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examBoard: Cambridge
examType: IGCSE
lessonTitle: Coastal Transportation and Deposition
Coasts are dynamic environments where material is constantly on the move. The sea doesn't just erode the coastline it also transports the eroded material and eventually deposits it elsewhere, creating new landforms in the process.
Key Definitions:
When waves break on a beach, they have energy that can move sediment. The size of sediment that can be moved depends on the wave energy stronger waves can move larger particles. As waves approach the shore at an angle, they push sediment up the beach at the same angle. However, the backwash (water returning to the sea) moves directly down the steepest slope, creating a zigzag movement of sediment along the coast.
Understanding coastal transportation is crucial because it affects beach formation, coastal defence strategies and harbour management. If we build structures that interrupt natural sediment movement, beaches downstream can be starved of sediment, leading to increased erosion and potential damage to coastal communities.
The sea transports material in four main ways, depending on the size of the sediment and the energy of the waves:
Large, heavy materials like boulders and large pebbles are rolled along the seabed by the force of the water. These materials are too heavy to be lifted but can be pushed along the bottom. This typically happens during storms when wave energy is highest.
Medium-sized particles like pebbles and gravel are bounced along the seabed. They're picked up by the water, carried a short distance, then dropped back down, creating a hopping or jumping motion. This is the most common form of transportation for beach materials.
Small, light particles like silt and clay are carried within the water. These tiny particles can remain suspended in the water for long periods, especially when wave energy is high and can be transported far from their original location.
Some materials, particularly from rocks like limestone and chalk, can dissolve in seawater and be carried as a solution. This dissolved material is invisible but can be transported great distances before eventually being deposited.
Longshore drift is the most significant process of coastal transportation, responsible for moving millions of tonnes of sediment along coastlines every year.
Longshore drift occurs when waves approach the beach at an angle (rather than straight on). This happens because:
Over time, this process moves large amounts of sediment in one direction along the coast, creating what coastal geographers call a 'sediment cell' or 'littoral cell'.
Along the Suffolk coast in eastern England, longshore drift moves sediment southwards. Material eroded from the soft cliffs at Covehithe and Dunwich is transported along the coast and eventually deposited to form features like Orford Ness, one of the largest shingle spits in Europe. This demonstrates how erosion, transportation and deposition are all linked in coastal systems.
When waves and currents lose energy, they drop the sediment they've been carrying, creating distinctive coastal landforms. The shape and size of these features depend on factors like wave energy, sediment supply and coastal shape.
Beaches are the most common depositional landforms, made up of material deposited by the sea. They form when waves lose energy and drop sediment, typically in sheltered areas like bays.
A typical beach has a steep upper section (berm) formed by storm waves and a gentler lower section shaped by normal waves. The size of material on a beach often decreases from top to bottom and from stormier to more sheltered areas.
Beaches vary widely in composition. Sandy beaches form in lower energy environments, while shingle (pebble) beaches typically form where wave energy is higher. The source rock of the local area also influences beach material.
Beaches act as natural buffers against wave energy, protecting cliffs and coastal settlements from erosion. This is why beach nourishment (adding new sand or shingle) is often used as a coastal management strategy.
Spits are elongated ridges of sand or shingle that extend from the land into the sea, often with a curved end. They form where:
Spurn Point is a dramatic 3.5-mile long spit stretching across the mouth of the Humber Estuary. Formed by longshore drift moving material southwards along the Holderness Coast, it's constantly changing shape. In 2013, a storm surge breached the spit, turning the end into a tidal island. Spurn Point demonstrates the dynamic nature of depositional features and their vulnerability to extreme weather events.
Bars are ridges of sand or shingle that form completely across a bay, connecting two headlands. They create a lagoon behind them and form when longshore drift deposits material across a bay mouth. Slapton Ley in Devon is a good example, where a bar has created the largest freshwater lake in southwest England.
Tombolos are depositional features that connect an island to the mainland. They form when waves refract (bend) around an offshore island, creating a zone of low energy behind it where deposition occurs. Over time, enough material builds up to form a land bridge. Chesil Beach in Dorset connects the Isle of Portland to the mainland, forming one of the UK's most famous tombolos.
Human activities can significantly disrupt natural coastal transportation processes, often with unintended consequences:
Groynes, sea walls and harbours can interrupt longshore drift, causing sediment to build up on one side and erosion to increase on the other. This is known as the 'terminal groyne effect' and can lead to 'sediment starvation' downstream.
Some coastal management now focuses on working with natural processes rather than against them. Managed realignment involves removing hard defences and allowing natural transportation and deposition to resume, creating new saltmarshes and mudflats that act as natural buffers.
Hurst Castle Spit is a 2.5km shingle spit in the western Solent. Natural longshore drift would continually reshape this feature, but because it protects Hurst Castle (a historic monument), it's artificially maintained. The 'Beach Replenishment Scheme' involves dredging shingle from offshore and placing it on the spit to replace material lost to storms. This demonstrates how valuable depositional features often require human intervention to maintain them when natural processes are disrupted.
Coastal transportation and deposition are essential natural processes that:
Understanding these processes is crucial for sustainable coastal management, especially as climate change brings rising sea levels and more frequent storms that will alter patterns of coastal transportation and deposition in the future.
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