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Unlock This CourseCoastal transportation is the movement of sediment (sand, pebbles and rocks) along the coastline by waves, tides and currents. This process is crucial in shaping our coastlines and creating many coastal landforms we see today. Understanding how sediment moves helps us predict coastal changes and manage coastal areas effectively.
Key Definitions:
Coastal transportation shapes beaches, creates spits and bars and affects coastal erosion. It's responsible for moving millions of tonnes of sediment each year, constantly reshaping our coastlines and affecting human activities like tourism and coastal development.
Waves transport sediment in four main ways, each suited to different sizes of material and wave conditions. Understanding these processes helps explain how coastlines change over time.
The largest and heaviest particles, such as boulders and large pebbles, are rolled along the seabed by strong waves. This happens during storms when waves have enough energy to move heavy materials. The particles bump and scrape against each other and the seabed, gradually becoming smaller and more rounded.
Medium-sized particles like small pebbles and coarse sand are picked up by waves and bounce along the seabed in a series of hops. This creates a bouncing motion where particles are lifted briefly before settling back down, only to be picked up again by the next wave.
Fine particles such as sand and silt are light enough to be carried within the water column. These particles remain suspended in the water, giving it a cloudy appearance. This is the most common form of transportation and can carry sediment over long distances.
Some minerals and chemicals dissolve in seawater and are transported invisibly. This includes calcium carbonate from shells and limestone, which dissolves in slightly acidic seawater. Though invisible, this process is important for transporting nutrients and building materials for marine life.
Boulders and large rocks moved by traction during powerful storms. Creates dramatic coastal changes but happens less frequently.
Pebbles and coarse sand moved by saltation. Common during moderate wave conditions and responsible for beach formation.
Fine sand and silt carried in suspension. Travels furthest and creates the muddy appearance of coastal waters.
Longshore drift is perhaps the most important coastal transportation process. It occurs when waves approach the shore at an angle, creating a zigzag pattern of sediment movement along the coastline.
When waves approach the shore at an angle (which happens most of the time due to prevailing winds), they push sediment up the beach diagonally during the swash. However, gravity pulls the backwash straight down the beach slope. This creates a zigzag pattern of movement, gradually transporting sediment along the coast.
The direction of longshore drift depends on the prevailing wind direction and the angle at which waves approach the shore. In the UK, the prevailing south-westerly winds mean that longshore drift generally moves sediment from west to east along the south coast and from south to north along the west coast.
Spurn Head is a 3.5-mile long spit formed by longshore drift moving sediment southwards along the Yorkshire coast. The spit grows by approximately 5cm per year as waves transport sand and pebbles from the eroding Holderness coastline. This demonstrates how longshore drift can create new landforms while contributing to erosion elsewhere. The spit is so dynamic that it has been breached by storms several times in history, only to reform naturally.
Several factors influence how much sediment is transported and how far it travels. Understanding these factors helps predict coastal changes and plan coastal management strategies.
Larger, more powerful waves can transport bigger particles over greater distances. Storm waves might move boulders that calm waves cannot budge. Wave energy depends on wind speed, wind duration and fetch (the distance over which wind blows).
Smaller, rounder particles are easier to transport than large, angular ones. Beach sand travels much further than cliff boulders. The shape affects how particles interact with water flow – smooth, rounded particles move more easily than rough, angular ones.
Soft rocks like clay and sandstone provide more sediment for transportation than hard rocks like granite. The type of rock also affects the size of particles produced – some rocks break into fine sand while others create large chunks.
Transportation rates vary seasonally. Winter storms create powerful waves that can move large amounts of sediment, while summer's gentler waves mainly transport fine materials. This seasonal pattern affects beach profiles and coastal landform development.
Coastal transportation doesn't just move sediment – it creates distinctive landforms that characterise our coastlines. These features provide evidence of transportation processes in action.
When waves lose energy, they deposit their sediment load, creating various landforms:
Beaches: Form where waves deposit sand and pebbles. The size of beach material reflects local wave energy and sediment supply.
Spits: Elongated ridges of sediment extending from the coast, formed by longshore drift. They often curve at the end due to wave refraction.
Bars: Ridges of sediment that can form across bays or river mouths, created when waves deposit material in areas of reduced energy.
Tombolos: Sediment deposits that connect islands to the mainland, formed by wave refraction around the island concentrating sediment deposition.
Chesil Beach is an 18-mile long pebble beach that demonstrates coastal transportation perfectly. The pebbles are graded by size – small pebbles at the western end gradually increase to large cobbles at the eastern end. This size grading occurs because longshore drift moves eastward and only the strongest waves can transport the largest pebbles the full distance. The beach connects the Isle of Portland to the mainland, forming a tombolo.
Human activities significantly affect coastal transportation processes, sometimes with unintended consequences for coastal management and development.
Sea walls, groynes and breakwaters interrupt natural sediment movement. While they protect specific areas, they can cause increased erosion elsewhere by starving beaches of sediment. Groynes trap sediment on their updrift side but cause erosion on their downdrift side.
Removing sediment from beaches or offshore areas for construction reduces the amount available for natural transportation. This can lead to increased coastal erosion and changes in beach profiles.
Buildings and infrastructure can alter wind patterns and wave energy, affecting transportation processes. Harbours and piers can trap sediment, changing local transportation patterns.
Coastal managers must balance protecting human activities with maintaining natural transportation processes. Solutions include beach nourishment (adding sand), managed retreat and designing defences that work with natural processes rather than against them.
Scientists use various methods to study coastal transportation, helping us understand and predict coastal changes.
Beach profiling tracks changes in beach shape over time, revealing patterns of sediment movement. Sediment tracing uses coloured sand or electronic tags to follow particle movement. Aerial photography and satellite imagery show large-scale changes in coastal features.
Wave measurement buoys record wave height, direction and frequency, helping predict transportation rates. Computer models combine this data to forecast coastal changes and assess the impact of proposed developments.
Understanding coastal transportation is essential for managing our coastlines effectively. As sea levels rise and storm patterns change due to climate change, these processes will become even more important for coastal communities worldwide.