Transcript Longshore_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=U9EhVa4
MmEs&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)
Beaches, 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
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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
Bar
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
Deposition feature
Spit
Bar
Tombolo
Example / Case-Study :
Spurn Head, E. Yorkshire (Spit)
Slapton Ley, Dorset (Bar)
Chesil Beach, Dorset (Tombolo)
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 sea
e.g Slapton Ley, Dorset
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
Longshore Drift
Peninsula
Possible Questions :
Is this a
Tombolo
Key Terms :
SPIT
TOMBOLA or
BAR ?
Describe and explain the
formation of a coastal
feature of deposition.
What is the difference
between a Bar and a Spit?
Coasts : Features of Deposition / Spit, bar and tombolo
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 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 saltwater LAGOON of still water.
Eventually, over time the
lagoon fills in with windblown sand, sediment, reeds
and other vegetation and
becomes a marsh.
This is SLAPTON LEY in
Dorset.
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.
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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?
Coasts : Features of Deposition / Evolution of Spurn Head, E. Yorkshire – A SPIT
A : Sea erodes the coast and Longshore Drift transports material down the coast
B : 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.
A
C
D
B
E
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
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Coasts : Features of Deposition / Features of the spit at Spurn Head
Direction of the River Humber current
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Coasts : Features of Deposition / Issues of coastal protection of a Spit
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 fulltime 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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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?
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