their energy and become transitional waves. The shallow depths interfere with the water particles at the base of the wave, causing it to slow down. Also, as one wave slows, the following waveform comes closer, reducing wavelength. Although some energy of the wave is lost to friction, the energy must go somewhere, and so wave height increases. When the wave steepness reaches the 1:7 ration, it breaks. If the surf has traveled from distant storms, breakers will develop relatively near shore in shallow water, and will be characterized by parallel lines of relatively uniform breakers. If the waves are local, they will not have been sorted into swell, and will be high energy and unstable, already nearing the 1:7 ratio and ready to break, breaking right after they feel the surface.

Wave dispersion

Sorting of waves by their wavelength.

Gravity waves

Symmetric waves that have wavelengths exceeding 1.74 cm, and are formed as capillary waves develop and catch more of the wind. Length is generally 15 to 35 times their height. As additional energy is gained, wave height increases more than wavelength. The crests become pointed, and the troughs are rounded, resulting in a trochoidal wavelength. When wave speed equals wind speed, neither wave height nor wavelength can change because there is no net energy exchange.

Crest

The high part of an idealized wave as it hits a permanent marker, such as a pier.

Wavelength

The horizontal distance between any two successive points in a wave, such as a crest and a trough.

Trough

The low part of an idealized wave as it hits an a permanent marker, such as pier.

Frequency

The number of wave crests passing past a fixed location over a unit of time, and is the inverse of the period. 1/P

Wave speed

The rate at which a wave travels. Wavelength/Period. More correctly known as celerity, which is used when no matter is in motion, just the waveform. Speed of deep water waves is dependent on wavelengths, and other factors, such as the gravitational attraction of the Earth.

Wave steepness

The ratio of wave height to wavelength. H/L. If the ratio exceeds 1/7, the wave will break and spill forward. In other words, if a wave is 1 meter high, it's maximum wave length can be 7 meters. Anything bigger, and it will break.

Wave period

The time it takes one full wavelength to pass a fixed position. Typical wave _____ range between 6 and 16 seconds.

Orbital waves

The type of energy motion at the surface. It involves components of both transverse and longitudinal waves, so particles move in orbits. Also a body wave because it transfers energy from the upper part of the ocean near the interface between the atmosphere and the ocean.

LIMPET 500

The world's first commercial wave power plant. Allows waves to compress in a partially submerged chamber that rotates a turbine for the generation of power. As the waves recede, air is sucked into the chamber and rotates the turbine in the other direction, generating power.

Factors that determine wave energy

These factors are the wind speed, duration, which is the length of time the wind spends in one duration, and the fetch, which is the distance the wind spends in one direction. Wave height is directly related to the energy of a wave. Wave heights in the sea are usually less than 2 meters, but waves with heights of 10 meters and periods of 12 seconds are not uncommon. When sea waves gain energy, their steepness increases, and when steepness reaches a critical mass of 1/7, open ocean breakers form, called whitecaps.

Longitudinal Waves

Transfer energy through all forms of matter. Also called push pull waves, because they push and pull back and forth in the direction where the energy is traveling. The shape of the wave, called the waveform, compresses and decompresses as it goes. Called a body wave because it transfers energy through a body of matter.

Rogue Wave

Twice the height of the average height of the tallest one third of the waves in an ocean. Chances of this occurring are 1 in several billion.

Deep Ocean Assessment and Reporting of Tsunamis

Utilizes sea floor sensors to pick up the pressure pulse of a tsunami passing above. Earthquakes below 6.5 are not considered tsunamigenic.

Wave Height

Vertical distance between a trough and a crest.

Circular orbital motion

Waves can travel great distances, in some cases 10000 kilometers and lasting over a week. The water itself doesn't travel the entire distance, but the waveform does, and the water passes the energy along by moving in a circle. An object in the water moves forward and backwards, allowing the waveform, the water's essential shape, to move forward while the water particles remain in essentially the same place. The diameter of circular orbital motion is equal to the wave height, and dies out somewhere below the surface.

Waves and Energy

Waves contain enormous energy, but there are many problems concerning how to effectively utilize them. First, one must consider how the devices could harness the power of the waves without being destroyed by them, the environmental effects, how they would affect the sediment flow into the coastline and how that would affect erosion, as well as the large amount of structures involved. Off shore generators might be able to harness the energy, but they would probably be damaged in large waves and be difficult to maintain. The best place to place power plants is most likely in the place where waves refract and converge, such as at headlands, which focuses wave energy. Internal waves are another good option. The best places for wind energy are in the 30 to 60 latitude, in the prevailing westerlies in the Southern Hemisphere, and along the western coasts, due to the west and east movement of storm systems.

Orthogonal line

Waves converge in headlands, and diverge in bays.

Fully developed sea

Waves in a fully developed sea cannot grow anymore, because they lose as much energy breaking as whitecaps under the force of gravity as they gain from the wind.

Shallow water waves

Waves in which depth is less than 1/20 of the wavelength. Also called long waves. They are said to touch bottom because they touch the ocean bottom, which interferes with the wave's orbital motion. Influenced by gravitational acceleration and water depth. The deeper the wave, the faster the wave travels. Since gravitational acceleration remains constant, depth is the only variable that must be calculated, and the equation becomes In Meters: 3.13 times square root of depth. In Feet: 5.67 times square root of depth. Examples of shallow water waves include tsunamis, tides, and wind driven waves that are now in near shore areas. The waves of shallow water waves are in a flat, almost elliptical motion, that flatten even more as ocean depth increases, because the vertical component of particle motion decreases with increasing depth.

Refraction

Waves seldom approach a wave at 90 degrees, but instead bend after hitting the bottom of the shoreline, resulting in the bending of each wave crest. No matter their original orientation, as the waves feel bottom, they will slow and bend, causing them to align somewhat before they hit the shore.

Progressive waves

Waves that oscillate uniformly and progress or travel without breaking. May be longitudinal, transverse, or orbital.

Wave trains

Waves with longer wavelengths travel faster than other waves and leave the sea area first. They are followed by smaller, slower groups of waves, which are given this name.

Standing Waves

What happens when two waves of the same wavelength are reflected off a barrier at a 90 degree angle and collide, resulting in no net movement. Anti nodes, which are crests that alternately become troughs, are the points in a standing wave in which the maximum vertical movement takes place. Nodes are the points in a standing wave in which no vertical movement takes place. Starts at complete flatness, at which water movement is greatest when it is horizontal, and then moves upward and crests, which is motionless, and then goes back down to horizontal flatness, but this time the water moves from right to left, and then moves back again to motionless cresting water, ex cept it crest from right to left, and then the cycle starts over again.

Interference patterns

When swells from different storms run together, the waves clash. It is the sum of the disturbance each wave would have produced individually. Happens when waves with similar characteristics but in different phases come together, and cancel each other out.

Surging breaker

When the ocean bottom has an abrupt shoreline, the energy is compressed into the wave, and it breaks right on the shoreline, deterring board surfers and attracting body surfers, as it is the most challenging of all waves.

Swells

When waves in a sea move toward it's margin, wind speeds diminish and the waves move faster than the wind. When this happens, steepness decreases, and waves become long crested. Swells move with little loss of energy over large stretches of the ocean surface, explaining why there are waves at shorelines even though there is no wind.

Surf Zone

Zone of breaking waves. Waves created in sea areas move across the oceans and release their energy on the margins of continents here. Energy from a storm can travel thousands of kilometers, and then expend their energy in a few powerful moments.