Physical Geography: Coasts
The coastal System
- The coast is considered an open system as it receives inputs from
outside the system and transfers outputs away from the coast and into
other systems; these systems can be atmospheric, oceanic or terrestrial.
Sediment Cells
- Sediment cells are used to split the coast into sections and are often
bordered by prominent headlands.
- Within them, the movement of sediment is almost contained and the
flows of sediment act in dynamic equilibrium (meaning inputs and
outputs are in a constant state of change but remain in balance).
- For a sediment cell, the dynamic equilibrium may be upset in the long
term by human interventions, or in the short term by natural variations.
- Within sediment cells are smaller subcells; these are often used when
planning coastal management projects.
Inputs refer to material or energy; coastal inputs are not limited but include
three main areas:
- Marine: waves, tides, salt spray
- Atmosphere: sun, air pressure, wind speed and direction
- Humans: pollution, recreation, settlement, defences
Energy refers to the power and driving force behind the transfers and flows in
the system:
- Wind
- Gravitational
- Flowing Water
Feedback Loops
A negative feedback loop:
1. When destructive waves from a storm lose their energy, excess
sediment is deposited as an offshore bar
2. The bar dissipates the waves energy which protects the beach from
further erosion
3. Over time the bar gets eroded instead of the beach
4. Once the bar has gone, normal conditions ensure and the system returns
to dynamic equilibrium
A positive feedback loop:
1. People walking over sand dunes destroys vegetation growing there and
causes erosion
, 2. As the roots of the vegetation have previously been holding the dune
together, damaging the vegetation makes the sand dunes more
susceptible to erosion; this increases the rate of erosion
3. Eventually the sand dunes will be completely eroded, leaving more of
the beach open to erosion taking the beach further form its original
state
Sediment Sources
Rivers:
- Most of the sediment in the coastal zone is a result of an input from
rivers, especially in high rainfall environments when significant river
erosion occurs. This sediment may then be deposited in estuaries which
are brackish areas where rivers flow into seas; they are important
wildlife habitats. This sediment is then transported throughout the
coastal system by waves, tides and currents.
Cliff Erosion:
- Cliff erosion is prominent in areas with unconsolidated cliffs, as their
instability means they are quickly eroded; some coastlines can retreat
by up to 10 meters per year, providing a significant sediment input.
Most erosion occurs during winter months due to more frequent
storms.
Wind:
- The wind is a coastal energy source and can cause sand to be blown
along or up a beach; sediment transport by wind may occur where there
are sand dunes or in glacial and desert environments, providing
sediment inputs.
Glaciers:
- In some coastal systems such as in Antarctica, Greenland, Alaska and
Patagonia, glaciers flow directly into the ocean depositing sediment
previously stored inside them; this occurs when glaciers calve, a process
where ice breaks off the glacier
Longshore Drift:
- Sediment is moved alone the beach by prevailing winds which alter the
direction of waves, allowing sediment to be transported from one area
of coastline to another; the swash approaches the coast at a 45 degree
angle. The backwash then pulls the sediment back at a 90 degree angle,
down the beach – the swash then transfers sediment alone the coastline
and repeats the process.
The coastal System
- The coast is considered an open system as it receives inputs from
outside the system and transfers outputs away from the coast and into
other systems; these systems can be atmospheric, oceanic or terrestrial.
Sediment Cells
- Sediment cells are used to split the coast into sections and are often
bordered by prominent headlands.
- Within them, the movement of sediment is almost contained and the
flows of sediment act in dynamic equilibrium (meaning inputs and
outputs are in a constant state of change but remain in balance).
- For a sediment cell, the dynamic equilibrium may be upset in the long
term by human interventions, or in the short term by natural variations.
- Within sediment cells are smaller subcells; these are often used when
planning coastal management projects.
Inputs refer to material or energy; coastal inputs are not limited but include
three main areas:
- Marine: waves, tides, salt spray
- Atmosphere: sun, air pressure, wind speed and direction
- Humans: pollution, recreation, settlement, defences
Energy refers to the power and driving force behind the transfers and flows in
the system:
- Wind
- Gravitational
- Flowing Water
Feedback Loops
A negative feedback loop:
1. When destructive waves from a storm lose their energy, excess
sediment is deposited as an offshore bar
2. The bar dissipates the waves energy which protects the beach from
further erosion
3. Over time the bar gets eroded instead of the beach
4. Once the bar has gone, normal conditions ensure and the system returns
to dynamic equilibrium
A positive feedback loop:
1. People walking over sand dunes destroys vegetation growing there and
causes erosion
, 2. As the roots of the vegetation have previously been holding the dune
together, damaging the vegetation makes the sand dunes more
susceptible to erosion; this increases the rate of erosion
3. Eventually the sand dunes will be completely eroded, leaving more of
the beach open to erosion taking the beach further form its original
state
Sediment Sources
Rivers:
- Most of the sediment in the coastal zone is a result of an input from
rivers, especially in high rainfall environments when significant river
erosion occurs. This sediment may then be deposited in estuaries which
are brackish areas where rivers flow into seas; they are important
wildlife habitats. This sediment is then transported throughout the
coastal system by waves, tides and currents.
Cliff Erosion:
- Cliff erosion is prominent in areas with unconsolidated cliffs, as their
instability means they are quickly eroded; some coastlines can retreat
by up to 10 meters per year, providing a significant sediment input.
Most erosion occurs during winter months due to more frequent
storms.
Wind:
- The wind is a coastal energy source and can cause sand to be blown
along or up a beach; sediment transport by wind may occur where there
are sand dunes or in glacial and desert environments, providing
sediment inputs.
Glaciers:
- In some coastal systems such as in Antarctica, Greenland, Alaska and
Patagonia, glaciers flow directly into the ocean depositing sediment
previously stored inside them; this occurs when glaciers calve, a process
where ice breaks off the glacier
Longshore Drift:
- Sediment is moved alone the beach by prevailing winds which alter the
direction of waves, allowing sediment to be transported from one area
of coastline to another; the swash approaches the coast at a 45 degree
angle. The backwash then pulls the sediment back at a 90 degree angle,
down the beach – the swash then transfers sediment alone the coastline
and repeats the process.
