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Wave Refraction - Keynote pdf
Sand - Keynote pdf
Longshore Drift - Keynote pdf
Seawalls and Beach Loss - Keynote pdf
Shoreline Features - Keynote pdf
Tsunamis - Keynote pdf

Coastal Geology

Wave Refraction

Wave refraction is the bending of waves which causes waves to change direction.
Wave refraction occurs when different parts of a wave travel at different speeds.

Wave refraction generally occurs because waves initially approach a beach at an angle.
Waves refract when one part of a wave enters shallow water and slows, whereas the rest of the wave still is in deeper water and traveling faster.

Waves tend to refract parallel to shore.

Wave Refraction and Irregular Coastlines

An irregular coastline has many headlands and bays.
Examples of headlands are Diamond Head and Koko Head.

Waves enter shallow water in front of a headland and slow.
The parts of the wave on either side of a headland (those entering the adjacent bays) still are in deep water and traveling fast.
Waves tend to refract towards headlands.
Wave energy is concentrated on points of land that extend into the ocean.
Therefore headlands tend to be areas of high wave energy and erosion.
Most headlands are rocky points as sand and small rocks are washed away.

The part of a wave that enters the center of a bay still is in deep water and traveling fast.
The parts of the wave on either side of a bay (those striking the adjacent headlands) are in shallower water and traveling slower.
Wave tend to disperse in bays.
Wave energy is dispersed in bays as waves spread throughout the entire bay.
Therefore bays tend to be areas of low wave energy and deposition.
Beaches tend to form in bays where the waves are smaller.

Sand

Several different types of beaches form in Hawaii.
Along with the most common type of beach sand, white sand, both black sand and green sand beaches form.

There are two types of black sand beaches:

  1. volcanic glass
  2. basaltic rock

Black volcanic sand beaches are made of Fe, Mg  silicate glass.
They form when basaltic magma flows into the ocean.
The vaporization of seawater blows the magma apart.
The magma cools instantaneously to form shards of volcanic glass.
Wave action rounds the glass shards.

Black basaltic rock beaches form when waves breakup coastal rocks.
Both of these types of sand come from the land.

Green sand beaches are made of crystals of an Fe, Mg silicate mineral, olivine.
Olivine is one of the most common minerals in basaltic rocks.
The olivine crystals can weather out of basalt and collect to form green sand beaches.
Green sand comes from the land.

The most common type of beach in Hawaii is made of CaCO3 (calcareous sand).
This sand comes from the ocean and forms primarily from organisms that live on the reef flats.
Primary organisms that contribute sand particles:

Longshore Drift

Longshore drift is the transport of sand along a beach face.
Sand generally flows from one end of a beach to the other, which explains why many beaches are thinner at one end.

Remember that when waves break water is moving in the direction of wave propagation.

Waves that approach a beach at an angle run up the beach face at an angle.
The water moves sand up the beach face at an angle, too.
After the wave stops, gravity pulls the water and the sand straight back into the ocean.

This pattern is repeated with each subsequent wave.
The result is that sand moves in a zigzag pattern along the beach, moving in the direction opposite to the direction that the waves approach from.

Longshore Drift and Shoreline Structures

If a structure along the shoreline, such as a groin, impedes the transport of sand, sand will collect (deposition) on one side of the structure.
On the opposite side of the structure sand starvation (erosion) occurs.
The direction of the waves determines which side undergoes deposition or erosion.
Sand builds up on the side of the structure that waves approach from.

Seawalls and Beach Loss

Beaches absorb the energy of waves.
Structures should be contructed away from the shoreline to allow room for erosion and flooding by large storm waves.
Regardless, erosion and the sinking of the islands results in the shoreline contacting structures.
So people construct seawalls to protect property; however, the seawall generally results in beach destruction.

If a wave strikes a relatively vertical structure along the shoreline, such as a seawall, it can reflect back into the ocean.
Seawalls tend to destroy beaches, which is why the construction of most seawalls is illegal.

Once the shoreline erodes to the point where the small, Trade Wind waves hit a seawall, generally sand will no longer deposit there.
The waves hit the seawall, reflect, and go back fast in to the ocean.
This fast moving wave eroded sand from the shore.
Beach cannot reform.

Shoreline Features

Waves erode by

  1. Abrasion
  2. Hydraulic action

Abrasion

Particles along the shoreline are moved by wave action which abrades the rocky coastlines.

Hydraulic Action

Waves force water into cracks and caves along the shoreline.
The water exerts pressure on the walls of the void which eventually breaks a rocky shoreline apart.

The attack of the waves is

  1. Directed horzontally
  2. Concentrated within a few meters of sea level

As the waves cut into a rocky coastline, a wave-cut terrace forms.
At the shoreward end of the wave-cut terrace, at sea cliff forms.
At the base of a sea cliff, the waves cut a wave nip.
As the wave nip enlarges, the face of the sea cliff is undercut and collapses.
Wave erosion makes sea cliffs migrate inland.

Waves erode a weak area of a headland to for a sea cave.
The sea cave can erode through the headland to form a sea arch.
With further wave erosion, the roof of the sea arch can collapse to for a sea stack.
The most famous sea stack on O'ahu is Moko li'i (Chinaman's Hat)

Eventually wave erosion will remove the entire subaerial part of each Hawaiian Island.

Tsunamis

Most tsunamis are seismic sea waves that are generated by earthquakes waves (seismic waves).
However, most earthquakes do not cause tsunamis.
To generate a tsunamis, the seafloor must fault (fracture and move), which moves seawater.
The movement of water by the faulting seafloor is the disturbing force.

The wavelength of tsunamis waves can be as long as 100 km.
However, in the open ocean, the wave heights of most tsunami waves are 1 m or less.
The wave period of tsunami waves average 12-15 minutes.
Tsunami wave speeds can be as high as 500-700 km/hr.

Most tsunamis waves are like a flood coming ashore.
Commonly a tsunami bore forms where the top of the wave breaks free, forming a wall of water separating two different levels of seawater.

The highest elevation that the wave reaches as it washes inland is called tsunami runup.

Historically, large tsunamis strike Hawaii about every 25 years.
The last two large tsunamis hit Hawaii in 1946 and 1960, so Hawaii residents should be prepared for a tsunami in the near future.

The 1946 tsunamis was generated by an earthquake in the Aleutian Islands, and the 1960 tsunamis was generated by an earthquake in Chile.

End of the final exam notes

 

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