Physical processes 2 (Edexcel GCSE Geography A): Revision Notes
Physical processes 2 - Coastal landscapes
Wave transport and deposition
Waves play a crucial role in moving sediment along coastlines and depositing it in new locations. Understanding how this happens helps explain the formation of many coastal features. When waves lose their energy, they drop the material they've been carrying, creating beaches, spits, and other depositional landforms.
Wave energy is the key factor controlling whether sediment is transported or deposited. High-energy waves pick up and carry material, while low-energy waves drop their sediment load.
Longshore drift
Longshore drift is one of the most important coastal processes for moving sediment along our shorelines. This process occurs when waves approach the beach at an angle, typically influenced by the direction of prevailing winds in the area.
Worked Example: The Longshore Drift Process
Here's how longshore drift works:
- Wave approach: Waves hit the beach at approximately a angle, driven by the prevailing wind direction
- Swash action: The wave swash pushes sediment up the beach at this angled approach
- Backwash return: Gravity pulls the backwash (and sediment) straight back down the beach at a angle to the shoreline
- Zigzag movement: This creates a distinctive zigzag pattern of sediment movement along the coast
This continuous process gradually transports beach material along the coastline in the direction of the prevailing wind. The overall movement can shift vast amounts of sediment over time, significantly affecting coastal landscapes.
Methods of wave transport
Waves transport different types of material using four distinct methods, depending on the size and weight of the sediment:
Traction
This method moves the largest and heaviest materials along the coastline. Large boulders and rocks are rolled along the seabed by the force of powerful waves. This typically occurs during storms when wave energy is at its highest.
Traction is most effective during storm conditions when wave energy is sufficient to move large materials that would otherwise remain stationary.
Saltation
Medium-sized materials like pebbles and small stones are transported through saltation. These materials are picked up by wave action and bounce along the seabed in a hopping motion. The stones are too heavy to remain suspended but light enough to be lifted temporarily.
Suspension
Finer materials such as sand and small particles are carried within the water column itself. These lightweight materials become suspended in the flowing water and can be transported over considerable distances before settling.
Suspended sediment can travel much further than materials moved by traction or saltation, making it important for long-distance coastal transport.
Solution
The finest transport method involves minerals that actually dissolve into the seawater. These dissolved materials are completely invisible but are carried along within the water flow. This process particularly affects certain rock types that are more chemically reactive with seawater.
Wave deposition
Deposition occurs when waves lose their energy and can no longer carry their sediment load. This process is most effective with constructive waves, which have specific characteristics that make them ideal for building up coastal features.
Characteristics of constructive waves
Constructive waves are relatively long and low, meaning their wavelength is much greater than their height. These waves have several important features:
- Strong swash: The forwards movement of water up the beach is powerful
- Weak backwash: The return flow down the beach is gentler
- Gentle approach: They break forwards rather than plunging down
The combination of strong swash and weak backwash is essential for deposition - it means more sediment is brought up the beach than is taken away.
Factors promoting deposition
Several environmental conditions encourage wave deposition:
- Sheltered locations: Areas protected from strong wave action, such as bays, allow sediment to settle
- Calm weather conditions: Reduced wind and wave energy means less disturbance of deposited material
- Gentle offshore gradients: Gradual underwater slopes create friction that reduces wave energy
All these factors work together to reduce the wave's energy, forcing it to deposit the sediment it has been carrying.
How longshore drift transports coastal material
The process of longshore drift creates a systematic movement of material along coastlines. The prevailing wind direction determines the overall pattern of sediment transport. As waves approach the shore at an angle, they bring sediment onto the beach through the swash. However, when gravity pulls this material back to sea through the backwash, it follows a perpendicular path to the shoreline.
This creates the characteristic zigzag movement pattern that continues along the entire coastline. Over time, this process can transport enormous quantities of sediment, potentially moving material for many kilometres along the coast. The continuous nature of this transport system means that coastal features are constantly evolving as material is removed from some areas and deposited in others.
The direction of longshore drift can change seasonally as prevailing wind patterns shift, affecting which parts of the coastline experience erosion or deposition.
Key Points to Remember:
- Longshore drift moves sediment along coastlines in a zigzag pattern caused by angled wave approaches and straight backwash
- Four transport methods work together: traction (rolling boulders), saltation (bouncing stones), suspension (floating particles), and solution (dissolved minerals)
- Constructive waves with strong swash and weak backwash are most effective at depositing sediment
- Deposition occurs in sheltered areas with calm conditions and gentle gradients where wave energy decreases
- Prevailing wind direction controls the overall direction of longshore drift and sediment movement along the coast