Coastal Systems - Features of Coastal Deposition

Depositional coastlines are characterised by an input of sediment from cliffs, beaches, river estuaries of sea-bed stores.

Material is redistributed by waves, tides and currents and in low-energy conditions is deposited along the shore or just off-shore to create distinctive features. These may be temporary and altered during high energy conditions, but re-formed once conditions calm and a state of dynamic equilibrium is restored.

Beaches: result from the dominance of constructive waves over destructive waves leading to a net gain of beach material over time creating a store of sediment.

Spit: the deposition of material transported along a coast by longshore drift (littoral movement) at a break in coast orientation and where the dominant current slows and weakens to produce an extended finger of depositional material, often with a recurved end.

Compound spits exhibit a number of recurved ‘spurs’ along their length as each recurvature represents a ‘break in coast orientation’ and the development of a new extension of the main spit under conditions of consistent longshore drift.

Bar: a continuation of a spit to join on to the coast across a bay. Usually where there is a break in coastline with longshore drift operating but without an intervening river current to disrupt continuous deposition.

Tombolo: where a spit happens to extend and join to an island, linking it to the mainland by a narrow beach of deposited material.

Offshore bar and barrier islands: characteristic of shores with a large influx of sediment, often from major rivers and currents which deposit and build accumulations of material into long, low mounds of sediment parallel to the coast. Offshore bars are largely submarine features while barrier islands are usually above high tide and contain sand dune formations.