PROCESSES
Waves are either constructive or destructive:
- Constructive waves create depositional landforms and occur when the swash of a wave is stronger than the backwash
- Destructive waves create erosional landforms and occur when the backwash of a wave is stronger than the swash
- Constructive waves create depositional landforms and occur when the swash of a wave is stronger than the backwash
- Destructive waves create erosional landforms and occur when the backwash of a wave is stronger than the swash
Coastlines are made up of a mixture of rock types - some rocks are soft and easily eroded (such as clay), others are harder and take much longer to erode (such as granite or sandstone). These differing rock types cause variations in how quickly the coastline erodes. The softer rock is eroded at a faster rate, meaning it retreats inland rapidly. This leaves the harder rock sticking out into the sea further than the softer rock - these are called Headlands. Where the softer rock has retreated inland are called Bays. The wave action becomes concentrated around the headlands as this is the first part of the coastline that the waves come into contact with. By the time the waves reach the bays they have considerably less energy than when they impacted on the headland. This is where depositional landform occur. |
There are four ways in which waves can erode rock in a coastal environment:
- Hydraulic Action - this is the process of the waves continually hitting the rock surface, causing the breakdown of the rock by mechanical weathering.
- Abrasion - this occurs when the rock surface is broken down by the impact of the pieces of rock and sand suspended in the wave.
- Attrition - the rock and sand suspended in the wave hit each other whilst being transported, meaning they become smaller and smoother.
- Solution - sea water is able dissolve certain rock types (such as limestone and chalk)
LANDFORMS
EROSIONAL COASTAL LANDFORMSOver time a cliff will erode as a result of hydraulic action.
If a harder type of rock lies on the top of softer rock then hydraulic action erodes the softer rock at the base of the cliff forming a wave cut notch. This weakening at the base of the cliff face eventually causes the harder, overhanging rock to fail and collapse. The process then starts again, with a wave cut notch forming until the cliff face fails etc. The retreating of the cliff leaves a horizontal platform at the base of the cliff known as a wave cut platform. At a headland, hydraulic action will erode weaker parts of the sediment and cause cracks at the base of the cliff. As the waves continue to erode the rock, the cracks will get larger and larger until a cave is formed. Continuous wave action causes the cave to become deeper and wider, until it eventually erodes through to the other side of the cliff, forming an arch. The arch continues to be eroded until its roof collapses. This results in a rock stack, which is now completely separate from the cliff. Once the roof of the arch has collapsed it leaves a new headland and the process starts all over again. The stack continues to be eroded until it becomes a stump. A great example of these processes in action are The Needles, off the coast of the Isle of Wight. As you can see this has been repeated over and over again, forming several stacks and stumps. |
DEPOSITIONAL COASTAL LANDFORMS
Deposition occurs when the energy of a wave isn't high enough to carry on transporting the sediment it is holding. As a result, the sediment is deposited and causes different landforms depending on the sediment being carried and where and how it is deposited.
SPITS and SALT MARSHES Spits are formed when a headland suddenly changes to another landform (usually a bay, or river mouth etc.) along a coastline. The waves weaken as they travel passed the headland and this drop in energy causes the sediment to be deposited. Over time this builds up and forms an elongated area of sediment which stretches away from the headland in the direction of longshore drift. The spit acts as a barrier against the waves and causes a calmer area of sea directly behind it. This often causes the formation of a salt marsh as the calmer conditions allow for a more diverse ecosystem to form. BARS, LAGOONS and TOMBOLOS If a spit continues to develop and grows across to another headland, this is called a bar. The area behind the bar (formally a bay) is completely cut off from the tide and becomes a lagoon. If a spit extends out to a stack or island, then this formation is known as a tombolo. |
SAND DUNES
Sand dunes are formed when there is a surplus of sediment and a strong prevailing wind to carry it.
At first, small obstacles (debris or rocks or grass) create a barrier that encourages additional sediment to accumulate around it. The sediment builds up and grass helps stabilise the dune against strong winds - this is called an Embryo Dune.
As the dune builds in size it becomes more stable and evolves into a Fore Dune. At this stage a new embryo dune may form in front of the fore dune.
A Dune Slack may occur if the dune becomes unstable and cause it to collapse. This can be caused by human intervention (walking along the dunes, building nearby etc.), by animals grazing on the vegetation or by strong storm winds eroding the sediment.
Sand dunes are an excellent example of a soft engineering technique for coastal protection.
Sand dunes are formed when there is a surplus of sediment and a strong prevailing wind to carry it.
At first, small obstacles (debris or rocks or grass) create a barrier that encourages additional sediment to accumulate around it. The sediment builds up and grass helps stabilise the dune against strong winds - this is called an Embryo Dune.
As the dune builds in size it becomes more stable and evolves into a Fore Dune. At this stage a new embryo dune may form in front of the fore dune.
A Dune Slack may occur if the dune becomes unstable and cause it to collapse. This can be caused by human intervention (walking along the dunes, building nearby etc.), by animals grazing on the vegetation or by strong storm winds eroding the sediment.
Sand dunes are an excellent example of a soft engineering technique for coastal protection.