SDME- Rivers and coasts

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  • Created by: amarag
  • Created on: 11-03-16 20:47

Features of a drainage basin

  • Tributary, small river that joins a main river
  • Source, where a river starts, usually in an upland area
  • Confluence, a point where two rivers join
  • Mouth, where a river flows into the sea or a lake
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Mechanical weathering

This is the breakdown of rock without changing its chemical composition

Freeze-thaw, type of mechanical weathering

  • Happens when temperature alternates above and below 0c
  • Water gets into rock that has cracks, eg, granite
  • When the water freezes it expands, which puts pressure on the rock
  • When the water thaws it contracts, which releases the pressure on the rock
  • Repeated freezing and thawing widens the cracks and causes the rock to break up
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Chemical weathering

This is the breakdown of rock by changing its chemical composition

Carbonating weathering- type of chemical weathering that happens in warm and wet conditions

  • Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid
  • Carbonic acid reacts with rock that contains calcium carbonate, eg, limestone, so the rocks are dissolved by the rainwater
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Biological weathering

  • This is the breakdown of rocks by living things,
  • Eg, plant roots break down rocks by growing into cracks on their surfaces and pushing them apart
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Rivers

  • Rivers erode the landscape, they wear it down, then transport the material to somewhere else where it's deposited
  • The shape of the valley and channel changes along the river depending on whether erosion or deposition is having the most impact (dominant process)
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River profile

Upper course

  • Has a steep gradient
  • V-shaped valley, steep sides
  • Narrow, shallow channel

Middle course

  • Medium gradient
  • Gently sloping valley sides
  • Wider, deeper channel

Lower course

  • Gentle gradient
  • Very wide, almost flat valley
  • Very wide, deep channel
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Four processes of erosion

Hydraulic action

  • The force of the water breaks rock particles away from the river channel

Corrasion

  • Eroded rocks picked by the river scrape and rub against the channel, wearing it away

Attrition

  • Eroded rocks picked up by the river smash into each other and break into smaller fragments
  • Their edges also get rounded off as they rub together

Corrosion

  • River water dissolves some types of rock, eg, chalk and limestone
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Four processes of transportation

Transportation is the movement of eroded material

Traction

  • Large particles like boulders are pushed along the river bed by the force of the water

Saltation

  • Pebble-sized particles are bounced along the river bed by the force of the water

Suspension

  • Small particles like silt and clay are carried along by the water

Solution

  • Soluble materials dissolve in the water and are carried along
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Deposition-

Happens when a river drops the eroded material it's transporting.

Deposition happens when a river slows down.

Why rivers slow down and deposit material

  • The volume of water in the river falls
  • The amount of eroded material in the water increases
  • The water is shallower, eg, on the inside of a bend
  • The river reaches its mouth
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Flood plains- flat areas of land that flood

  • The flood plain is the wide valley floor on either side of a river which occasionally gets flooded
  • When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting. This builds up the flood plain
  • Meanders migrate across the food plain, making it wider
  • The deposition that happens on the slip-off slopes of meanders also builds up the flood plain
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Levees

  • These are natural embankments along the edges of a river channel
  • During a flood, eroded material is deposited over the whole flood plain
  • The heaviest material is deposited closest to the river channel because it gets dropped first when the river slows down.
  • Over time, the deposited material builds up, creating levees along the edges of the channel due to repeated flooding
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Deltas- where river meets sea/lake- low-lying area

  • Rivers are forced to slow down when they meet the sea or a lake
  • This causes them to deposit the material it builds up and the channel gets blocked
  • If the sea doesn't wash away the material it builds up and channel gets blocked. This forces the channel to split up into lots of smaller rivers called distributaries
  • Eventually the material builds up so much that low-lying areas of land called deltas are formed
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Prolonged rainfall- why rivers flood

  • After a long period of rain, the soil becomes saturated
  • Any further rainfall can't infiltrate which increases runoff into rivers
  • This increases discharge quickly, which can cause a flood
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Heavy rainfall

  • Heavy rainfall means there's a lot of runoff
  • This increases discharge quickly, which can cause a flood
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Snowmelt

  • When a lot of snow or ice melts it means that a lot of water goes into a river in a short space of time
  • This increases discharge quickly, which can cause a flood
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Relief

  • If a river is in a steep-sided valley, water will reach the river channel much faster because water flows more quickly on steeper slopes.
  • This increases discharge quickly, which can cause a flood
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Geology

  • When a river is in an area of permeable rock, more water percolates into the rock instead of flowing on the surface.
  • This means there's less runoff, so the risk of flooding is lower
  • When a river is in an area of impermeable rock, water doesn't percolate into the rock but flows on the surface.
  • This means there's more runoff, so the risk of flooding is higher
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Deforestation- human causes

  • Trees intercept rainwater on their leaves, which then evaporates
  • Trees also take up water from the ground and store it
  • This means cutting down trees increases the volume of water that reaches the river channel, which increases discharge and makes flooding more likely
  • Deforestation also causes soil erosion- there are no trees to hold the soil together, so it gets washed away
  • The soil ends up in the river, which raises the riverbed.
  • This reduces the volume of water the river channel can hold, so the risk of flooding is increased
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Urbanisation- human causes

  • Urban areas have lots of buildings made form impermeable materials like concrete and they're surrounded by roads made form tarmac.
  • Impermeable surfaces increase runoff and drains quickly which takes runoff to rivers
  • This increases discharge quickly, which can cause a flood
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Flood impacts

  • People are killed by flood waters or by the others of flooding, eg, by the spread of disease when water supplies have been contaminated by sewage from broken sewage pipes
  • Buildings are damaged or destroyed, making people homeless and causing businesses to shut down
  • Jobs are lost because of damage to premises and equipment, or pollution of farmland
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Why effects of flooding are worse in LEDCs than in

  • There's less money to spend on flood protection ad to help people after a flood
  • Also, more people live and work in areas that are likely to flood and poorer transport links mean it's more difficult to get help to places that have been affected
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Urban and rural areas

Urban areas

  • Have drainage systems and they're covered with impermeable materials, these increase discharge so hydrographs for rivers in urban areas are steep

Rural areas

  • Have more vegetation, which decreases discharge.
  • There are also more reservoirs in rural areas- they store water and release it slowly, decreasing discharge in the river below
  • This means hydrographs for rivers in rural areas are more gently sloping
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Hard and soft engineering-rivers

Hard engineering

  • Man-made structures built to control the flow of rivers and reduce flooding

Soft engineering

  • Schemes set up using knowledge and processes of a river to reduce the effects of flooding
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Dams and reservoirs

They are huge walls built across the rivers, usually in the upper course. A reservoir is formed behind the dam

Benefits

  • Reservoirs store water and release it slowly, which reduces the risk of flooding
  • The water in the reservoir is used a drinking water and can be used to generate hydroelectric power.
  • Reservoirs are also attractive and can be used for recreation

Disadvantages

  • Dams are very expensive to build
  • Creating a reservoir can flood existing settlement
  • Eroded material is deposited in the reservoir and not along the river's natural course making farmland downstream less fertile
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Channel straightening

The river's course is straightened-  meanders are cut out by building artificial straight channels

Benefits

  • Water moves out of the area more quickly because it doesn't travel as far- reducing the risk of flooding

Disadvantages

  • Flooding may happen downstream of the straightened channel instead, as flood water is carried there faster
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Man-made levees

Man-made embankments along both sides of a river

Benefits

  • The embankments mean that the river channel can hold more water, which reduces the risk of flooding.

Disadvantages

  • If the levees fail it can cause catastrophic flooding
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Flood warnings- soft engineering

People are warned about possible flooding through TV, radio, newspapers and the internet

Benefits

  • The impact of flooding is reduced- warnings give people time to move possessions upstairs, put sandbags in position and to evacuate

Disadvantages

  • Warnings don't stop a flood from happening.
  • People may not hear or have access to warnings (LEDCs where communications are less developed)
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Preparation- soft engineering

Buildings are modified to reduce the amount of damage a flood could cause. Or people make plans for what to do in a flood, eg, keep a blanket and torch in a handy place.

Benefits

  • The impact of flooding is reduced, buildings are less damaged and people know what to do when a flood happens

Disadvantages

  • Preparation doesn't guarantee safety from a flood and it could give people a false sense of security
  • It's expensive to modify homes and businesses
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Flood plain zoning- soft engineering

Restrictions prevent building on parts of a flood plain that are likely to be affected by a flood

Benefits

  • The risk of flooding is reduced- impermeable surfaces aren't created like roads
  • The impact of flooding is reduced- there aren't any houses or roads to damage

Disadvantages

  • The expansion of an urban area is limited if there aren't any other suitable building sites.
  • It's no help in areas that have already been built on
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Sustainable flood and coastal strategies

  • Sustainable strategies meet the needs of people today without stopping people in the future getting the things they need.
  • This means not using up the resources or damaging the environment
  • Hard engineering strategies are usually less sustainable than soft engineering strategies because they generally cost more to build and maintain and they damage the environment more
  • Hard engineering strategies are also less suitable for LEDCs as they're usually really expensive
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Four processes of erosion- coast

Hydraulic action

  • Waves crash against rock and compresses the air in the cracks. This puts pressure on the rock. Repeated compression widens the cracks and makes bits of rock break off

Corrasion

  • Eroded particles in the water scrape and rub against rock, removing small pieces

Attrition

  • Eroded particles in the water smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together

Corrosion

  • Weak carbonic acid in sea water dissolves rock like chalk and limestone
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Destructive waves

  • They're high and steep
  • Their backwash is more powerful than their swash. Means material is removed from the coast.

Factors that affect the size and power of destructive ways

  • Wind- the force of the wind on the water's surface is what creates waves. A strong wind gives large, powerful waves.
  • Fetch- is the distance of water over which the wind has blown to produce a wave. The greater the fetch, the bigger and more powerful the wave.
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Economic reasons to protect coastlines

  • Loss of tourism- many coastal areas are popular tourist destinations. Flooding and erosion can put people off visiting. Fewer tourists means businesses that rely on tourism may close.
  • Businesses near cliffs that are eroding are at risk of collapsing into the sea. Coastal flooding can damage or destroy business premises. Businesses have to relocate, make costly repairs or can be forced to shut down.
  • Coastal flooding damages agricultural land because sea water has a high salt content. Salt reduces soil fertility and so crop production can be affected for years after a flood. Farmland is also lost to coastal erosion, which has a huge effect on farmers' livelihoods.
  • Property prices can fall if houses are affected by flooding or are at risk form erosion. People may also be unable to get their properties insured.
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Social reasons to protect coastlines

  • Deaths- coastal floods have killed thousands of people in the past.
  • Water supplies affected- floodwater can pollute drinking water with salt or sewage
  • Loss of housing- homes near cliffs affected by erosion are at risk of collapsing into the sea. Over time whole villages can be lost die to erosion. Many people are made homeless because of floods.
  • Loss of jobs- coastal industries may be shut down because of damage to equipment and buildings, eg, fishing boats can be destroyed.
  • Damage to infrastructure- roads near cliffs affected by erosion are at risk of collapsing into the sea
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General reason to protect coastlines

Coastal areas are at risk from erosion and flooding by the sea.
Both can cause a lot of problems for the environment and the people living there

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Environmental reasons to protect coastlines

Ecosystems are affected

  • Seawater has a high salt content.
  • Increased salt levels due to coastal flooding can damage or kill organisms in an ecosystem
  • The force of floodwater also uproots trees and plants, and standing floodwater drowns some trees and plants.

Sites of Special Scientific Interest (SSSIs) are threatened by coastal erosion

  • Can ruin any ongoing research
  • Research may be very important
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Hard and soft enigneering - coasts

Hard engineering

  • Man-made structures built to control the flow of the sea and reduce flooding and erosion

Soft engineering

  • Schemes set up using knowledge of the sea and its processes to reduce the effects of flooding and erosion
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Sea wall

A wall made out of a hard material like concrete that reflects waves back to sea

Benefits

  • It prevents erosion of the coast.
  • Also acts as a barrier to prevent flooding

Disadvantages

  • It creates a strong backwash, which erodes under the wall.
  • Sea walls are very expensive to build and to maintain
  • Ugly- may put people off visiting and so the town won't make income from tourists
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Rip rap

Boulders that are piled up along the coast

Benefits

  • The boulders absorb wave energy and so reduce erosion and flooding

Disadvantages

  • Boulders can be moved around by strong waves, so they need to be replaced
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Groynes

Wooden or stone fences that are built at right angles to the coast. They trap material transported by longshore drift.

Benefits

  • Groynes create wider beaches which slow the waves
  • This gives greater protection from flooding and erosion

Disadvantages

  • They starve beaches further down the coast of sand, making them narrower.
  • Narrower beaches don't protect the coast as well, leading to greater erosion and floods
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Revetments

Slanted structures made of concrete, wood or rocks built at the foot of cliffs.

Benefits

  • They absorb wave energy and so reduce erosion

Disadvantages

  • Expensive to build and they create a strong backwash that erodes under the barrier
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Breakwaters

Concrete blocks or boulders deposited on the sea bed off the coast

Benefits

  • They force waves to break offshore so their erosive power is reduced before they reach the shore

Disadvantages

  • They're expensive and can be damaged by storms
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Beach replenishment- soft engineering

Sand and shingle from elsewhere is added to beaches

Benefits

  • Beach replenishment creates wider beaches which slow the waves.
  • This gives greater protection from flooding and erosion
  • A bigger beach, that looks natural, may attract more tourists, increasing income form the tourist industry

Disadvantages

  • Taking material from the seabed can kill organisms like sponges and corals.
  • It's a very expensive defence
  • It has to be repeated as the replenishment gets eroded
  • Soft engineering processes don't always work- beach material may be removed by winter storms and have to start all over again, so costly
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Managed replenishment - soft engineering

Removing an existing defence and allowing the land behind it to flood

Benefits

  • Over time the land will become marshland- creating new habitats.
  • Flooding and erosion are reduced behind the marshland
  • It's a fairly cheap defence
  • Would allow the cliff line to erode to a point of equilibrium

Disadvantages

  • People may disagree over what land is allowed to flood and who should get the compensation
  • Like flooding valuable farmland would affect the livelihood of farmers and ruins potentially good resource
  • Doesn't stop initial erosion and loss of beach material will affect tourist industry
  • Who knows where the coast will have eroded before equilibrium is reached, may be too inland
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Deposition

  • This is when material being carried by the sea water is dropped on the coast
  • Coasts are built up when the amount of deposition is greater than the amount of erosion
  • Low energy waves carry material to the coast but they're not strong enough to take a lot of material away- this means there's lots of deposition and very little erosion.

Amount of material that's deposited on an area of coast is increased when:

  • There's lots of erosion elsewhere on the coast, so there's lots of materials available
  • There's lots of transportations of material into the area
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Constructive waves

Waves that deposit more material than they erode and build up the coast are called constructive waves

  • They have a low frequency
  • They're low and long
  • The swash is powerful and it carries material up the coast
  • The backwash is weaker and it doesn't take a lot of material back down the coast. This means material is deposited on the coast
  • They're made by weaker winds and have a shorter fetch than destructive waves
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Hydrographs

  • River discharge is the volume of water that flows in a river per second. Measured in cumecs.
  • Hydrographs show how the discharge at a certain point in a river changes over time
  • Storm hydrographs show the changes in river discharge around the time of a storm
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Why lag time happens?

  • Lag time happens because most rainwater doesn't land directly in the river channel
  • There's a delay as rainwater gets to the channel
  • It gets there by surface runoff or by infiltration and flowing slowly underground.
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Short lag time

  • If more water flows as runoff the lag time will be shorter.
  • This means discharge will increase and the hydrograph will be steeper because more water gets to the river in a shorter space of time
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Comments

GCSErevision2016

gr8!

shanai100

gr8

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