SDME- Rivers and coasts
- 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
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
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
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
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)
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
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
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
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
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
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
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
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
Heavy rainfall
- Heavy rainfall means there's a lot of runoff
- This increases discharge quickly, which can cause a flood
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
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
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
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
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
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
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
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
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
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
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
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
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)
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
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
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
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
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.
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.
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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.
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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