Introduction to Coastal Deposition
Coastal deposition happens when waves lose energy and drop the sediment they've been carrying. This creates some of the most beautiful and important coastal features we see today. Unlike erosion which wears away the coast, deposition builds it up, creating new land and protecting existing coastlines from wave attack.
Key Definitions:
- Deposition: The process where waves drop sediment when they lose energy.
- Sediment: Small particles of rock, sand and other materials carried by waves.
- Longshore drift: The movement of sediment along the coast by waves hitting the shore at an angle.
- Constructive waves: Low-energy waves that deposit more material than they erode.
🌊 When Does Deposition Occur?
Deposition happens when waves lose energy. This occurs in sheltered areas like bays, behind headlands, or where the coastline changes direction. Constructive waves with long wavelengths and low heights are particularly good at depositing sediment rather than eroding it.
The Process of Coastal Deposition
Understanding how deposition works is key to understanding coastal landforms. When waves approach the shore, they carry sediment in suspension. As they break and lose energy, this sediment settles out of the water and accumulates on the seabed or shore.
Factors Affecting Deposition
Several factors control where and how much deposition occurs along a coastline:
⚡ Wave Energy
Low-energy waves deposit more than they erode. Sheltered coasts and areas with gentle offshore slopes tend to have lower wave energy.
🌊 Sediment Supply
Rivers, cliff erosion and offshore sources provide the raw materials for deposition. More sediment means more potential for depositional landforms.
🌬 Coastal Shape
Bays, estuaries and areas where the coastline changes direction create calm conditions perfect for deposition.
Depositional Landforms
Coastal deposition creates several distinctive landforms. Each forms under specific conditions and tells us about the processes operating along that stretch of coast.
Beaches
Beaches are the most common depositional landform. They form where waves deposit sand, shingle, or pebbles. The size of beach material depends on wave energy - high-energy waves create pebble beaches, while low-energy waves form sandy beaches.
Beach Profile Features
Berm: The flat area at the back of the beach formed by storm waves. Beach face: The sloping area where most wave action occurs. Offshore bar: Underwater ridge of sediment parallel to the shore.
Spits
Spits are narrow ridges of sand or shingle that extend from the coast into the sea. They form where longshore drift carries sediment along the coast until it reaches a bay or river mouth where the coastline changes direction.
➡ How Spits Form
1. Longshore drift carries sediment along the coast
2. At a bay or estuary, the sediment continues in the same direction
3. The spit grows out into the water
4. Wind and waves may curve the end, forming a hook
Case Study Focus: Spurn Head, Yorkshire
Spurn Head is a 3.5km long spit on the Yorkshire coast. It formed where the River Humber meets the North Sea. Longshore drift from the north has carried sediment southwards, creating this distinctive curved spit. The spit protects the Humber estuary from wave attack but is constantly changing shape due to erosion and deposition.
Bars and Lagoons
Bars form when spits grow completely across a bay, cutting off the water behind to create a lagoon. This happens when there's no major river to keep the bay mouth open.
🌊 Bar Formation
Bars need specific conditions: a good sediment supply, consistent longshore drift and a bay without a large river. Once formed, the lagoon behind gradually fills with sediment and may become marshland.
Tombolos
A tombolo is a ridge of sand or shingle that connects an island to the mainland. They form when waves are refracted (bent) around an island, creating an area of calm water where deposition occurs.
Case Study Focus: Chesil Beach, Dorset
Chesil Beach is an 18km long tombolo connecting the Isle of Portland to the mainland. It's made of pebbles that get larger from northwest to southeast. This size sorting happens because larger waves from the southwest can move bigger pebbles further along the beach. Behind the beach is the Fleet Lagoon, an important wildlife habitat.
Longshore Drift
Longshore drift is crucial for understanding coastal deposition. It's the process that moves sediment along the coast, creating many depositional landforms.
How Longshore Drift Works
When waves approach the shore at an angle, they push sediment up the beach diagonally. The backwash then carries the sediment straight back down the beach due to gravity. This creates a zigzag movement of sediment along the coast.
↗ Swash
Waves push sediment up the beach at an angle, following the direction of the approaching wave.
↘ Backwash
Water flows straight back down the beach due to gravity, carrying sediment with it.
➡ Net Movement
The result is a zigzag movement that transports sediment along the coastline in the direction of the prevailing wind.
Human Impact on Coastal Deposition
Human activities can significantly affect coastal deposition processes, sometimes with unexpected consequences.
Coastal Management Effects
Sea walls, groynes and other coastal defences can interrupt longshore drift, causing deposition in some areas and erosion in others. Groynes trap sediment on their updrift side but starve the downdrift coast of sediment.
🚧 Coastal Engineering
Harbours, piers and breakwaters can act like giant groynes, completely blocking longshore drift. This often causes serious erosion problems downdrift of these structures.
Climate Change Impacts
Rising sea levels and changing storm patterns affect coastal deposition. Higher sea levels can push depositional features inland, while more intense storms may erode previously stable deposits.
Managing Depositional Coasts
Many depositional coastlines are important for tourism and wildlife. Management strategies include beach nourishment (adding sand), dune restoration and careful planning of coastal developments. The key is working with natural processes rather than against them.
Summary
Coastal deposition creates some of our most valuable coastal environments. Understanding these processes helps us manage coasts sustainably and predict how they might change in the future. From the beaches we enjoy to the spits that protect our estuaries, depositional landforms are essential parts of our coastal landscape.