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Unlock This CourseThe coast is one of the most dynamic environments on Earth. Every day, powerful forces shape and reshape our coastlines through a combination of wave action, weathering and human activity. Understanding these physical processes is crucial for managing coastal areas and protecting communities from coastal hazards.
Key Definitions:
Waves are created by wind blowing over the sea surface. The size and power of waves depend on three main factors: wind speed, wind duration (how long it blows) and fetch (the distance over which wind blows). The longer the fetch, the more powerful the waves become.
Not all waves are the same. Understanding the difference between constructive and destructive waves is essential for understanding how coastlines change.
Waves can be classified into two main types based on their characteristics and effects on the coastline:
Characteristics: Low height (under 1m), long wavelength, low frequency (6-8 per minute). Strong swash, weak backwash. Build up beaches by depositing sediment.
Characteristics: High height (over 1m), short wavelength, high frequency (10-14 per minute). Weak swash, strong backwash. Erode coastlines by removing sediment.
Swash: Water rushing up the beach. Backwash: Water flowing back down the beach. Wavelength: Distance between wave crests.
Coastal erosion occurs through several different processes, each working to wear away the coastline in different ways. These processes often work together to create dramatic coastal landscapes.
The sheer force of waves hitting the coast. Water gets forced into cracks in rocks, compressing air inside. When the wave retreats, the compressed air expands explosively, widening the cracks. This is the most powerful form of erosion during storms.
Waves pick up stones, pebbles and sand and hurl them at the cliff face, acting like sandpaper. This grinding action wears away the rock surface. The larger the material, the more effective the abrasion.
Rocks and pebbles carried by waves knock against each other, gradually becoming smaller, smoother and more rounded. This process creates the smooth pebbles found on many beaches.
Chemical erosion where acids in seawater dissolve certain types of rock, particularly limestone and chalk. This process is slow but continuous, even during calm weather.
Flamborough Head demonstrates all four erosion processes in action. The chalk cliffs show clear evidence of hydraulic action in the form of caves and blowholes. Abrasion has created a wave-cut platform visible at low tide. The white chalk is particularly susceptible to solution, whilst attrition has created the smooth pebbles found on nearby beaches.
Once material has been eroded, it needs to be moved along the coast. This transportation happens through several different processes, depending on the size and weight of the material being moved.
Dissolved chemicals are carried in the water itself. You can't see this happening, but it's constantly occurring, especially with limestone and chalk coastlines.
Fine particles like clay and silt are carried in the water column. This gives seawater a murky appearance after storms when lots of material is suspended.
Small stones and pebbles bounce along the seabed. They're too heavy to stay suspended but light enough to be picked up and dropped by wave action.
Large boulders and rocks are rolled along the seabed by the force of the waves. This only happens during very powerful storms when wave energy is at its highest.
One of the most important transportation processes is longshore drift, which moves sediment along the coastline in a zigzag pattern.
Longshore drift occurs when waves approach the beach at an angle due to the prevailing wind direction. The swash carries material up the beach at an angle, but gravity pulls the backwash straight down the beach. This creates a zigzag movement of sediment along the coast.
1. Waves approach beach at an angle
2. Swash moves sediment up beach diagonally
3. Backwash pulls sediment straight down
4. Process repeats, moving material along coast
Spurn Head is a 5.5km long spit formed by longshore drift. Sediment is transported southwards along the Yorkshire coast by waves approaching from the northeast. The spit has grown across the mouth of the Humber Estuary, demonstrating how powerful longshore drift can be in shaping coastlines over time.
When waves lose energy, they drop the sediment they've been carrying. This deposition creates many of our coastal landforms, from beaches to spits.
Deposition happens when wave energy decreases. This can occur for several reasons:
Beaches form where deposition exceeds erosion. Constructive waves build up beaches by depositing sand, shingle and pebbles. The size of material depends on wave energy - high energy beaches have larger pebbles.
Whilst waves attack the coast from the sea, weathering and mass movement attack cliffs from above, working together to shape coastal landscapes.
Water enters cracks in rocks and freezes. Ice expands by 9%, widening cracks. Repeated freeze-thaw cycles gradually break rocks apart. Common in areas with cold winters.
Salt crystals grow in rock cracks as seawater evaporates. The growing crystals create pressure, widening cracks. Particularly effective in warm, dry climates.
Rainwater is slightly acidic and reacts with certain minerals in rocks. This is particularly effective on limestone and chalk, creating features like caves.
Once weathering has weakened cliff materials, gravity can cause mass movement - the downslope movement of material under the influence of gravity.
Individual rocks fall from cliff faces, often after freeze-thaw weathering. Creates scree slopes at cliff base. Common on hard rock coastlines.
Large sections of cliff slide downwards along a slip plane. Often triggered by heavy rainfall saturating cliff materials. Can be very destructive to coastal communities.
The Holderness Coast is Europe's fastest eroding coastline, losing 2 metres per year on average. The soft boulder clay cliffs are vulnerable to both marine erosion and mass movement. Heavy rainfall saturates the clay, making it heavy and unstable, leading to frequent landslides. Wave action then removes the fallen material, undermining the cliff further.
Tides and currents play crucial roles in coastal processes, affecting when and where erosion, transportation and deposition occur.
Tides are the regular rise and fall of sea level caused by gravitational pull from the moon and sun. They affect coastal processes in several ways:
As tides flow in and out, they create currents that transport sediment. These currents can be very strong in narrow channels and estuaries, moving large amounts of material.