Simple, compound spits, Tombolos, Offshore Bars and Bars

• What is the difference between simple and compound spits

  Simple spit:

  • a long, narrow ridge of sand or shingle that extends from the coastline into the sea, often at a point where the coastline changes direction. It forms when longshore drift moves sediment along the coast, and deposition occurs where the current loses energy (e.g., in areas of calm water).

  Compond spit:

  • has multiple "barbs" along its length, often due to changes in direction of the prevailing winds or currents. Over time, a compound spit can develop from a simple spit if longshore drift shifts direction or if multiple phases of deposition occur.

  
  

  Example of Compound: Entrance Poole Harbour

  
• How do tombolos form

  Tombolo is a beach (or ridge of sand & shingle) that has formed between a small inland and the mainland.

  • Deposition occurs where waves lose their energy and the tombolo builds up.
  • Tombolos may be covered at high tide, for example at St Ninian's in the Shetland island

  
• How do Bars/ Barrier beaches form

  • When a beach or spit extends across a bay to join two headlands, it forms a barrier beach or bar.
  • Formed from rounded shingle shingle deposits
  • Bars can also trap water behind them to form lagoons such as Slapton Leg

  
• How do Offshore bars form

  • They are submerged or partially exposed ridges of sand or coarse sediment created by waves offshore from the coast.
  • Destructive waves erode sand from the beach with their strong backwash & deposit it offshore.
  • Offshore bars act as both sediment sinks and potentially sediment input stores.
  • They can absorb wave energy thereby reducing the impact of waves on the coastline