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Properties of Ocean Waves

An ocean wave is an undulation of the sea surface.

Wave parameters:
Wave crest
Wave trough
wave height (H)
wave amplitude (1/2H)
wave length (L)
wave period (T).
Progressive waves move across the sea surface.
Surface waves
Internal waves
Tsunamis Surface waves
Standing waves oscillate about a fixed point.
Seiches

Defintions and symbols

Definition of terms- elementary, sinusoidal, progressive wave

Properties of Ocean Waves
Idealized Wave Spectrum
 

This is a wave spectrum showing wave height versus wave period.  So its relating how high a wave is to how long it takes successive wave crests to pass a given point.  BE AWARE, THE Y-AXIS SCALE IS INCORRECT!!!
SWELLS CANNOT BE 200 METERS HIGH! (EXCEPT IN THE MOVIES).
I think correct heigh scale is a factor of 10 less.
Log log plot, meaning both axes are divided into sections of magnitude 10
Wave types are: capillary, chop, swell, seiche, tsunami and tide.
Capillary waves very short, less than 10cm high.
Chop waves are everything between capllary waves up to ca 5000 cm, or 50 m high.

 

Properties of Ocean Waves
Wind Generation of Waves

The type of wave generated by wind is determined by:
Wind velocity
Wind duration
Fetch (distance over which wind blows)
Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.

Wave Motions

Progressive waves are waves that “move” forward across a surface.

As waves pass, wave form and wave energy move forward, but not the water.
Water molecules move in an orbital motion as the wave passes.
Diameter of orbit increases with increasing wave size and decreases with depth below the water surface.
 

Now let’s talk about the motion of waves.
Progressive waves move forward across a surface.
Movement of a wave is not a movement of mass but a movement of energy, or a ‘wave form’  (do the rope demo).
This type of progressive wave motion causes circular or elliptical motion on particles at the surface and beneath the surface of the wave.

 

 

Progressive Wave Types

Sea – irregular waves in the area of generation
Swell – more regular waves beyond area of generation
Surf - waves that have reached the coast, grow in height, and break

Orbit Diameter and Stokes Drift

Wave Motions

Here is a picture of a wave passing.
Notice the wavelength, wave height.
As the wave progresses, a particle on the surface remains fairly stationary with respect to forward motion.
The surface particle describes a ciruclar orbit whose diamter is equal to the wave height.
Deep- and Shallow-Water Motion

Wave base is the depth to which a surface wave can move water.
If the water is deeper than wave base:
orbits are circular
no interaction between the bottom and the wave.
If the water is shallower than wave base
orbits are elliptical
orbits become increasingly flattened towards the bottom.

 

So here is a comparison between wave motion in deep water versus wave motion in shallow water.
Deep water, orbit of particles is circular. Diamter of circle is equal to the wave heigh and diamete of orbit decreases as you approach the base of the wave. Below this no more energy is felt. The wave base is about one half the wavelength.
In shallower water, the particle’s orbits are flattened circles (ellipses) and become increasingly elliptical as you approach the seafloor. The motion of particles is more back and forth rather than up and down, and become nearly back and forth on the ocean bottom. This can result iinsignificant disturbance of oceans sediments and resuspension of sediments into water column.
This type of particle motion isfound when the wave is in water that is shallower than ½ the wavelength.

There are three types of waves defined by water depth

 

Deep-water wave (d>or=1/2 of L)
Intermediate-water wave (d>1/20 and <1/2 of L)
Shallow-water wave (d<or= 1/20 of L)
Celerity is the velocity of the wave form and not of the water.
Celerity equations.

There are three types of waves defined by the water depth.
Deep water wave, intermediate and shallow wave.  We will talk usually about the two extremes, shallow and deep and not intermediate. Just keep in mind that these represent two extremes of a gradient and between these extremes there is waves that have characteristics that are the hybrid of the two.
This is also a relative definition. You can have deep water waves in a few inches of water and you can have shallow water waves in the deep ocean, if the wave high gets big enough

 

 

Flow of Energy,not Mass

Life History of Ocean Waves

Fetch is the area of contact between the wind and the water and is where wind-generated waves begin.

seas inside fetch area

Swells:  wave found outside the fetch.

Progressive Wave Types
Sea
Surf
Swell
 

Sea – irregular waves in the area of generation
Swell – more regular waves beyond area of generation
Surf - waves that have reached the coast, grow in height, and break

 

 

 

 

above graph shows a fetch area with chaotic seas. Once outside the fetch area the wave form patterns become more organized into swells as the longer wavelength waves outrun the shorter wavelength waves.
Eventually the waves in this area outside the fetch would become shorter ands shorter, as the slower waves move through
 

Wave interference is the momentary interaction between waves as they pass through each other. Wave interference can be constructive or destructive.
 

The waves you observe at the surface are the net result of wave interfreence, or the momentary interaction of waves as they pass through one another.
Wave interference can be constructive or destructive.
Constructive wave interference:  two or more waves whose waveforms lineup perfectly, crest to crest, trough to trough, such that the resulting wave is the sum of the heights.
Destructive interference: two or more wates whose wave forms line up out of phase with one another such that the wave heights of the crest of one is canceled out by the shorter wave height of the other’s trough.
More often there is some intermediate situation between the two, resulting in a waveform that is complex.
When you are in the fetch area, the energy is not consistent, and the wave forms produced have a large variety of wavelengths, periods and heights. This will result in complex wave interference and a chaotic sea.

Life History of Ocean Waves
 

Transformation : Dispersion
 

Dispersion: Gradual separation of wave types based on their relative wavelengths and speeds
Because celerity increases as wavelength  increases:
-long waves travel faster than short waves.
-This causes dispersion outside of the fetch and regular ocean swell.
 

Transformation : Shoaling
 

Here’s a graphic that is not in the text.
When waves reach shallow waters where the depth is less than ½L ,then the wave begins to “feel” the bottom. When the depth is less than 1/20 L, the circular orbits of the particles become flattened or elliptical and the water movement becomes more back and forth.
When the wave is in shallow water a number of changes occur tot eh shape of the wave. The wave H increases, wavelength L decreases and celerity decreases.
 

Transformation : Shoaling
 

The shallower the water, the greater the interaction between the wave and the bottom alters the wave properties, eventually causing the wave to collapse.
 

Wave speed decreases as depth decreases.
Wavelength decreases as depth decreases.
Wave height increases as depth decreases.
Troughs become flattened and the wave profile becomes extremely asymmetrical.
Period remains unchanged. Period is a fundamental property of a wave.
Celerity equation of shallow water wave.
 

To review, the wave speed decreases
Wavelength decreses
Wave height increases
Period remains unchanged. Period is a fundemental property of a wave.
Wave profile becomes very asymmetrical
 

Transformation : Refraction
 

Wave refraction is the bending of a wave crest into an area where it travels more slowly.

 

 

 

 

 

 

Now let’s look at the form of the wave from above, as it approaches land.  Due to the decrease in wave celerity as a wave moves into shallow water.
Bathymetry is not equal everywhere, as you approach land the bottom depth is very variable and this has influence on the way the energy of waves is dissipated onto land.
Wave orthogonals are lines at right angles to the direction of wave crest. Bending of orthogonals is referred to refraction and indicates focussing or dispersion of wave energy as waves approach shore
 

Transformation : Breaking
 

Wave steepness is a ratio of wave height divided by wavelength (H/L).
 

In shallow water, wave height increases and wave length decreases.
When H/L is larger than or equals 1/7 (H/L ³ 1/7), the wave becomes unstable and breaks.
There are three types of breakers: spilling breakers, plunging breakers, and surging breakers.
 

How do waves break?
Wave break when wave heigh is 1/7 the wavelength.
This is the wave Steepness, the ration of H/L. When H/L>1/7, wave becomes unstable and breaks.
Three types of breakers
 

Transformation : Storm Surge
 

Storm surge is the rise in sea level resulting from low atmospheric pressure and the accumulation of water driven shoreward by storm winds.
 

Water is deeper at the shore area, allowing waves to progress farther inland.
Storm surge is especially severe when superimposed upon a spring high tide.

 

 

 

 

Resonance amplifies the displacement at the nodes and occurs when the period of the basin is similar to the period of the force producing the standing wave.


 

 

Standing waves or seiches consist of a water surface “seesawing” back and forth.

Node : The line about which the surface oscillates.
Located in centers of enclosed basins and toward the seaward side of open basins.
Antinodes: Points where there are the maximum displacement of the surface as it oscillates.
Antinodes usually located at the edge of the basin.

Geometry of the basin controls the period of the standing wave. A basin can be closed or open.
Standing waves can be generated by storm surges.
Resonance amplifies the displacement at the nodes and occurs when the period of the basin is similar to the period of the force producing the standing wave.

Standing wave

A first order seiche. The undisturbed sea level is indicated by the broken yellow line. Three water particles are shown as an indication of water movement in the seiche.
Note the node in the centre and that water under the node moves only horizontally, while water at both ends of the basin moves vertically.

A first order seiche in a basin open to the deep ocean. The undisturbed sea level is indicated by the broken yellow line. Two water particles are shown as an indication of water movement in the seiche.
Note the node at the connection of the basin to the deep ocean, and that water under the node moves only horizontally, while water at the end of the basin moves vertically

Internal waves form within the water column on the pycnocline.

Because of the small density difference between the water masses above and below the pycnocline, wave properties are different compared to surface waves.
Internal waves display all the properties of surface progressive waves including reflection, refraction, interference, breaking, etc.
Any disturbance to the pycnocline can generate internal waves, including: Flow of water related to the tides., Flow of water masses past each other, Storms, or Submarine landslides.

Internal Waves

An internal wave propagating on the interface between two layers. The undisturbed sea level is indicated by the yellow line. Water particles are shown as yellow and magenta dots. Yellow dots sit in the middle of the water column and move only up and down. Magenta dots sit at the top and bottom of the water column and move only in the horizontal.

By watching a yellow dot you can see how a water particle in the middle of the water column moves up and down, but does not move horizontally, as the wave passes through.

 

Internal waves form within the water column along the pycnocline.

 

Tsunamis were previously called tidal waves, but are unrelated to tides.
 

Tsunamis consist of a series of long-period waves characterized by very long wavelength (up to 100 km) and high speed (up to 760 km/hr) in the deep ocean.
Because of their large wavelength, tsunamis are shallow-water to intermediate-water waves as they travel across the ocean basin.
They only become a danger when reaching coastal areas where wave height can reach 10 m.
Tsunamis originate from earthquakes, volcanic explosions, or submarine landslides.
 

Generation of a Tsunami
 

Generation of a Tsunami
 

Tsunami speed & ocean depth:
“The speed at which tsunamis travel depends on the ocean depth.
§
Tsunamis are shallow-water waves, which means that the ratio between water depth and wavelength is very small. These shallow-water waves move at a speed equal to the square root of the product of the acceleration of gravity (9.8m/s/s) and the water depth.
§
Notice how the waves become larger as they reach shore, where the water is shallower. When the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and the wave height increases (shoaling effect). It is in these shallow waters that a large tsunami can crest to heights exceeding 100 feet (30 m) and strike with devastating force.
A tsunami can exceed 500 mph (and a period of about one hour) in the deep ocean but slows to 20 or 30 mph in the shallow water near land. In less than 24 hours, a tsunami can cross the entire Pacific Ocean.
In summary,
The deeper the water, the faster the wave is, vise versa.
Shoaling effect results in a tsunami that was imperceptible in deep water may grow to be several feet or more in height in the coastlines.”
Citation source:http://www.owlnet.rice.edu/~esci108/108_EQ_Lec_4.ppt
 

What Is a Rogue Wave?

A “rogue wave” is typically used to describe a wave that is significantly larger than other waves during a given time, at a given location. Several factors can create rogue waves. Multiple waves can intersect to create a much larger wave- up to the sum of the heights of the waves that combine. Additionally, currents can contribute to the formation of rogue waves. And rogue waves can result as a normal part of the wave spectrum.  In other words, waves are not created individually but rather, in large groups. And a wave(s) within this group can be significantly larger than the rest. Rogue waves have been witnessed all around the world. The North Sea, Gulf of Alaska, S. African coasts are infamous for producing rogue waves- some that have approached 100 feet in height

 

 

 

 

 

تعريف ناحية ساحلي
 

*از نظرجغرافيايي: ساحل منطقه وسيعي از خشكي و دريا است كه در آن، عوامل مختلف خشكي و دريا با يكديگر در تعامل بوده و شرايطي را ايجاد مي‏كنند كه با هر يك از مناطق خشكي و دريايي متمايز است.
* از نظر مهندسي سواحل: منطقة ساحلي از پشت تلماسه‏ها (در سواحل ماسه‏اي) و يا پرتگاه (در سواحل صخره‏اي) شروع شده و تا منطقه شكست امواج ادامه مي‏يابد.
* از ديدگاه حقوقي: اراضي ساحلي منطقه‌اي است كه به عرض دو كيلومتر از بالاترين مد دريا در طول ساحل شروع شده و تا عمق 6 متر از پايين ترين تراز جزر در داخل دريا ادامه دارد.

 

 

 

 

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  1. نعیمه نعیمی
    آوریل 30, 2012

    این مقاله بسیار عالی بود.من دانشجوی ارشد زمین شناسی زیست محیطی هستم ومطالب این مقاله جزیی از درس های اصلی من هست و من بسیار به آن علاقه دارم.

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  2. سعيد اكبري
    دسامبر 25, 2011

    خوبه

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  3. سعيد اكبري
    دسامبر 25, 2011

    خوب بود

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