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Coastal Erosion Lesson

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Coastal Processes

The coast is a dynamic environment, and is subject to many processes, including waves, tides, currents, storms, wind, subsidence, and the rise of sea level due to global climate change. Natural disasters may be especially severe in this area as a result.

A huge population lives in coastal areas. Half of tshe US population lives within 50 miles of the ocean or the Great Lake. Millions of people are at risk from various coastal erosion problems or natural disasters that affect the coast.


Waves

ALL waves are the movement of energy through a medium. They transmit energy, not water mass; the water is not actually moving. Energy transferred from one water particle to another in orbits. This is what causes the wave form to move. Waves of this nature are called orbital waves. Orbital waves occur between two fluid media (like the air and the water). They are also progressive waves – waveform moves forward.


Wave Anatomy (true for all waves)

Crest . . . . . . . . . . . . . . . . . . . .
Trough. . . . . . . . . . . . . . . . . . .
Wave Height . . . . . . . . . . . . . .
Wavelength . . . . . . . . . . . . . . .
Amplitude. . . . . . . . . . . . . . . . .
Baseline. . . . . . . . . . . . . . . . . .
Period . . . . . . . . . . . . . . . . . . .
Frequency . . . . . . . . . . . . . . . .
Steepness . . . . . . . . . . . . . . .
Velocity = wavelength/T . . . . . .
Highest part of a wave
Lowest part of a wave
Vertical distance from crest to trough
Distance from Crest to Crest
Distance from the Crest or Trough to the baseline
aka “still water” line
Time required for a wave crest at point A to reach point B
Number of waves which pass point A per second
Ratio of height of wave to its wavelength (Height/wavelength)
Example:
Given T=10 sec; wavelength = 100m
V=wavelength/T = 100m/10 sec = 10m/sec


Wavelength determines the size of the orbits; Water depth determines the shape of the orbits. There are two broad categories:

Wave development

Most ocean waves wind-generated. Wave size depends on the fetch, the wind velocity, and the duration of wind.

As the wind begins to blow, energy is transfered from the air to the water, the surface begins to ripple. All waves start as capillary waves, very small waves less than 1.73 cm high. The wind pushes the water up, and gravity pushes it down. The wind pushes the wave forward, so the crest becomes a trough. This is what causes circular orbits of individual water molecules. So capillary waves are also forced waves as the waves are dependent upon the force that creates them to exist.





As the wind continues to grow, larger waves are formed. Sometimes the wind blows very strongly over a short distance; when this happens the wave becomes too tall and will break upon itself. Wave steepness is the ratio of the wavelength to height; waves with a ratio greater than 1:7, the wave will break and excess energy dissipated as turbulence. These waves are referred to as "white caps" and are an indication that a storm may be approaching.


Waves Approaching a Shore

In the open ocean, the wave train begins to move towards the shore. So long as the wave is moving through water (1) that is greater than 1/2 their wavelength the orbits will remain circular (2). However, the circular motion of water molecules is interrupted and the wave slows as water becomes more shallow. Energy is no longer disappated with depth, so the energy goes up and the wave becomes too high for its wavelength. The water at the top of the wave (3) is now moving faster than that at the bottom of the wave. The orbits are still trying to form, and the wave breaks (4) and forms the surf (5) as it rushes on shore.






Waves help to erode the shoreline by physically pounding on the rocks and sediments that make up the coastline. Waves do not always travel in a straight line - they can bend in response to changes in the coastline or other factors.


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