Coastal Processes and Landforms » Bar Development

Introduction to Bar Development

Bars are some of the most fascinating coastal landforms you'll encounter. They're essentially ridges of sand, shingle, or other sediment that form in coastal waters. These features can dramatically change the shape of coastlines and create unique environments for both wildlife and human activities.

Bar development is a perfect example of how coastal processes work together to create landforms. The combination of waves, currents and sediment supply creates these impressive features that can stretch for kilometres along our coasts.

Key Definitions:

• Bar: A ridge of sand, shingle, or sediment that forms parallel to the coast, either attached to the shore or separated by water.
• Longshore drift: The movement of sediment along the coast caused by waves approaching the shore at an angle.
• Sediment transport: The movement of sand, pebbles and other materials by water, wind, or ice.
• Spit: A narrow ridge of sediment that extends from the coast into the sea or across a bay.

Types of Bars

Not all bars are the same! Understanding the different types helps us appreciate how varied coastal environments can be and how different processes create different landforms.

Offshore Bars

These bars form parallel to the coast but are separated from the shore by a stretch of water called a lagoon. They're created when waves break in shallow water, depositing sediment in ridges.

🌊 Characteristics

Usually submerged at high tide, visible at low tide and can extend for several kilometres along the coast.

⚓ Formation

Waves break in shallow water, losing energy and depositing sediment in parallel ridges to the shore.

🌏 Examples

The Outer Banks of North Carolina, USA, are a famous example of offshore barrier islands.

Bay Bars (Baymouth Bars)

These impressive features form when sediment is transported across the mouth of a bay, eventually creating a complete barrier. They're excellent examples of how longshore drift can dramatically alter coastal geography.

Bay bars develop when longshore drift carries sediment from one side of a bay to the other. Over time, this sediment builds up until it completely blocks the bay mouth, creating a lagoon behind the bar.

The Role of Longshore Drift

Longshore drift is the engine that powers bar development. Understanding this process is crucial for grasping how bars form and change over time.

How Longshore Drift Works

When waves approach the shore at an angle (which they usually do), they push sediment up the beach diagonally. However, when the wave retreats, gravity pulls the sediment straight back down the beach. This creates a zigzag movement of sediment along the coast.

→ Swash Action

The forward movement of water up the beach carries sediment diagonally up the shore, following the angle of the incoming wave.

↓ Backwash Action

The return flow of water moves straight down the beach due to gravity, carrying sediment perpendicular to the shoreline.

Factors Affecting Bar Development

Several factors work together to determine whether bars will form and what type they'll be. Understanding these factors helps explain why bars are found in some locations but not others.

Wave Energy and Direction

The strength and direction of waves are crucial for bar formation. Constructive waves (low energy, long wavelength) are better at building bars because they deposit more sediment than they erode.

🌊 Wave Energy

Lower energy waves are more effective at depositing sediment and building bars.

➔ Wave Direction

Consistent wave direction helps maintain longshore drift and bar development.

⏱ Tidal Range

Areas with smaller tidal ranges often have better conditions for bar formation.

Sediment Supply

Bars need a constant supply of sediment to form and maintain themselves. This sediment can come from rivers, cliff erosion, or offshore sources.

The size and type of sediment also matter. Sand creates different bar types compared to shingle or pebbles. Finer sediment can be transported further, while coarser material tends to form steeper, more stable bars.

Human Impact and Management

Bars don't exist in isolation from human activities. People have been using, modifying and sometimes struggling with bars for centuries.

Benefits of Bars

Bars provide natural coastal defence, protecting the land behind them from wave attack. They also create unique habitats like lagoons and salt marshes, which are important for wildlife.

🛡 Coastal Protection

Bars act as natural sea walls, absorbing wave energy and protecting coastal communities from flooding and erosion.

🐺 Wildlife Habitats

The lagoons and marshes behind bars provide crucial breeding and feeding areas for many species of birds and marine life.

Challenges and Management

Bars can also cause problems. They may block harbours, change drainage patterns, or be vulnerable to storm damage. Managing these features requires understanding their natural processes.

Common management strategies include beach nourishment (adding sediment), building groynes to control longshore drift and creating managed retreat zones where bars can migrate naturally.

Climate Change and Future Challenges

Climate change is affecting bar development through sea level rise, changing storm patterns and altered sediment supply. Rising sea levels may cause bars to migrate landward or disappear entirely.

Understanding these changes is crucial for coastal management. Some bars may need active management to survive, while others might be allowed to change naturally as part of coastal adaptation strategies.