Coastal Features - Transportation & Deposition

Coasts Page

EROSION – TRANSPORTATION - DEPOSITION

TRANSPORTATION AND DEPOSITION • • • • •

Transportation This is the movement of materials like sand and pebbles by the sea. Deposition This is the material dropped by the sea, usually on a beach. Spit A spit is an area of sand or shingle which either extends at a gentle angle out to sea or which grows across a river estuary. Bar A bar is a barrier of sand stretching across a sheltered bay. Beach This is a landform feature caused by the sea depositing sand and pebbles.

TRANSPORTATION AND DEPOSITION

• • •

Swash The water which rushes up a beach after the wave breaks. Backwash The water which returns down the beach to the sea. Longshore drift The zig-zag movement of material along a beach caused by the sea's swash and backwash.

LONGSHORE DRIFT • ~ Sediment is moved along a shore by Longshore drift. The waves break on the shore at an angle – the • 1. SWASH pushing material up the beach • 2. gravity pulls the wave and sand back down the • 3. beach in a more or less straight line - the BACKWASH • 4. The next wave picks up the sediment and the process is repeated. If the coastline changes direction away from the • 5. sea, the sediment is still carries along the coast until the change of direction and then it is carried out into the sea and quickly deposited in deeper water. The result is the formation of a spit. • 6.

Process of Longshore Drift Waves approach the shore at an angle T

Sediment gets moved along the beach BACKWASH

IF R D RE O H GS N O L

SW

AS H

SW

AS H

A spit

a. b.

c.

•

A spit is a narrow ridge of sand and shingle attached to the coast at one end and ending in open water at the other. The best example in Britain is Spurn Head on the Humber Estuary. Spits are formed by longshore drift - the movement of sand along the coast by waves hitting the coast at an angle. Imagine throwing a ball at an angle to a wall. To get the ball you would have to walk, it would not come back to you. Keep moving throwing the ball at an angle and eventually you would have moved a long way from where you started. Now replace the wall with a beach, you with a wave and the ball with sand and you can imagine how longshore drift works. Spits are formed when the coastline changes direction but the sand carries on moving the same way and builds out into the sea. The best example in Britain is Spurn Head on the Humber Estuary - have a look in an atlas. The sand is moving down from the North and continues moving building out to the South. The spit ends when the water is too deep for the sand to build up above the surface. Water behind the spit is quite sheltered, allowing deposition to take place. Mud, sand and silt builds up and plants begin to colonise the area forming salt marshes.

Formation of a spit A lagoon forms behind the spit

Sediment moves along the beach

Longshore Drift

As the coast changes direction material is deposited to form a spit

The spit grows and the end becomes hooked

A change in the prevailing wind direction makes the end hooked Sediment builds up in the sheltered water of the lagoon Settlement and farmland grow on the land behind the lagoon.

Spurn Head Spit Spurn Point is a narrow sandy promontory approximately five kilometres long. The photo shows Spurn Point from the south. The buildings in the foreground are a lifeboat station and Humber Pilots base. Direction of Longshore Drift ~ Source of deposition from Holderness coast

Narrow neck nearly breached

Sheltered lagoon filling with mud

Spit hooks across the Humber Estuary

Deep water estuary

Original coast Sheltered lagoon

Hurst Ness Spit, Hampshire Direction of Longshore drift

Sheltered lagoon

Groynes to stabilise the spit

Direction of Longshore drift

Hooked end of the spit

BARS ~ a ridge of sediment which is parallel to the coast. It is not always exposed. It may be slightly off-shore or it may be a ridge blocking an estuary. The lagoon is trapped behind

This is the Bar

TOMBOLO ~ a bar joining an island to the mainland.eg. Chesil Beach

COASTAL DUNES

~ Sand dunes form on the landward side of some beaches. If lots of beach is exposed at low tide, the wind can dry out and then carry the sand inland. Obstacles catch the particles causing a ridge to form and this is a dune. They are a very valuable natural sea defence. Often they need stabilising with plants like marram grass or netting and need protection from human erosion.

RIAS ~ flooded river valleys formed by sea level rise usually at the end of an ice age. Eg. FAL Estuary, Falmouth.

RAISED BEACHES ~ as the result of a change in sea level beaches can be left some height above present sea level eg. Marazion, Mounts Bay. Islay, Scotland

30m Raised beach

Former cliffs

Present beach

Lower raised beach

Storm ridge At this location there is a 30 metre raised beach and below it a line of former cliffs and caves

FJORDS ~ flooded glaciated valleys formed in the same way.

Factors at work on coasts

Back to Start

Coasts Page