The coast is a strip of land bordering the sea. It's ever-changing and diverse because constant wave action means that nothing can stay the same for long. Breakers erode cliff faces while the sand is continually being moved up and down the beach. Even on a calm, quiet day, the smallest ripple across a beach will change the coastline.
Waves
The coastline is shaped by the interaction between the wind and waves. The wind builds up energy in waves far out at sea, and this energy then breaks on the shore, with two types of waves sculpting the coastline in new ways. The two types are constructive and destructive waves. A constructive wave is a strong upward rush of water, known as a swash, which delivers sand and gravel up the beach. A destructive wave is a wave that has a weak swash but a strong backward rush, known as a backwash, which carries the sand and gravel out to sea. Waves hit the beach from one side or the other and return at a right angle to the beach, creating a zigzag motion that moves sand and gravel along the shore.
Deposition and transportation
The ebb and flow of the tides create depositional landforms, such as narrow spits, offshore bars and tombolos, cuspate forelands, and sand dunes. Sediment from muddy rivers and streams maintains the process of coastal construction. There are four main processes involved in the transportation of the sediment:
| Sediment transportation |
|---|
| Process | Explanation |
| Solution | Substances are dissolved and carried along in the water. |
| Saltation | Larger particles are too heavy to be carried, so they are bounced along the sea bed. |
| Suspension | This is where very fine material is carried along in the water. |
| Traction | Very large particles are pushed along the sea bed by the force of the water. |
| Table 1 |
The transportation of sediment along the shore is known as longshore drift or littoral drift.
Erosion
The constant pounding of waves wears away at the base of cliffs, forging platforms and making chunks of rock crumble into the sea. This is known as mass movement. The solid rock eventually breaks away, and the waves scoop out the weaker clays left behind, creating a bay or cove. Headlands are formed where the coastline gives way on either side. This leaves stacks and stumps.
The waves erode the coast in a number of ways:
| Coastal erosion ways |
|---|
| Coastal erosion | Explanation |
| Hydraulic action | Air in cracks in the cliff is compressed when waves crash in. The pressure caused by this action breaks off pieces of rocks. |
| Attrition | Pieces of rock in the water bash against each other and break into smaller bits. |
| Abrasion | Rock and sediment is transported by the waves, smashing and grinding against rocks and cliffs. This breaks bits off and smoothes the surface. |
| Corrosion | Soluble rocks get gradually dissolved by seawater. |
| Table 2 |
Weathering
Along with the marine forces, sub-aerial processes (land-based) help to alter the coast. These can be broken down into weathering, the gradual breaking down of rocks in situ, to mass movement, which is the movement of material downslope through gravity.
Coastal systems
The steady supply of building material via what is known as the coastal system (it has inputs, outputs, stores, and transfers) to the littoral zone (the area of shoreline from the sea to the land) is how this ever-changing coastal landscape is achieved. The coastline can be classified according to its formation process (primary and secondary) as a result of sea-level changes (submergent and emergent) or as a result of plate activity (primary and secondary).
The coast can be steep cliffed (rocky coastline) to relatively flat (coastal plain) depending on if the waves are high or low energy. The geological structure has a massive influence on the morphology (for example, strata, the different layering of rocks) and erosion rates of a coast.
Structure
Rapid coastal erosion is caused by geological factors such as lithology (soft rock type, weak cohesive bonds, and porous rocks) and geological structure combined with marine factors that quickly remove collapsed sediment, allowing erosion to restart.
Long-term sea-level changes occur as a result of a complex interplay of eustatic (the rise and fall of the sea levels which are influenced by the ice age), tectonic (where the land sinks or rises because of changes at tectonic plates boundaries), and isostatic changes (these refer to changes in the level of the land which is also influenced by the ice age).
Coastal Management and Engineering
Coastal management (such as hard and soft engineering) disrupts the sediment cells. Human activity, such as the construction of major dams or dredging along the coastline, interrupts the operation of the sediment cell. This can increase rates of coastal recession. Economic and social issues arise where the coast has eroded (on the U.K. Holderness coastline, a holiday park lost over 400 holiday homes through coastal retreat).
Fig. 1 - Groynes at Mundesley in Norfolk, UK. This is a hard engineering method in coastal management.
Coastal flooding
Coastal flooding is largely a natural event. However, human influences on the coastal environment can exacerbate this. Engineered protection structures along the coast, such as a seawall, alter the natural processes of the beach, often leading to erosion on an adjacent stretch of the coast which also increases the risk of flooding. Sea level changes and the extreme weather caused by climate change have increased the intensity and occurrences of coastal flooding.
Fig. 2 - Flood gate on Chesil Beach in Dorset, UK.
Coasts - Key takeaways
- Coasts are an ever-changing, diverse place.
- Waves are the busiest sculptors on the coastline.
- The three marine processes that affect the coast are erosion, transportation, and deposition.
- There are four processes by which eroded material is moved, solution, saltation, suspension, and traction.
- There are four processes by which waves erode the coast, hydraulic action, attrition, abrasion, and corrosion.
- Rapid coastal erosion is caused by geological factors such as lithology and geological structure combined with marine factors.
- Long-term sea-level changes occur as a result of a complex interplay of eustatic, tectonic and isostatic changes.
References
- Fig. 1: Groynes at Mundesley in Norfolk, UK. This is a hard engineering method in coastal management (https://en.wikipedia.org/wiki/File:Groyne_at_Mundesley,_Norfolk.JPG) by MichaelMaggs (https://commons.wikimedia.org/wiki/User:MichaelMaggs) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
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