Coasts

The coast as a system

Open system, sediment is transferred across the boundary of the system

The creation of waves

Transfer of energy from the wind to the sea surface

The energy of waves depends on

Strength of the wind, length of time it is blowing, and the distance it is blowing over

Fetch

The distance that the wind blows over the sea surface

Wave breaking cycle

Shallower water slows down the base of the wave, causing the wave to grow in height, until the top part plunges forward and breaks onto the beach

Swash

The forward movement of the wave

Backwash

The backwards movement of the wave

Formation of constructive waves

Light distant winds in the open ocean

Wave form of constructive waves

Long wavelength with low height

Wave break of constructive waves

Strong swash with weak backwash

Impact on beach with constructive waves

Beach gain creating a wide, flat beach

Formation of destructive waves

Strong winds and local storms

Wave form of destructive waves

High crashing waves with a narrow wavelength

Wave break of destructive waves

Weak swash and strong backwash

Impact on beach of destructive waves

Loss of beach leaving a steep and narrow beach profile

Solution (erosion)

Chemical reaction between sea water and certain types of rocks, dissolving them

Hydraulic action / power

Shear force of the wave hitting the rocks

Attrition

Rocks hitting against one another, breaking them down into small sediment

Abrasion

Pieces of rock are picked up by waves and thrown against the cliff

Rockfall

Material breaks off of the cliff and rolls down it

Slump

Material moves down the slope with rotation, permeable rock on top of impermeable rock

Slide

Material moves down a slope in a straight line

Solution (transportation)

Dissolved sediment is carried in the water

Suspension

Small sediment is carried within the flow of the water

Saltation

Smaller sediment is bounced along the sea bed

Traction

Large sediment is dragged along the sea bed by the currents

1 - Wave cut platform

The wave hits the cliff between high and low water marks

2 - Wave cut platform

The water erodes the cliff by hydraulic action and abrasion only between the high and low water marks

3 - Wave cut platform

The erosion creates a wave cut notch which undercuts the overhang / cliff and continually grows

4 - Wave cut platform

The overhang collapses under its own weight causing the cliff to retreat

5 - Wave cut platform

The process repeats and restarts, hitting the newly exposed rock

6 - Wave cut platform

The wave cut platform remains and can be seen at low tide

Wave cut platform feedback

Negative feedback, as the waves have further to travel each time, losing more energy each time

1 - CCASS

Air and water widens the line of weakness by hydraulic action and abrasion

2 - CCASS

The crack widens into a cave, which continues to erode

3 - CCASS

Hydraulic action and abrasion break through the back wall of the cave, forming an arch

4 - CCASS

The overhang of the arch collapses due to its own weight and the reduced weight that the arch legs can hold, forming a stack

5 - CCASS

The stack continues to erode and eventually falls, leaving a stump behind

Blowhole

Lines of weakness grows upwards and eventually reaches the top of the cliff. During storms, water may be pushed upwards out at the top of the cliff

1 - Spit

Longshore drift moves the sediment down the coast

2 - Spit

The coastline changes direction

3 - Spit

Longshore drift continues in the same direction as the prevailing wind, depositing sediment as a spit

4 - Spit

When the wind changes direction for a significant length of time, the spit curves

5 - Spit

The end of the spit begins to curve as wave refraction carries sediment into the sheltered water

6 - Spit

Low energy gentle waves go behind the spit and deposit finer sediment, which eventually forms a salt marsh

Single spit

Straight or curved, do not have minor spits or recurved edges

Compound spit

Straight or curved, has one or more minor spits or recurved edges

Mature spit features

Sand dunes and salt marshes

Sand dunes on a spit

Deposited sand dries out and is blown inland of the spit

Salt marshes behind spit

Low energy gentle waves deposit finer sediment behind the spit, allowing salt resistant vegetation to grow. The roots bind the sediment together, forming usable land

Bar

Spit that develops across a bay with no strong water flowing out of it.

Lagoon

Former bay that a bar has formed across

Tombolo

A spit that grows out to and joins an island to the mainland

Offshore bars

Submerged, or partially exposed, ridges of sand / coarse sediment that is created by waves offshore of the coast - absorb wave energy and act as sediment sinks and input stores

Conditions needed for a sand dune

Abundance of dry sand, wind, large tidal range so sand can dry out and obstacle to trap the sand around

1 - Sand dune

An obstacle is in the beach, such as seaweed, rocks or driftwood

2 - Sand dune

Wind blows sand, which then gets trapped by the obstacle

3 - Sand dune

More sand builds up around it, forming an embryo dune

4 - Sand dune

Dune continues to grow upwards to become a foredune, and then a yellow dune with marram grass

5 - Sand dune

More vegetation grows, and the humus from decaying plants slowly turns the sand grey, and into a grey dune

6 - Sand dune

The sand supply is cut off at the back of the dune, leaving smaller, wasting dunes

Dune slack

The dune surface is removed to below the water table, leaving it to flood

Blowout

Wind has been funnelled between the dunes, removing the sand