Depositional Features - Drift Aligned
Spits
- A spit is an embankment of sand and shingle extending from the mainland out into the sea.
- The formation of a spit begins where the is a sudden change in the shape and direction of the coastline, for example at a headland.
- Normally longshore drift would carry sediment around the coast, however when there is a substantial change in the shape longshore drift continues to deposit material in the same direction rather than following the coastline.
- Overtime the ridge grows in size with material being deposited faster than it can be removed.
- Deposition of sediment can be brought about earlier near estuaries as the flow of water into the sea is stronger than the drift, forcing the sediment to be dropped.
- The shape of a spit can be changed by a couple of things:
- Firstly wave refraction curves around the spit, forming a 'hook' on the end known as a recurved spit.
- Change in the prevailing wind can also form the recurved end.
- Sand dunes often form on spits as well as salt-loving vegetation colonising the sheltered part of the sea behind the spit creating a salt marsh.
- Spits are eroded by the sea and wind but as longshore drift continues along the coast, a constant supply of sediment ensures their continued existence.
- Good examples of spits in the United Kingdom include Blakeney Point in Norfolk and Spurn Head in Humberside.
Bars and Tombolos
- Bars are formed in similar ways to spits.
- As longshore drift transports sediment down the coast, it deposits in low energy zones, such as a bay.
- Bars are particularly noticeable at low tide where they can be exposed in relation to the rest of the beach.
- During high tide, the bar can make the water shallow resulting in the waves breaking earlier before they reach the shore.
- When a spit or a bar has enough sediment to continue to build up, it is possible that it may reach an island just off the coast. This is then known as a tombolo.
- Good examples of tombolos include Chesil beach, which connects the Isle of Portland to mainland Britain, and Llandudno in North Wales.