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Coasts Overview

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by

Claire James

on 11 August 2014

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Transcript of Coasts Overview

Coastal Regions
A coastal region extends inland to the limit of the salt water, sea spray, and wind-blown sands, and seaward to where the waves are responsible for the movement of material on the seabed.
Physical and environmental factors
- Coastal landforms are largely formed as a result of the geographical structure, the direction, and the strength of waves.
- Rock composition also influences differences in landforms. Different types of rock are more resistant to erosion, such as Basalt.
- The type of rock and its jointing pattern can affect the rate of erosion and the shape of coasts. Cracks or joints in rocks are lines of weakness which waves can enter and erode the rock.

- Factors that add to the geology in determining the coastal soils, vegetation types, and landforms include: the slope of the land, the direction it faces, and the degree of exposure of rocks to the waves. These factors contribute to various coastal regions having distinctive characteristics.
- Uplift and subsidence along fault lines are also major factors in changing coasts.
o Subsidence is when the Earth’s surface shifts downwards relative to sea level.
o Uplift is when there is an increase in elevation.

Climate and weather
- Winds determine wave direction and strength.
- Gale-force winds, severe storms with high tides, may cause severe coastal erosion, depleting beaches and this in turn may threaten any present buildings.

Weathering and Erosion
- Weathering is the breakdown of rocks into smaller particles by chemical reactions or physical forces.
- These rock fragments may weather further and eventually crumble into tiny particles of gravel, sand, and silt.
- Some of this material is carried away by wind, wave action and runoff.
- Erosion is the removal or transport of these articles.
- Erosion may be slow or rapid depending on the hardness of rocks present, and the intensity of the erosion process.

Erosion processes
- Abrasion or corrosion is a mechanical or physical process where sand is used as a tool of erosion. Large stones and pebbles can be carried by waves and when hurled at cliffs create cliff retreats.
- Attrition is a mechanical process where sand grains and pebbles rub against each other and reduce in size.
- Hydraulic action is erosion caused by the force of moving water. It occurs when waves are forced into cracks and compress the air. When the water flows out again, the trapped air suddenly expands with a force which is capable of shattering the surrounding rock.
- Sub-aerial erosion is when rain water runs down the side of a cliff. The water wears away small grooves called rills. Over time these rills can become gullies.
- Mass movement is when cliffs give way and cause landslides.

Year 10 Geography
Coasts Overview
Landforms caused by erosion
The shape of coasts depends upon geology, climate, and wave exposure. Cliffs and bluffs are caused when waves attack the land.

When land slopes steeply into the water, the water undercuts the shore and deposits the material beneath water level. With continued erosion the platform widens.

A beach may form because waves are slowing down and depositing rather than eroding.

If a platform widens far enough the force of the waves no longer affect the cliff and equilibrium occurs. Equilibrium is a state of balance.

Caves are formed when there is a line of weakness at the base of the cliff, and this line is exposed to waves. Water erodes this line of weakness and a tunnel is formed. If there is a weakness from above the tunnel to the top of a cliff a blowhole may form.

If waves attack a headland from both sides an arch may form. If the arch collapses a stack is formed.

Headlands and bays occur when there are alternating resistant and less resistant rocks. The softer rock erodes first, and a bay is formed. The more resistant rocks remain and headlands are formed.

Deposition
Deposition is the when material is transported by water from one location to another and deposited.

Sand deposited by waves form beaches, spits, and barriers.

Longshore drift is the movement of sediment along the beach due to wave action.

Beaches are regions where material is in constant motion.

Spits
are long, narrow accumulations of sand, where one end is joined to the mainland and the other projects into a body of water. Spits are the result of the accumulation of material delivered by longshore drift behind an obstacle. They may also form where the coastline abruptly changes direction, such as at an estuary.

Sandbars
are submerged depositional features caused by near shore currents.

Barrier islands
are long, narrow, offshore islands built by wave action and are separated from the mainland by a body of water. They are called bars when they are submerged.

Coastal sand dunes
are large accumulations of sand on the inland side of beaches. They provide a vital buffer zone for the coastline from destructive storm waves. Dunes are formed when onshore winds blow sands inland. The sand is trapped by vegetation.

Wave Action
Waves, tides, and currents continuously change coastal places.
Waves are created by the transfer of energy from winds blowing over the ocean surface. Waves can erode or deposit material on beaches.

The two kinds of waves that affect most coasts are:

Swell waves
which are produced by storms in the ocean.

Local waves
which are generated by local winds and are confined to a section of the coast.

An understanding of wave action is essential for coastal management such as recognising the need for sea walls, harbours, and planning for erosion control and beach safety.

Tides
Tides are the alternating rise and fall of the surface of the sea.

The tidal range is the vertical distance between high and low water.

In Victoria the average tidal range is between 1 and 1.5 metres.

King Tide is when the tidal range is up to 3 metres.

When tidal water moves inwards it is the
flood tide
and when the tidal water moves outwards is the
ebb tide
.

Sea Level Change
In the short term tides and storm surges can change sea levels on a small scale.
In the long term coastal features such as raised beaches, fjords and rias, depending
upon whether the sea level has risen or fallen.

*A fjord is a long, narrow inlet with steep sides/cliffs.
* A rias is an estuary at the mouth of a river where a valley has flooded.

Climate and earth movements affect global sea levels. Twenty thousand years ago the seal level was
100 metres below what it is presently, and a land ridge connected Tasmania to Victoria.

Coastal landforms and deposition
Deposition occurs where sand build up more quickly than it is eroded.

This usually happens in the following conditions:
- In sheltered areas with low energy waves.
- Where longshore drift or rivers move and deposit sediment.
- Where waves move sediment from offshore banks onto the shore.
- When the wind blows sand to the back of the beach to form dunes.

The global average is 530 tonnes per second of sediment is carried into the sea by rivers. This sediment helps to create rich and flat deltas at the mouth of rivers and alluvial plains.

*A delta is the mouth of a river where it flows into the ocean, sea, estuary, lake, or reservoir.
* Alluvial plains are the flat land areas adjacent to streams and rivers.

Beaches
In Australia there are 10685 different beaches according to the Coastal Studies Unit at the University of Sydney.
A beach is a collection of loose material along the coast. Beaches form where the waves are constructive and the material is able to stay in place.

Backshore
is only covered by water during the highest tide.
Foreshore
is covered and uncovered daily by the tides.
Offshore
is below the level of low tide.
Berm
is a narrow ledge or ridge of material deposited at the back of the beach by the highest tides.
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