Continuation of the notes for the sixth exam
Greenhouse Effect
Wave Refraction
Sand
Beaches
Longshore Drift
- Keynote pdf
Seawalls and Beach Loss
- Keynote pdf
Shoreline Features
- Keynote pdf
The sun radiates most of its energy as visible light.
Most visible light passes through the atmosphere.
Approximately 70% of the solar radiation is absorbed by Earth's surface
and lower atmosphere.
As Earth's temperature is relatively low, averaging 18oC, most absorbed
radiation is reradiated as IR radiation.
Over geological time, inputs of solar radiation = outputs of solar radiation.
Atmospheric gases absorb IR radiation and hold this energy
in the atmosphere.
The gases are called greenhouse gases.
The primary greenhouse gases are H2O and CO2.
The concentration of CO2 in the atmosphere is increasing, >30% in recent history.
This increase results primarily from the burning of fossil fuels: coal, oil,
gas, etc.
Increasing CO2 levels results in more IR radiation trapped in the
atmosphere.
More energy trapped in the atmosphere should lead to higher atmospheric temperatures.
The phenomenon of increasing atmospheric temperatures is called global warming.
The many detrimental effects of rapid global warming include the melting of the polar ice caps and the flooding of the nearshore environment; significant changes in weather patterns; ; and increased rates of extinction due to habitat loss.
End of the notes of the sixth quiz
Start of the notes for the seventh quiz
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.
Beaches changes seasonally.
Large winter waves move sand offshore.
Winter beach profiles are narrow and steep.
Small summer waves move sand towards the shore
Summer beach profiles are wide and less steep.
Large waves stir the ocean deeper than small waves.
Wave stir the ocean to 1/2 their wavelength.
Winter wave transport some sand into water too deep for summer waves to return
to the shore.
Beaches undergo annual sand loss.
For beaches to maintain a relatively constant state, there must be inputs
of sand to balance the lost of sand (outputs).
Over geological time inputs
= outputs.
Most of Hawaii's beaches come from the ocean.
Therefore a constant supply of sand requires a health reef system.
Reefs require clean, clear, warm water.
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:
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:
End of the notes for the seventh quiz
Start of the notes for the final
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.
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.
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.
Waves erode by
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
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.
The notes for the final exam are complete