1. Introduction to Longshore Drift
- Definition: Longshore drift is the process by which sediments, including sand, gravel, and pebbles, are transported along a coastline by wave action. It occurs parallel to the shore, driven by the angle at which waves approach the coast.
- Importance in Coastal Geography: This process is fundamental in shaping the coastline, contributing to the formation and evolution of various coastal features like spits, bars, and beaches. Understanding longshore drift is crucial for managing coastal erosion and sediment deposition.
2. How Longshore Drift Works
- Wave Approach:
- Angle of Incidence: Waves rarely approach the shore straight on. Instead, they typically hit the coast at an angle due to prevailing wind patterns and ocean currents.
- Oblique Wave Direction: This angled approach is key to initiating longshore drift. The direction of wave approach influences the direction of sediment movement along the shore.
- Swash and Backwash:
- Swash:
- Action: After a wave breaks, the water that rushes up the beach is called the swash. It moves at the same angle as the incoming wave.
- Sediment Transport: The swash carries sediments such as sand and pebbles up the beach at an angle, contributing to the lateral movement of materials.
- Backwash:
- Action: Once the swash loses its energy, gravity pulls the water back down the beach. This return flow is known as backwash.
- Straight Path: Unlike the swash, backwash moves directly down the slope of the beach, perpendicular to the shoreline, pulling sediments back toward the sea.
- Swash:
- Zigzag Pattern of Sediment Movement:
- Continuous Process: The combined action of swash and backwash results in the sediments being moved in a zigzag pattern along the coastline. This continuous process gradually transports sediment laterally along the coast, a movement known as longshore drift.
- Sediment Transport Direction: The overall direction of longshore drift is determined by the direction of the prevailing waves and can vary depending on seasonal changes in wave patterns.
3. Factors Influencing Longshore Drift
Wave Direction:
- Prevailing Wind: The direction of the prevailing wind significantly influences the angle at which waves approach the coast, thereby affecting the direction of longshore drift.
- Storms and Seasonal Changes: Changes in wind patterns, such as those caused by storms or seasonal variations, can alter the direction and strength of longshore drift.
Wave Energy:
- High-Energy Waves: Stronger waves, often generated by storms, can move larger sediment particles and transport them over greater distances along the shore.
- Low-Energy Waves: During calmer weather, smaller waves with less energy may only move fine sediments, resulting in slower sediment transport.
Beach Slope:
- Steep Slopes: Beaches with steep slopes tend to experience more rapid swash and backwash, leading to faster sediment movement along the coast.
- Gentle Slopes: On beaches with gentler slopes, sediment movement is slower, and the longshore drift process is less pronounced.
Human Interventions:
- Groynes: These are structures built perpendicular to the coastline to trap sediments and reduce the effects of longshore drift. While they can help prevent erosion on one side, they may cause sediment depletion and erosion on the down-drift side.
- Breakwaters: These are structures built offshore to break the force of waves. They can alter the natural flow of longshore drift, leading to sediment accumulation in some areas and erosion in others.
- Sea Walls: Although designed to protect the coast from wave action, sea walls can reflect wave energy, potentially increasing erosion at the base of the wall and disrupting longshore drift.
4. Effects of Longshore Drift
- Beach Formation and Maintenance:
- Sediment Supply: Longshore drift is essential for supplying sediment to beaches. This continuous supply helps maintain beach width and volume, especially in areas where erosion is a concern.
- Beach Nourishment: In some areas, artificial beach nourishment projects rely on an understanding of longshore drift to distribute sand along the shore effectively.
- Coastal Erosion:
- Sediment Imbalance: When the rate of sediment removal by longshore drift exceeds the rate of sediment deposition, erosion can occur. This can lead to the retreat of the coastline and loss of valuable land.
- Erosion Hotspots: Areas down-drift of groynes or other barriers often experience increased erosion because the natural supply of sediment is cut off.
- Sediment Deposition and Landform Creation:
- Spits: Longshore drift can lead to the formation of spits, narrow landforms that extend from the coast into the sea. Spits form where the coastline changes direction or where there is a significant decrease in wave energy.
- Bars and Barrier Islands: When longshore drift carries sediment across a bay or estuary, it can create bars or barrier islands. These features can protect the coast from wave action and create sheltered areas behind them.
- Tombolos: A tombolo is a landform that forms when longshore drift connects an island to the mainland with a narrow strip of land.
5. Diagrams of Longshore Drift
- Longshore Drift Process Diagram:
- Visual Components: The diagram should clearly show waves approaching the shore at an angle, with arrows indicating the movement of swash up the beach and backwash down. The zigzag pattern of sediment movement should be highlighted, showing the direction of longshore drift along the shore.
- Annotations: Include labels such as ‘Swash,’ ‘Backwash,’ ‘Wave Direction,’ and ‘Longshore Drift Direction.’ Brief explanatory notes can help students understand how the process works.
- Spit Formation Diagram:
- Visual Components: A diagram showing a coastline with a spit extending out into the sea, formed by the deposition of sediment due to longshore drift. The direction of longshore drift and wave approach should be indicated.
- Annotations: Label the spit, beach, and sea, and include arrows showing the movement of sediment along the coast that leads to the formation of the spit.
