Beaches - Keynote pdf
Sand - Keynote pdf
Longshore Drift and Groins - Keynote pdf
Seawalls and Beach Loss - Keynote pdf
White sand beaches are the most type of beaches in Hawaii.
White sand is biogenous marine
sediment.
Biogenous sediment forms from the hard parts of organisms.
The chemical composition of white sand is CaCO3.
Geologist use acid to test for CaCO3.
CaCO3 effervesces in acid.
These organisms live on the reef flats, and, when they die, waves roll their
hard parts towards the shoreline.
Most sand particles come from
Clams are an example of a bivalve and Opi'hi are monovalves.
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:
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.
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.
Beaches allow the waves to slow down and deposit sand.
People used to build their dwellings back from the beach, because big wave can cross the beach and flood the forebeach area.
However eventually the beach will recede inland.
People build seawalls to protect property.
The construct of seawalls eventually leads to the destruction
of the beach.
One day, big waves will cross the beach, strike
the seawall going fast, reflect, and return to ocean going fast.
The return flow prevents the deposition of sand and results
in erosion.
The notes for the second exam are continued in Climate