Coastal deposition

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Coastal deposition

Where sand/shingle is deposited on a beach rather than removed - inputs are greater than outputs.

The most common form of coastal deposition that occur as a result of sediment being deposited, that may have come from rivers, and cliff erosion. Human impact may increase the supply of material available.

Spits

Narrow, long stretches of sand/shingle that extend out to sea, or partway across a river estuary. One end is more protected than the other, and mud flats/salt marshes may develop in sheltered areas behind them. One of the most famous examples is Chesil Beach in Dorset:

  • Sandy spits form as a result of dominant constructive swell waves.
  • Shingle spits are a result of dominant destructive waves.

Why do spits develop hooked ends?

Two explanations are offered:

  1. A change from the prevailing wind direction, coinciding with the direction of second most dominant fetch and wave direction.
  2. Wave refraction occurs at the end of the spit which carries some material into more sheltered areas.

Tombolos

This is where a spit or bar connects the mainland to an island.

Such features are uncommon in the UK, but are the most common feature of coastal deposition in the world, shown by their presence on the Eastern Seaboard of the USA from New Jersey south to Florida. They are a number of sandy beaches that are totally separate to the main land, but run parallel to it, meaning that lagoons may develop behind them.

Can be described as triangular beaches. Their origin is due to longshore drift operating on a coastline from two different directions. The two sets of storm waves build up a series of ridges, each protecting the material behind it, creating the triangular feature.

Not strictly a feature resulting directly from marine action, but the blowing of sand from a beach inland.

Conditions for formation:

  1. Strong on-shore winds.
  2. Large expanses of dry sand (spits, cuspate forlands, bays).
  3. Obstacles to limit sand movement.

Sand movement (saltation):

Is helped or hindered by:

  1. Wind velocity.
  2. Grain size and shape.
  3. Dampness of sand.
  4. An obstacle present around which deposition of sand occurs and vegetation grows.

Sand dunes

Sand dune characteristics:

These apply to the diagram above.

Name: Characteristics:
Embryo dune The first part of the dune to develop. Stabilisation occurs via marram and lyme grass, which act as traps for sand. Conditions are dry and plants adapt to this via long roots, or thorny leaves to reduce evapotranspiration.
Yellow dune Colour is due to a lack of humus, but with distance inland they become increasingly grey due to greater amounts of humus. Heights can reach 5m and plants include sand sedge, sea holly, and red fescue.
Fixed grey dunes Limited growth due to distance from beach. Far more stable as shown by existence of thistle, evening primrose, bracken, bramble and heather.
Dune slacks Depressions between dune ridges, which will be damp in summer and water-filled in winter. Species include water mint, rushes, and weeping-willow.
Blow outs Often evidence of over use by humans. Large 'holes' that appear in the dunes.

The most important component for their development is shelter, usually provided by estuaries, barriers, and spits. This is followed by fine sediment in the form of silt and clay grains that is the main input into the system. Over time, sediment is deposited and is not easily removed, especially as flow velocities are low, and the length of time the area is not covered by water increases.

Common vegetation includes algae and Salicornia due to their ability to withstand both being underwater and high levels of salinity. Eventually, Spartina grass may dominate.

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