Longshore drift a process of coastal transportation deposition for web.pdf · 4. Tombolo Tombolos...

• Longshore drift – a process of coastal transportation

• The sea transports its load of sand and shingle in two main ways:

1. Up the shore in swash

2. Back down the shore in backwash

• Processes of coastal deposition

Coastal deposition occurs when waves lose energy and therefore their ability to carry material

The sea deposits its load on the coastal area

Constructive waves have an ability to move the load inland

Wave refraction in bays result in wave energy decreasing and this results in deposition occurring

Deposited material may include shingle, sand and sediment

• Landforms of coastal deposition

1. Beach

A beach is an area of sand and gravel located in an area between high tide and low tide mark

Waves move up the sea shore, slow down and their load of sand and rocks is deposited

Heavier load of rock and shingle is deposited on the backshore

Finer, lighter material such as sand is deposited on the foreshore

Over time a beach is formed

A storm beach forms when waves are strong enough to push large rocks and boulders above the high tide mark

Rossnowlagh beach

Longshore Drift

Bundoran

Sand Spit

• Landforms of coastal deposition

2. Sand spit

A sand spit is a long narrow ridge of deposited sand and shingle that is connected to the coast at one end

Sand spits develop due to longshore drift and deposition

Sand spits form where the process of longshore drift is interrupted

Sediment is deposited in sheltered and shallow water

Over time this sediment builds up and becomes visible above the level of the sea

• Landforms of coastal deposition

2. Sand spit (continued)

On the seaward side of the sand spit constructive waves deposit fine sediment such as sand and form a beach

Over time the sand spit gets wider

As sand spits grow the area of water behind it is often then sheltered by waves and wind

This often leads to the formation of a salt marsh

Wave refraction may occur at the end of a sand spit which results in the sediment being carried around the end of the sand spit to form a recurved spit (i.e. hook)

• Landforms of coastal deposition

. Sandbars - formation

A sand bar forms when a sand spit extends across a bay and over time connects two land masses

Sand bars will eventually seal off an area of coastline and the water directly behind the sand bar will then be called a lagoon

There are two types of sand bar:

1. Offshore bar

Located away from the coastline

Parallel to the coastline

Generally only exposed at times of low tide

Breaking waves deposit sand on the offshore bar

Size of the offshore bar increases over time

May eventually block or close off the bay creating a lagoon

Sand Bar in Jersey

Bartragh Island in Killala Bay Co. Mayo

2. Baymouth bar

Sand spit extends from one side of a landmass across a bay and reaches the other landmass

Blocks/closes off the bay

Over time the water in the lagoon will dry up and the area will be colonized by salt loving vegetation – it is now called a SALT MARSH

Australia

Rio de Janeiro

Tampa Florida

builders

Alaska

Turkey

Lady’s Island

Lake

Salt Marshes

• Landforms of coastal deposition

4. Tombolo

Tombolos are formed due to wave refraction and longshore drift

Result from a sand spit extending out to an island and connecting the island to the mainland

Waves approach the island, wave refraction occurs

Sediment is deposited

Results in the spit connecting the island to the mainland and – a tombolo is formed

Coastal Features