Mechanical Weathering Example: Freeze-thaw weathering

Stage One

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Water seeps into cracks and fractures in the rock.

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Stage Two

When the water freezes, it expands about 9%. This wedges apart the rock.

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Stage Three

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With repeated freeze-thaw cycles, the rock breaks off.

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Types of Erosion

The break down and transport of rocks – smooth, round and sorted.

Attrition

Rocks that bash together to become smooth/smaller.

Solution

A chemical reaction that dissolves rocks.

Abrasion

Rocks hurled at the base of a cliff to break pieces apart or scraped against the banks and bed of a river.

Hydraulic Action

Water enters cracks in the cliff, or river bank, air compresses, causing the crack to expand.

Types of Transportation

Longshore Drift

Step 1:

Swash moves the load (materials in the sea e.g. stones) up the beach in the direction of the prevailing wind.

Step 2:

Backwash then pulls the load off the beach at right angles to the shore.

Step 3:

Suspended load is carried in a zig-zag motion.

Types of Weathering

Weathering is the breakdown of rocks where they are.

Biological

Breakdown of rock by plants and animals e.g. roots pushing rocks apart.

Mechanical

Breakdown of rock without changing its chemical composition e.g. freeze thaw

Case study: the Holderness Coast

• The fastest eroding coastline in Europe – it retreats at a rate of 1-2 metres every year.
• Strong prevailing winds and soft boulder clay means it erodes quickly.
• Mappleton is perched on a cliff top and has approximately 50 properties which means it needs to be protected.
• 1991 the decision was taken to protect Mappleton; rock armour and groynes were put in place. The whole scheme cost around £2million.
• This has stopped erosion and trapped material at Mappleton.
• However, this has caused increased erosion south of Mappleton.
• Groynes and a sea wall are also in place at Hornsea – these protect the coastline but are expensive to maintain and replace.

Types of Waves

Constructive Waves

Destructive Waves

This wave has a swash that is stronger than the backwash. This therefore builds up the coast.

This wave has a backwash that is stronger than the swash. This therefore erodes the coast.

Size of waves

Affected by:

• Fetch how far the wave has travelled
• Strength of the wind.
• How long the wind has been blowing for.

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Beach Nourishment

Beaches built up with sand, so waves have to travel further before eroding cliffs.

• Cheap
• Beach for tourists.
• Storms = need replacing.
• Offshore dredging damages seabed.

Managed Retreat

Low value areas of the coast are left to flood & erode.

• Reduce flood risk
• Creates wildlife habitats.
• Compensation for land.

Formation of Coastal Stack

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1. Hydraulic action widens cracks in the cliff face over time.
2. Abrasion forms a wave cut notch between high tide and low tide.
3. Further abrasion widens the wave cut notch to from a cave.
4. Caves from both sides of the headland break through to form an arch.
5. Weather above/erosion below –arch collapses leaving stack.
6. Further weathering and erosion eaves a stump.

Formation of Coastal Spits - Deposition

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1. Swash moves up the beach at the angle of the prevailing wind.
2. Backwash moves down the beach at 90° to coastline, due to gravity.
3. Zigzag movement (Longshore Drift) transports material along beach.
4. Deposition causes beach to extend, until reaching a river estuary.
5. Change in prevailing wind direction forms a hook.
6. Sheltered area behind spit encourages deposition, salt marsh forms.

Coastal Defences

Hard Engineering Defences

Groynes

Wood barriers prevent longshore drift, so the beach can build up.

• Beach still accessible.
• No deposition further down coast = erodes faster.

Sea Walls

Concrete walls break up the energy of the wave . Has a lip to stop waves going over.

• Long life span
• Protects from flooding
• Curved shape encourages erosion of beach deposits.

Gabions or Rip Rap

Cages of rocks/boulders absorb the waves energy, protecting the cliff behind.

• Cheap
• Local material can be used to look less strange.
• Will need replacing.

Coastal defences – soft engineering

Formation of Ox-bow Lakes

Step 1

Step 2

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Erosion of outer bank forms river cliff. Deposition inner bank forms slip off slope.

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Further hydraulic action and abrasion of outer banks, neck gets smaller.

Step 3

Step 4

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Erosion breaks through neck, so river takes the fastest route, redirecting flow

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Evaporation and deposition cuts off main channel leaving an oxbow lake.

Upper Course of a River

Near the source, the river flows over steep gradient from the hill/mountains. This gives the river a lot of energy, so it will erode the riverbed vertically to form narrow valleys.

Formation of a Waterfall

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1) River flows over alternative types of rocks.

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2) River erodes soft rock faster creating a step.

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3) Further hydraulic action and abrasion form a plunge pool beneath.

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4) Hard rock above is undercut leaving cap rock which collapses providing more material for erosion.

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5) Waterfall retreats leaving steep sided gorge.

Middle Course of a River

Here the gradient get gentler, so the water has less energy and moves more slowly. The river will begin to erode laterally making the river wider.

Formation of floodplains and deltas

Floodplains

Deltas

A large area of flat land either side of the river that is prone to flooding.

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Lower Course of a River

Near the river’s mouth, the river widens further and becomes flatter. Material transported is deposited.

Formation of levees

When a river floods, fine silt/alluvium is deposited on the valley floor. Closer to the river’s banks, the heavier materials build up to form natural levees.

• Nutrient rich soil makes it ideal for farming.
• Flat land for building houses.

Case Study: York floods

Causes

• 50mm of rain fell in 24 hours; which is the average for the whole month of September.
• The drains couldn’t cope with the water.
• The River Ouse burst its’ banks

Effects

• The A1M was closed in North Yorkshire due to flooding, about 100 cars were stranded
• In York the fire service was unable to rescue a narrow boat which sunk in the River Ouse.
• Two-Thirds of the York Dungeon tourist attraction was flooded and stayed closed for over a month
• Fortunately it is believed no one killed

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Responses

• The fire brigade had to pump water out from riverside homes and businesses.
• Shop keepers and residents built up sandbags to try to keep the water out.
• More than 20 people were rescued or assisted with cars stuck in the flood water.

Types of Transportation

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A natural process by which eroded material is carried/transported.

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Solution

Minerals dissolve in water and are carried along.

Suspension

Sediment is carried along in the flow of the water.

Saltation

Pebbles that bounce along the sea/river bed.

Traction

Boulders that roll along a river/sea bed by the force of the flowing water.

Erosion processes

Attrition

Rocks that bash together to become smooth/smaller.

Solution

A chemical reaction that dissolves rocks in the river.

Abrasion

Rocks hurled at and scrape against the banks and bed of a river.

Hydraulic Action

Water enters cracks in the river bank, air compresses, causing the crack to expand.

Name a landform in the upper course of a river .

Define hydraulic action

Define abrasion

Define hard engineering

What is a Spit?

Define swash

Describe how beach nourishment works?

Give one advantage of a Seawall

Draw three diagrams to show freeze-thaw weathering

How fast is the Holderness Coast eroding?

What are gabions and how do they work?

What is a meander and how does it become an ox-bow lake?

Define Saltation

Draw a diagram to show traction

Define a waves 'fetch'

Describe two processes taking place to form a Spit

What is Managed Retreat and how does it work?

Define erosion

Give one advantage of beach nourishment

Name a landform found in the lower course of a river

What feature often forms behind a Spit?

Give one disadvantage of beach nourishment

Who was Annie Taylor and what is she famous for?

Define suspenson

What is the dominant rock type along the Holderness Coast?

Give one specific effect of the York floods

What is a levee?

Give one disadvantage of groynes

Give one factor which affects the size of waves

Define deposition

Complete the sequence - Crack, cave , ... , ...., .....

Give one disadvantage of gabions.

Define soft engineering

Give one specific cause of the York floods.

What feature can be found at the bottom of a waterfall?

Define biological weathering

What happens to the width of a river as it flows downstream?

What are groynes and how do they work?

What happens to the speed of a river as it flows from the upper to the middle course?

Describe the process of longshore drift

Define attrition

Freeze-thaw weathering is an example of which type of weathering?

Define lateral erosion

What is a floodplain?

What is a Seawall and how do they work?

Give one advantage of Managed Retreat

Give one specific response to the York floods

What landform is left behind when a waterfall retreats?

Give one difference between constructive and destructive waves