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Unit 1 Section A - How physical processes have created coastal landforms

Page history last edited by K J Hutchinson 10 years, 9 months ago

Lesson 1 - Introducing the coast

 

Learning objectives:

- to define the key terms coast and inter-tidal zone

- to be able to produce a diagram showing the processes operating at the coast (this acts as an overview for Section A)

- to understand why tides occur

 

The coast is the narrow strip of shoreline that separates the land from the sea. Land, sea and air meet at the coast, making it a constantly-changing environment. The coast is also affected by human activity - over 4 billion people worldwide now live at the coast.

 

The coast is an open system because it has inputs from outside (eg. sediment from rivers) and outputs into other systems (eg. sediment transferred into deep seas). Many processes operate at the coast - for example, sub-aerial weathering, erosion, transportation and deposition. Some of the inputs, processes and outputs are shown on the diagram below. Try to keep this diagram in mind as you work through Section A - it is a very useful overview of the key parts of the topic.

 

 

The inter-tidal zone is sometimes known as the surf zone or the foreshore. It is the part of the coast between high-water and low-water marks i.e. the area that is sometimes covered in water and sometimes exposed to the air. The backshore is inland of the inter-tidal zone and is usually above the influence of the waves. The nearshore (sometimes called the breaker zone) is where the waves break; the offshore zone is further out to sea and is beyond the influence of the waves. These areas are marked on the diagram below.

 

 

Tides are created because the Earth and the moon are attracted to each other. You could think of them like magnets. The moon tries to pull at anything on the Earth to bring it closer. However, the Earth is able to hold onto everything except water (the earth can't hold onto water as it is always moving).  Each day, there are two high tides and two low tides because the moon is pulling at the water. The ocean is constantly moving from high tide to low tide, and then back again. There are about 12 hours and 25 minutes between the two high tides.

 

Useful weblinks:

National Maritime Museum GCSE guide to tides

Wikipedia definition of inter-tidal zone

 

Lesson 2 - Waves

 

Learning objectives:

- to be able to explain how waves form using key geographical terminology such as prevailing wind, fetch, friction, break

- to be able to define the swash and backwash of a wave

- to know the key characteristics of constructive and destructive waves

- to be able to produce an annotated diagram showing the key characteristics of constructive and destructive waves

 

Waves get their energy from the wind. As the wind blows over the surface of the sea, it creates friction. This frictional drag causes water particles to rotate and their energy is transferred forward in the form of a wave. Whilst the water moves forward, the water particles return to their original position.

 

As a wave reaches shallow water, friction between the sea bed and the bottom of the wave causes it to slow down. Its shape becomes more elliptical (rather than circular). The top of the wave (called the crest) is not affected by the friction, and it becomes steeper until it eventually breaks. When the wave breaks, water rushes up the beach. This is called the swash. The movement of water back down the beach is called the backwash.

 

There are three main factors that affect the strength of a wave:

 

1. The strength and speed of the wind.

2. The duration of the wind - this is the length of time for which the wind has blown.

3. The fetch - this is the distance over which the wind has blown. 

 

You can see the influence of these factors by using the Wave Machine Simulator.

 

 

Waves are one of the most significant forces in shaping the coastline. There are two main types of wave - constructive waves and destructuve waves.

 

Constructive waves are low energy waves that tend to arrive at the coast at a rate of less than 8 waves per minute. Constructive waves are small in height. They have a strong swash and a weak backwash. This means that constructive waves tend to deposit material and build up a beach.

 

Destructive waves have much higher energy and tend to arrive at the coast at a rate of more than 8 per minute. They are much larger in height than constructive waves, often having been caused by strong winds and a large fetch. Destructive waves have a weak swash but a strong backwash so they erode the beach by pulling sand and shingle down the beach as water returns to the sea. This means that less beach is left to abosrb wave energy.

 

         

 

Useful weblinks:

BBC Bitesize article about waves, including an animation of swash and backwash

Wave machine simulator - create your own wave

Wycombe High School - animation of constructive and destructive waves

Curriculum bits - explanation and animation of constructive and destructive waves

 

Lesson 3 - Weathering and erosion 

 

Learning objectives:

- to be able to define the terms weathering and erosion, marine erosion and sub-aerial processes

- to be able to explain how the processes of marine erosion operate - abrasion (corrasion), attrition, solution and hydraulic action

- to be able to explain how the sub-aerial processes operate - solution (corrosion) and wetting/drying

 

Weathering is the breaking down of rocks by the action of the weather, plants or chemical action. In happens in situ - this means 'without movement'. This is the main difference between weathering and erosion. Erosion refers to rocks being broken down and moved away. Marine erosion is the wearing away and removal of rocks by the action of the sea.

 

You need to be able to define the four main processes of marine erosion. They are corrasion (abrasion), attrition, solution and hydraulic action.

 

Corrasion (abrasion) this is where the bits of rock and sand that are carried in the waves are hurled at cliffs and grind them down. It is sometimes described as sandblasting.

 

Attrition is where the waves cause rocks and pebbles that they are carrying to smash into each other and break down. They become smaller and rounder.

 

Solution is where acids contained in sea water slowly dissolve certain types of rock. Sea water is very corrosive and slowly dissolves chalk and limestone. This increases the size of the cracks and joints in the rock so that the other forces of erosion can become more effective.

 

Hydraulic Action is the constant force of waves crashing on the shore. When waves crash against the cliffs they force air into cracks in the rock. The air is trapped, and pressure builds up. As the waves move back, pressure is released and the trapped air expands. Small explosions take place and weaken the rock, blasting fragments away.

 

You might find it useful to remember the 4 processes using the phrase CASH. Can you work out which image in the cartoon represents each of the four processes?

 

                                 

 

Note: The AQA(B) course classifies solution (corrosion) as a form of weathering. This is because salt spray gets in cracks in the rock, and when it evaporates it leaves salt crystals behind. As the crystals grow in size, they can force the rocks apart. This happens 'in-situ'. The process of solution described above, however, is erosion as the rock is carried away.

 

Wetting/drying is another form of weathering. Here, softer rocks like clay expand when they are wet and contract when they are dry (think about your pottery lessons if you need an easy way to remember this!). Over time, the continual expansion and contraction can weaken the rocks and make them more easily eroded by other processes.

 

Sub-aerial processes are processes that attack the face and top of a cliff - i.e. those parts that are exposed to the atmosphere. Sub-aerial processes include rainfall, surface runoff and freeze-thaw weathering.

 

Useful weblinks:

Animation of processes of coastal erosion from BBC Bitesize

Video clip from the BBC showing how sub-aerial processes and erosion at the Bullers of Buchan

Coastal Kung-Fu video from Portchy (SLN)

 

Answers to cartoon quiz above: A Hydraulic Action; B Abrasion; C Solution; D Attrition

 

Lesson 4 - The influence of geology at the coast - bays and headlands

 

Learning objectives:

- to understand how rock type affects coastal erosion, focusing on chalk and clay

- to be able to produce an annotated diagram to show how headlands and bays form along a discordant coastline

- to examine how the Dorset coastline (Isle of Purbeck) has been influenced by geology

 

Rock type (geology) is one of the main factors affecting the shape of a coastline. Headlands and bays form along coasts that have alternating bands of harder and softer rock. The hard rock is more resistant to erosion, so the coastline is worn away less quickly leaving a headland sticking out into the sea. Where there is softer rock, erosion is quicker and a bay forms. This is known as differential erosion.

 

 

Coastlines with alternating bands of hard and soft rock are sometimes called discordant coastlines or Atlantic coastlines.

 

Scarborough is a really good example of how headlands and bays form on a discordant coastline. You can see the North Bay, Castle Headland and South Bay on the map and photo below. Make sure that you can identify these features on both the map and the satellite photo. We will see in a later lesson how the Castle Headland is now having to be protected against the power of the waves.

 

 

Over time, as the headland juts out more into the sea it becomes more exposed to the full force of the wind and the waves. This means that it becomes more vulnerbale to erosion that the sheltered bay which has a wide beach to protect it. Wave refraction focuses the force of the waves on the headland. This protects the surrounding bays from erosion, and wide beaches can often form in these bays as a result.

 

 

Limestone, chalk and granite are examples of hard, resistant rock. Clay is a much less resistant rock and it erodes much more quickly.

 

Our main case study for this section of the course is the Dorset coast. The Dorset coastline is a great example of how geology can affect the shape of the coast. In the stretch of coastline between Studland Bay and Peveril Point (south of Swanage), four different bands of rock form the coastline. Studland Bay, which has been eroded quite quickly, is made of sand and clay. Moving southwards, where the rock type changes to chalk, 'The Foreland' sticks out into the sea as a headland. This is where Old Harry Rocks have formed. Further south, we come to Swanage Bay which has been eroded quickly because the rock type is clay which is soft and less resistant. Peveril Point is the next headland to the south. It sticks out into the sea because it is made of limestone, a more resistant rock which has managed to withstand the erosive power of the waves more effectively than the clay.

 

 

 

Useful weblinks:

S-Cool guide to the influence of geology at the coast

BBC Northern Ireland video clip about the formation of headlands and bays

Where's the Path - a great web application that allows youi to view OS maps and Google imagery side-by-side

BBC Bitesize on headlands and bays

 

Lesson 5 - Old Harry's mystery

 

Learning objectives:

- to be able to locate Old Harry Rocks

- to be able to explain how Old Harry and his wife formed

- to practise OS map skills - focusing on four and six figure grid references

 

Old Harry Rocks have resulted from the erosion of a chalk headland called The Foreland, between Swanage Bay and Studland Bay in Dorset. They are a stunning piece of coastal scenery and well worth a visit if you happen to be in the area. Old Harry and his (late) wife were formed by marine erosion and sub-aerial weathering. You need to be able to explain how this happened. Old Harry's wife was eroded away in a storm in 1896 - all that is left now is a stump.

 

 

Old Harry Rocks are one end of a band of chalk. A similar looking coastal feature is at the other end of the chalk band. Can you identify what it is? You could use this Google Earth file to help you.

 

One of the key skills we practice in this lesson is using grid references. You need to be able to give and find both 4-figure and 6-figure grid references. If you have any doubts about how to do this, you should look at the Slideshare presentation below (thanks to Slidehsare user jdmcd) and then work through the grid references activities on the Ordnance Survey Mapzone website or read through the OS leaflet 'Map reading made easy peasy'. If you are still having problems, please tell your geography teacher NOW so that we can get things sorted out before the exam! The examiner will expect you to be able to use grid references accurately.

 

 

 

Remember:

 

- Babies crawl before they walk

- Planes go along the runway before taking off into the sky

- Bottoms up!

 

                                  (especially for Mr Hogan!)

 

Useful weblinks:

BBC Class Clips video about Old Harry Rocks

Another BBC Class Clips video about Old Harry Rocks

Andrew Stacey's photos of Old Harry (with accompanying notes)

Jurassic Coast tour by Noel Jenkins (opens in Google Earth)

OS mapzone

 

Lessons 6 and 7 - Landforms associated with 'hard' coastlines

 

Learning objectives:

- to know what is meant by the terms 'hard coastline' and 'coastal landform'

- to learn the theory of the erosion of a headland and to be able to produce a fully annotated diagram to show the development of fault, crack, cave, arch, stack, stump

- to be able to explain how wave-cut notches and wave-cut platforms are created

 

A hard coastline is one that is made up of well-structured rocks with few lines of weakness. This is often the case with igneous rocks, but it can also happen with sedimentary rocks such as limestone and chalk. As mentioned above, these rocks are more resistant to the power of the waves, so rates of erosion are slow. A number of coastal landforms are associated with hard coastlines.

 

Chalk rocks often produce spectacular cliffs. These cliffs tend to be very tall, with a vertical drop to the sea from the top of the cliff. An example is Bempton Cliffs, in North Yorkshire. Bempton Cliffs are an RSPB reserve.

 

                    

 

You should make sure that you can describe and explain the process of headland erosion in detail. As it is quite a long process, you can pick up plenty of marks when answering an exam question about the erosion of a headland. To get full marks, you will need to use geographical terminology and write about the sequence of formation in the correct order! You should make sure that you can describe this sequence both in words and by producing an annotated diagram (similar to the one below).

 

Remember:

 

DESCRIBE means say what you see.

EXPLAIN means give reasons.

 

Examiners look for very different types of answer when using these two command words.

 

                           

 

The first step in the process of headland erosion is for the sea to attack the foot of the cliff. It will begin to erode natural weaknesses in the rock (eg. a crack or joint) causing them to be widened by hydraulic action, wave pounding and solution. Abrasion and hydraulic action then widen these cracks still further, creating small sea caves at the foot of the cliff. Over time, continued erosion of these caves leads to an arch being formed when the sea caves break through the headland. A combination of wave attack at the base of the arch and weathering of the roof of the arch (by frost, wind and rain) weaken the structure until eventually the roof of the arch collapses. This creates a stack which is a column of rock that stands separate to the main headland. Sea spray then weakens the stack, and eventually it is eroded away leaving a stump. This will be covered by the water at high tide.

 

 

The Coastal Erosion Animation on the BBC Northern Ireland website shows this process very clearly. You could test yourself on this topic by predicting what stage comes next before advancing the presentation.

 

You also need to know how wave-cut notches and wave-cut platforms are formed.

 

Marine erosion will be greatest when big waves actually break against the foot of a cliff. The foot of the cliff is then undercut - this means that there is a notch cut in the bottom, with the cliff overhanging it. This is known as a wave-cut notch. As the wave-cut notch gets bigger and bigger, the cliff above it becomes more and more unstable as it is not supported by anything. Eventually it collapses. The process keeps repeating, and the cliff retreats (moves inland) over time. The gently sloping land left at the foot of the retreating cliff is called a wave-cut platform. The photo below shows the wave-cut platform at Southerndown, near Bridgend in Wales.

 

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Remember: a coastal landform is a physical feature (eg. cliff, arch, stack, wave-cut platform) that has been shaped by erosion and/or weathering. It is very common for exam questions to ask you about how a coastal landform has been formed.

 

Useful weblinks:

BBC Northern Ireland coastal landforms

GeoNet - still images of the animation above, with captions

BBC Bitesize on the erosion of a headland

BBC Class Clips video about headland erosion creating the Green Bridge of Wales - includes a superb commentary

RSPB guide to Bempton Cliffs

BBC Class Clips on the development of a wave-cut platform

 

Well done to Collette making this animation to help fellow students with their revision:

 

                   

 

Lesson 8 - Why are 'soft' coastlines vulnerable to rapid erosion?

 

Learning objectives:

- to know what is meant by a 'soft' coastline

- to be able to explain why the cliff slumped under Holbeck Hall in Scarborough

- to know how the cliff has been stabilised since the slump

 

The rocks on a soft coastline do not have a strong structure and they become very unstable when wet. Examples of these type of rocks are gravel and clay.  Boulder clay (a deposit left by a retreating ice-sheet) is found along much of the Norolk Coast and the Holderness Coast in the UK. Cliff slumping and landslides are very common in these locations. They result from heavy rainfall (which is becoming increasingly common in the UK) soaking through the rock (a sub-aerial process) combined with wave attack at the base of the cliff. Abrasion is the main process of marine erosion on soft cliffs. The rock (eg. clay) quickly crumbles as it is hit by pebbles and stones carried by waves. Hydraulic action is important too.

 

Clay cliffs may suffer from rotational slumping. This happens when rain falls on the cliffs and water seeps through the permeable upper layers of the cliff (often sand or gravel) into the clay below. The clay is impermeable (this means that it doesn't let water through) so the water collects above the clay, making it more and more unstable. The cliff gets heavier and heavier with the addition of the water. Eventually, the top layer of rock slips down the cliff face in a rotational manner, under the force of gravity. The cliff rotates downwards towards the beach as the clay layer slumps.

 

One of the most spectacular landslides in the UK happened at Holbeck Hall in Scarborough in 1993. The four-star Holbeck Hall Hotel slid down the cliff and into the sea between the night of 4th and 5th June due to a rotational landslide! The cliff was about 60 metres high, and it was made of glacial till (a sandy, silty clay). This is a very soft material. The cliff retreated about 70 metres, and over 1 million tonnes of rock slumped to flow across the beach to form a semi-circle about 200 metres wide jutting out to sea for about 130 metres (see photo below). The cause of the landslide was a combination of heavy rainfall (140mm in April and May 1993), poor drainage on the slope, a build-up of water pressure in the cliff and the geology of the area. 

 

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In 1997, Holbeck Hall became the subject of a significant court case in English civil law when the owners of the hotel sued Scarborough Borough Council for damages, alleging that as owners of the shoreline they had not taken all practical measures to prevent the landslip. The claim was rejected on the grounds that the Council was not liable for the causes of the landslip itself.

 

Since the landslip occurred, the cliff has been strengthened at a cost of over £1.5 million.  The following work has been done (see diagram below):

 

(a) the slope where the landslide occurred has been made flatter - the flatter it is, the less likely it is to give way

(b) new vegetation has been planted (a salt-resistant type) to stabilise the cliff top

(c) drainage pipes have been dug into thje cliff to help rainwater drain away rather than collecting on top of the impermeable clay layer in the cliff

(d) a layer of large rocks have been put at the toe (base) of the cliff - this is rock armour to protect the base of the cliff from wave action - it has been cemented into place

(e) the cliff has been infilled with a layer of small rocks to allow the rainwater to pass through easily

(f) permeable geotextile bags filled with clay from the original landslide have been packed against the slope

(g) geotextile sheets (a special type of strong plastic which lets the water through) have been placed within the cliff

 

Can you match up each of the steps (a)-(g) with the labels on the diagram above?

 

Useful weblinks:

British Geological Survey article about Holbeck Hall landslide (including a photo gallery)

BBC News revisit Holbeck Hall 15 years after the cliffslide - video

BBC Class Clips video about blowholes and cliff slumping

 

Lesson 9 - Longshore drift

 

Learning objectives:

- to be able to define longshore drift

- to be able to explain the process of longshore drift using both words and diagrams

- to know the difference between swash-aligned beaches and drift-aligned beaches

- to know what a groyne is and be able to explain how it prevents longshore drift

 

Beach sediment (sand, stones, pebbles etc) is moved up and down the beach (in the swash and backwash). It may also be moved along the beach, by a process called longshore drift (LSD).  Longshore drift happens when the waves approach the beach at any angle other than parallel to the beach.

 

Usually, the waves approach the beach at an angle. This angle is determined by the prevailing wind. The swash moves the sediment up the beach at the same angle as the wind, while the backwash moves it back down the beach towards the sea under the force of gravity (i.e. straight back down the beach at an angle of 90 degrees). As a result, the sediment is moved along the beach. You must make sure that you can describe and explain this process both in words and by using a fully annotated diagram. This diagram will help you to answer a variety of questions in the exam.

 

 

Longshore drift may be slowed down by building groynes on a beach. Groynes are usually wooden structures that are built at right angles to the beach (they stick out into the sea). Groynes work by trapping the sediment on the updrift side and helping to keep it on the beach. They are spaced about 50-100 metres apart. As a wide beach is a great way of absorbing the waves' energy, this can help prevent cliff erosion. Some groynes are made of concrete, steel or large rocks.

 

 

Groynes are expensive to build.  They are usually made of tropical hardwoods which are more resistant to marine erosion.  Groynes may have a life of 15-20 years but often have to be replaced rather than repaired.

 

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When waves approach parallel to the coastline, the swash and backwash move the material up and down the beach. This creates a wide beach known as a swash-aligned beach. During storms, ridges of beach sediment can form along the beach, parallel to the sea. These are called berms.

 

 

Useful weblinks:

Curriculum Bits animation of longshore drift

BBC Bitesize on longshore drift

BBC Class Clips video - Longshore drift and groynes on Borth Beach

 

Lesson 10 - Spits, bars and saltmarshes

 

Learning objectives:

- to be able to explain how spits, bars and saltmarshes

- to develop the skills of annotating an oblique aerial photo and a vertical aerial photo using an OS map

- to produce a case study of Hurst Castle spit

- to practice key mapwork skills - using the key, using the scale, giving and finding 4- and 6-figure grid references, finding map evidence of human activity at the coast

 

A spit is an area of sand or shingle that has been transported by longshore drift and then deposited as the coastline has changed direction. It is attached to the land at one end. It is a depositional landformd. Hurst Castle Spit in Hampshire is a very famous example.

 

 

 

Where the coastline changes direction, sediment is deposited in the same direction as the original coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is built above the level of the sea. Drift continues along the seaward side of the spit extending it further down the coast while salt marsh develops in the slow-moving water on the landward side.

 

 

Occasionally strong winds blow from a different direction. Some material is then pushed inland causing the end of the spit to curve. This can happen on a number of occasions over time, producing a spit with several hooked ends.

 

 

Spits can become a permanent feature. This happens when the prevailing wind picks up sand from the beach and blows it inland across the spit to form sand dunes. These dunes will then be colonised by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area of water behind the spit. Water is trapped behind the spit, creating a low energy zone. As the water begins to stagnate, mud and marsh begin to develop behind the spit.

 

A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be diverted so that it follows the coastline for some miles before reaching the sea.

 

A bar is a barrier of sand or shingle that stretches right the way across a sheltered bay, joining to the land at both ends.

 

Bars can form in several ways: 

 

(a) a spit grows the whole way across a bay

(b) a sandbank devlops offshore, parallel to the shore, and is moved towards the coastline by the waves and wind until it joins the mainland

 

 

Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of the bar is called Slapton Ley.

 

 

A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland which is joined to the mainland by a shingle ridge known as Chesil Beach

 

 

Useful weblinks:

 

BBC Class Clips video - Blakeney Point

BBC Class Clips video - Kaitorete Spit, New Zealand

BBC Scotland video about spits, bars and tombolos

BBC Bitesize - Spits

BBC Bitesize - Tombolos

Lesson 11 - Mock exam question

 

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