CoastsLongshore Drift

Lesson Objectives• To recognise the impact of longshore drift on

the coastline.

How did the beach ball end up here?

• http://www.youtube.com/watch?v=U9EhVa4MmEs&feature=player_embedded

Longshore drift

The movement of material along the coast in the direction of the prevailing winds

Benefits of Longshore Drift :

•Builds up wider beaches at coastal resorts with groynes so more people can use the beach

•Provides more beach material further along the coast to help protect coastline from erosion

Waves approach at an angle to the shore

Beach material is Higher on the side of the groyne facing the prevailing wind and

waves

Beach is Wider on the side of the groyne

facing the approaching waves

Eroded material (chalk pebbles from Flamborough head)

are found down the coast where the natural cliff

material is Boulder Clay

Longshore Drift

Draw a diagram showing the process of Longshore Drift.

Make sure to include all the labels.

Self assessment: Give yourself a level

Homework: Exam Question

Have a go at answering this exam question from June 2011 Past Paper.

Expect full and accurate definition for max marks (e.g. the movement of material (1) parallel to coast (1) by/because of ...(1)). Partial statements conveying idea of material moving of sediment worthy of 1 mark.

Direction – accept eastwards, easterly, east, northeast or west-to-east (1). Accept - in direction of prevailing wind (1). Reason – credit any valid Figure 2a/b observation e.g. build-up of deposition on west side of Cobb(1); more beach on west side of groynes(1)... Accept likely fieldwork finding that beach height lower on east side of groynes(1); prevailing wind (if not offered previously unless qualified/developed)

CoastsBeaches, spits

and bars

Lesson Objectives• To be able to describe and explain the

formation of beaches, spits and bars.

Coasts : Features of Deposition / Spit, bar and tombolo

Key Terms :

Longshore Drift

Deposition feature

Spit

Bar

Tombolo

Peninsula

Example / Case-Study :

Spurn Head, E. Yorkshire (Spit)

Slapton Ley, Dorset (Bar)

Chesil Beach, Dorset (Tombolo)

Possible Questions :

Describe and explain the formation of a coastal feature of deposition.

What is the difference between a Bar and a Spit?

As cliff and beach material is transported along a coast by Longshore Drift, if the flow is interrupted or slowed down, the material is DEPOSITED to form a range of Features of Deposition. A Spit, bar and tombolo are all low-lying beaches made up of sand, gravel, pebbles and rise just a few metres above high tide. Beach grasses may grow in them (marram grass) and the roots then stabilise the features and wind-blown sand may accumulate further. But their low height may mean they are submerged, altered or washed away in severe storms when Erosion is more powerful than Deposition.

Spit A linear beach that extends from the shore across a river estuary or along the coast – but doesn’t join on to any other feature. It is recurved at its end depending on which is the most powerful current.e.g Spurn Head, E. Yorkshire

Bar A Spit which goes right across a bay and joins onto a shore at the other side. Usually where there is no strong river current trying to get to the seae.g Slapton Ley, Dorset

Tombolo A Spit which extends out to join on to an island so that it forms a Peninsula rather than an Island.e.g. Chesil Beach, Dorset

Is this a

SPIT

TOMBOLA or

BAR ?

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Coasts : Features of Deposition / Spit, bar and tombolo

Key Terms :

Longshore Drift

Deposition feature

Spit

Bar

Tombolo

Recurved

Lagoon

Salt Marsh

Example / Case-Study :

Spurn Head, E. Yorkshire (Spit)

Slapton Ley, Dorset (Bar)

Chesil Beach, Dorset (Tombolo)

Possible Questions :

What is the environmental value of a spit, bar or tombolo?

Are these permanent or temporary features of coastal deposition?

A SPIT is unable to extend right across an estuary due to the force of the river current finding a route out to the sea.It is RECURVED at the end in the direction of the strongest of the two currents. In Spurn Head’s case, the North Sea current is more powerful than the River Humber current, so the spit curves in towards the estuary.

A TOMBOLO – where a spit extends out to an island and joins it to the mainland.The most famous is CHESIL BEACH in Dorset at 11 miles long.

A BAR is a spit which extends across a bay and reaches the shore on the other side. The water which is cut off becomes a salt-water LAGOON of still water. Eventually, over time the lagoon fills in with wind-blown sand, sediment, reeds and other vegetation and becomes a marsh.This is SLAPTON LEY in Dorset.

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River Humber Current North Sea Current

A

E

D

C

B

A : Sea erodes the coast and Longshore Drift transports material down the coastB : The North Sea and R. Humber currents meet, slow down & deposit their material. The N.Sea current is stronger so the recurved spit bends in to the Humber channel.

C : As the Spit head grows southwards, erosion still takes place up-coast

D : The neck of the spit has more erosion than deposition – so gets thinner

E : The sea breaks through the neck, the spit head is an island and is eroded away

Coasts : Features of Deposition / Evolution of Spurn Head, E. Yorkshire – A SPIT 14

Direction of the River Humber current

Direction of th

e North Sea cu

rrent

Deposited river silt (mud)

Turbulence where 2 currents meetRecent Deposition

Thin neck of spitLikely breach

Spurn Lighthouse

Spurn Lifeboat station

Coasts : Features of Deposition / Features of the spit at Spurn Head 15

Coasts : Features of Deposition / Issues of coastal protection of a Spit

Key Terms :

Longshore Drift

Deposition feature

Spit

Estuary

Example / Case-Study :

Spurn Head, E. Yorkshire (Spit)

Possible Questions :

Why are there disagreements about whether to protect coasts from further erosion?

What are the issues surrounding the management of sensitive coastal environments?

The thin neck of Spurn Head is only a few metres wide. Victorian coastal engineers tried to stop the spit eroding away at its thinnest point over a hundred years ago, but their wooden groynes have now collapsed and are no longer effective. The issue is whether to protect the spit from further erosion, or let the sea carry out its operations, and erode it away.

Issues if the spit is allowed to be eroded away:•The 7 homes of Britain’s only full-time lifeboat crew of the Humber lifeboat are located at the southern end. They would have to be relocated – possible to Grimsby if the spit eroded away – meaning a longer journey time into ships in distress in the N. Sea

•The Humber pilots are based there too – they go out to ships entering the Humber estuary – one of the busiest in Britain – to guide ships around the shifting and dangerous sand-banks of the estuary.

•The spit is an important route for migrating birds from the continent which, in spring, use the spit to guide them into Britain, and in autumn on their flights south for the winter.

•The Spit is managed by the Yorkshire Wildlife Trust and is a vital habitat for many plants & insects

Issues if the spit is protected from further erosion•The costs of protecting the entire length of the spit would be huge.

•The Victorian attempts to keep the spit in one position have increased its current problems as the Holderness coast to the north continues to erode westwards – leaving the spit more exposed to NE winds and waves.

•The erosion of material from the spit provides important material to help protect the beaches south of the Humber in Lincolnshire and even the Netherlands. Stopping the erosion and movement of this material from Spurn would cause faster erosion elsewhere.

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Things you could include…..•Direction of longshore drift•Arrows to show swash and backwash•Area of calm water behind spit

•Saltmarsh

•Vegetation on spit

•Hooked end because of wind changes