Chemical weathering: decomposition of rocks by changing chemical composition such as rain
Biological weathering: Plants’ roots growing or animals burrowing into joints or cracks and force apart or loosen the rock.
Unit 2.3 Rivers
The Water cycle:
The amount of water of earth never changes. It is only moved and stored in different ways.
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Evaporation: when the temperature of water or air changes water is turned to gas and rises into the atmosphere
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Evapotranspiration: Plant suck up the water in the earth and then water can be evaporated from their leaves called transpiration.
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Condensation: water vapour blown towards mountains is forced to rise and then cools into droplets of water which form clouds and fall as rain or snow. (precipitation)
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Interception: some rainfall is intercepted (caught and absorbed) by plants or soil and some flows on the surface of the earth. The water that is absorbed can saturate the land and the water that run on top forms streams and rivers.
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Overland flow: streams flow on top of the ground and join to form rivers which feed into lakes and streams.
Load: The material carried by the river
Types of erosion:
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Types of river transport:
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Hydraulic action: the impact of the moving river
Corrasion: (abrasion) the wearing away of the bed and river bank by the load being covered
Attrition: the wearing away of the load as particles bump together while being carried by the river
Solution: (corrosion) the dissolving of material by the river water
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Solution: materials dissolved in the river water
Suspension: very light materials carried near the surface of the river
Saltation: large particles bounced along the river bed
Traction: heavy rocks and boulders rolled along the river bed
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River land forms:
Erosional land forms: shapes in the landscape formed by erosion
The river’s long profile:
Upper course:
Starts at the source
Valley sides are steep
Lots of vertical erosion
Heavy loads of boulders and large rocks
Water falls
Potholes
Interlocking spurs
V shaped valleys
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Middle course:
Valley widens, slopes are not as steep sand the gradient of the river is less.
More lateral erosion (side to side)
Meanders
Levees
Flood plains
Oxbow lakes
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Lower course:
River comes to an end and slows down as it meets the sea or lake – mouth of the river
Lots of deposition
Deltas
Flood plains
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Waterfalls
Falling water and rock particles wear
away soft rock
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The hard rock is undercut as
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erosion continues
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Hard rock collapses and if moved
by the flow. The waterfall moves backwards
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Erosion continues and the waterfall
continues to move upstream
leaving a gorge of recession
Interlocking spurs
In the upper course the river does not have a huge amount of energy to erode as it does not have a high discharge and it has to transport large pieces of sediment.
When the river meets areas of harder rock that are difficult to erode it winds around them. A series of hills form on either side of the river called spurs. As the river flows around these hills they become interlocked. So, a series of interlocking spurs are often found in the upper course of a river valley.
V-shaped valleys
Formed by vertical erosion when potholes grow and join together eroding the rock beneath the river.
 
Potholes
Formed by large stones getting trapped in the river bed causing corrosion which drills holes into rock bed which will eventually grow and join together.
Rapids – places where the water is shallow and river bed is rocky and uneven. The water is rough and the gradient is varied causing water to run faster. These can be used by white water rafters.
Meanders
Rivers with big sweeping bends with water flowing in corkscrew motions and causing lateral erosion on the outsides of bends and deposition on the insides.
Depositional land forms: shapes in the landscape formed by deposition
Floodplains
Land next to the river which is liable to flood. Often very marshy and poorly drained. River deposits silt, gravel as it floods.
Meanders (inside of bends) – as above
Deltas
Area of flat low lying, marshy land where a river meets the sea or lake. They can form their own lakes or lagoons. The loss of speed means the river deposits its load which is usually mud or silt. As this deposition takes places parts of the river are cut off leaving small lakes or lagoons. The river breaks up into distributaries.
Continued erosion on the outside of meanders or in the neck of the river, may cause two parts of the river to create a shorter path for the water. The deposition on the inside of the bend may cause the old path of the river to become cut off and a lake is formed. An oxbow lake.
Oxbow lakes
Levee
Naturally formed when rivers flood. When the river floods it loses energy and deposits its load. This makes the banks of the river higher than the river or the flood plain. Sometimes the natural levees occur and other times they are built to prevent against river flooding.
Creating meanders and oxbow lakes
In this picture, soil and mud is being eroded from various points on the bank. It’s being transported in the direction of the white arrows and deposited downstream (the sandy patches). This is changing the course of the river
This picture shows the same river many years later, the erosion and deposition have created such a deep meander that it has nearly formed a circle
Eventually, the river erodes so much that it cuts off part of the meander and creates an oxbow lake
Case study: formation of a waterfall Niagara Falls
Where?
Two waterfalls in the Niagara River between New York State and Ontario, Canada
What?
Spectacular waterfall carrying 90% of the world’s water
12,000,000 visit every year
Producer of hydroelectric power
Benefits:
Money from tourism, hydroelectric power, fame, water supply
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How was it formed?
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Glaciers melted 12,000 years ago
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Melted water poured down into the great lakes
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As the lake overflowed it caused the Niagara River to flow downhill and fell down the escarpment (cliff)
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The rocks at the falls are made of different layers of soft and hard rock
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The soft rock (shale) was eroded from underneath the hard rock (sandstone)
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The water could now fall freely
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The force of the water eventually eroded away so much rock that the top rock was undermined and fell
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This process happened over and again and is still happening!
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This means that the waterfall is retreating every year.
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The force of the water falling creates a plunge pool at the bottom of the falls.
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Case study: Living by Deltas Ganges, LEDC, Bangladesh
Deltas: Where the river slows as it reached the sea a large amount of deposition takes place as the load can no longer be carried by the force of the water. The load deposited causes the river to split up into smaller distributaries which flow to the sea. The sediment dropped by the river is often very fertile and therefore much vegetation grows there.
Where: Bangladesh, bordered with India
Delta is at the end of the Ganges river which flows from the Himalayas
Formed: Deposition of load at the end of the Ganges river as it arrives at the coast and slows down.
How? A) River carries a large amount of silt which builds up to form islands
B) As more silt builds up flooding occurs and creates small distributaries (small little streams winding to the sea)
C) Between these distributaries land is rich and fertile
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Advantages: flooding and irrigation all crops to be grown all year round such as rice and vegetables. Preferable to city slums. Jute (used for making burlap sacks) is grown and there are many fish to catch.
Disadvantages: monsoons (heavy rain fall), cyclones (strong winds and rain), floods
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Case study: Flooding of the Brahmaputra and Ganges Rivers, Bangladesh (LEDC)
Causes of 1998 flooding:
Monsoon season- 80% of rain falls June to September
Deforestation in the Himalayas increases runoff below
Urbanization – building on floodplains
1998 both rivers peaked at the same time
Silt had been deposited near the mouth blocking the main channel
Global warming melting Himalayas
Poorly maintained embankments
Flat low lying land over 80% of Bangladesh
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Effects in 1998:
70% of land in Bangladesh affected
2/3rds of people affected
Dhaka 2ms deep in water
Electricity supply cut off for several weeks
Wells contaminated and not safe for drinking
7 million homes destroyed
25 million homeless people
1300 approximate death toll
2 million tonnes of rice destroyed
Roads, bridges, airports and a third of the railway destroyed
$1.5 billion damages
Management: (how to prevent it)
Since 1989 Bangladesh has been trying to:
Build 5000 flood shelters with stilts to save lives
Improve forecasting with satellite technology
Early warning system with megaphones
Build dams
Control water with sluice gates and water pumps
Heighten embankments on side of river to 7m- more than 7500km already in place
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Case study: Flooding MEDC Boscastle UK
Where: Cornwall UK
When: 2004
What: settlement was left in ruins by floods
Short term Causes: intense rainfall caused local rivers to burst their banks, heaviest rains in living memory, 185 mm fell in just five hours, three million tonnes of water was added to a tiny drainage basin
Long term causes:
The soils were already saturated from previous rainfall earlier in the week, encouraging overland flow to begin even sooner.
The three river valleys are very steep and narrow. A broader floodplain would have helped to soak up water and river energy more effectively.
The steep valley sides mean that soils are thin, with limited water storage capacity when heavy rain comes.
Surrounding vegetation includes agricultural land with limited interception storage, although there is some forestry along the riverbanks.
The rain coincided with high tide in the bay. This restricted the rate of exit of floodwater into the harbour.
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Effects: motor vehicle damage, shops were carrying greater levels of stock due to tourist season and were lost.
No one died but property damage was high. At least thirty cars were washed straight into the harbour and many more were left upturned and badly damaged.
A three-metre high wave of water was reported to have crashed through one street at 80 kilometres per hour. Fridge-freezers were picked up and swept out of kitchens as water entered properties. Six properties collapsed entirely.
Infrastructure disruption – Both bridges in the village were destroyed and sections of road were swept away. Telephone, water, electricity and gas supplies were all interrupted.
Irreplaceable loss of historical artefacts – The ‘Witch Museum’ – which is fifty years old and receives 50,000 visitors a year – had some of its unique contents damaged.
Physical injury No-one died, but at least one resident suffered a heart attack.
Mental injury Many residents suffered stress and anxiety in the year that followed. It was six months before many properties were sufficiently repaired for homeowners to permanently return home.
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Case study: Flood management: Responses to Boscastle floods
Case study: River management in MEDC: Mississippi River, USA
Where/what is it?
Mississippi is 3800km long
Flows through ten states
Has over 100 tributaries
Has a drainage basin covering 1/3 of the USA
Causes of 1993 flooding:
Heavy rain in April 1993 saturated the upper Mississippi basin
Thunderstorms in June caused flashfloods
Mid July 180mm of rain in one day
Levees in nearby towns collapsed
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Management:
6 huge dams and 105 reservoirs
Afforestation to delay runoff
Strengthening the levees with concrete mattresses 25mx8m
Making the course shorter and straighter - from 530km to 300km by cutting through the neck of meanders to get the water passed towns more quickly to the sea
Diversionary spillways – overflow channels 9km long
Less construction on the floodplain e.g. St Louis.
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Little task for you ….
Unit 2.2.3 Marine Processes
Coast: Where land meets the sea
Fetch: the distance the wind has travelled over the sea – the longer the fetch the bigger the waves
Constructive waves: swash is stronger than backwash causing deposition
Destructive waves: backwash is stronger than swash causing erosion
Marine transport:
Suspension
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Solution
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Traction
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Saltation
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Fine sediment carried in the water
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Dissolved material carried in the water
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Large pebbles and stones rolling along sea bed
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Small pebbles hitting one another and bouncing along the sea bed
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Marine erosion:
Hydraulic action
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Air forced between cracks on rocks
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Corrosion
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Sea water dissolving parts of rocks
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Attrition
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Large rocks and sediment in water collide and wear each other down
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Corrosion
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Large rocks and sediment thrown against the cliffs
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Long shore drift: The movement of sediment along the beach
Groynes: beach protection against longshore drift
Headlands and bays: Formed where there hard and soft rock. The soft rock is eroded away and the hard rock is not.
Formation of caves, arches and stacks.
A line of weakness called a fault appears in the rock
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This fault increases in size until it becomes a cave
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The waves erode the cave until the water breaks through the other side creating an arch
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The roof of the arch falls into the sea creating a stack
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The stack is eroded away to form a stump
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Case study: Coastal erosion: The twelve apostles Victoria, Australia
Where:
Port Campbell National Park, Victoria, Australia
Limestone cliffs formed in layers from the sediment on the sea floor forming sedimentary rock.
9 remaining stacks of rocks off the Victoria coast
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Formed:
Headlands formed where the rock was harder
Hydraulic action, corrosion and corrosion eroded along the fault lines
Cliff base eroded away to form WAVE CUT PLATFORMS, notches, arches, caves and stacks
When the arches collapsed stacks were formed
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