Coastal Landscapes

Landforms of Coastal Deposition

Introduction

Depositional landforms are created when wave energy decreases, allowing sediment to accumulate and shape the coastline. These features include beaches, spits, onshore bars, tombolos, and salt marshes. Understanding their formation requires recognising the interplay of transport, deposition, and wave dynamics.

1. Beaches

  • Description: Beaches are stretches of sand, shingle, or pebbles found along coastlines, acting as natural buffers between the sea and the land.
  • Formation and Processes: Beaches form when constructive waves transport sediment to the shoreline, depositing it due to their strong swash and weak backwash. Longshore drift brings additional material from other parts of the coast. In low-energy environments, fine sand forms gently sloping beaches, while high-energy conditions result in coarser material like pebbles creating steeper profiles. Seasonal changes also affect beaches, with destructive waves removing sediment during winter and constructive waves rebuilding them in summer.

2. Spits

  • Description: Spits are narrow ridges of sand or shingle that extend from the coastline into open water, often curving at their end.
  • Formation and Processes: Spits form where longshore drift transports sediment along the coast and deposits it when wave energy decreases, such as at estuaries or where the coastline changes direction. Over time, sediment accumulates, and the spit grows outward. The hooked end of a spit is shaped by wave refraction or changes in the prevailing wind direction. In estuaries, tidal currents may halt the spit’s extension, leaving an open end.

3. Onshore Bars

  • Description: Onshore bars are ridges of sediment that stretch across a bay, connecting two headlands and often enclosing a lagoon.
  • Formation and Processes: Onshore bars form when longshore drift transports sediment across a bay, and deposition occurs in the relatively calm waters. This process continues until the sediment ridge spans the entire bay, joining the headlands. Bars require low-energy environments to prevent wave action from removing the deposited material.

4. Tombolos

  • Description: Tombolos are ridges of sand or shingle that connect an offshore island to the mainland.
  • Formation and Processes: Tombolos develop as wave refraction around an island reduces wave energy in the sheltered zone between the island and the mainland. This calm area allows sediment carried by longshore drift or waves to be deposited, gradually building a land connection. Over time, continuous sediment accumulation strengthens the tombolo.

5. Salt Marshes

  • Description: Salt marshes are coastal wetlands formed in sheltered intertidal zones, often behind spits or bars, and are characterised by vegetation adapted to saline conditions.
  • Formation and Processes: Salt marshes form in low-energy environments where fine sediment is deposited by rivers or tides. Initial mudflat formation occurs as deposition settles out of the slow-moving water. Pioneer plants like cordgrass colonise these areas, stabilising the mudflats and trapping more sediment. This raises the marsh surface, allowing for the succession of other vegetation species. Over time, the marsh expands and develops complex drainage channels.

Processes Driving Deposition

  • Constructive Waves: Deposit sediment onshore with their strong swash.
  • Longshore Drift: Moves sediment along the coast to supply material for features like spits and bars.
  • Wave Refraction: Reduces wave energy in sheltered areas, allowing deposition.
  • Tides and Currents: Transport sediment and deposit it in low-energy zones, such as estuaries and lagoons.
  • Vegetation Stabilisation: In salt marshes, plants trap sediment and contribute to landform growth.

Conclusion

Depositional landforms such as beaches, spits, onshore bars, tombolos, and salt marshes showcase how sediment accumulation shapes coastlines. The balance between sediment transport and deposition highlights the dynamic interactions within coastal systems. For OCR A-level Geography students, these landforms illustrate the role of deposition in creating diverse and valuable coastal features.