Water on the Land revision notes and case studies
Processes of erosion –
hydraulic action, abrasion,
attrition, solution; vertical
and lateral erosion.
Processes of transportation
– traction, saltation,
suspension and solution.
Deposition and reasons for
it.
Hydraulic action involves the force of water against the
bed and banks.
Abrasion is the process by which the bed and banks
are worn down by the river’s load. The river throws
these particles against the bed and banks, sometimes
at high velocity.
Attrition is when material (the load) carried by the
river bump into each other and so are smoothed and
broken down into smaller particles.
Solution is the chemical action of river water. The acids
in the water slowly dissolve the bed and the banks.
Traction. Boulders and pebbles are rolled along the
river bed at times of high discharge.
Saltation. Sand sized particles are bounced along the
river bed by the flow of water.
Suspension. Fine clay and sand particles are carried
along within the water even at low discharges.
The amount of water in the upper
course of a river is low so the river
only has energy to erode downwards.
This is vertical erosion.
As the course of a river approaches its
middle stages it flows over flatter land
and lateral (sideways) erosion starts
Deposition. This is where the river
dumps the material it has been
carrying because it doesn’t have the
energy to carry it any further. The
largest particles will be deposited first
as they are too heavy to carry. Smaller
particles will be carried further.
Solution. Some rocks can be dissolved by the river
water eg. limestone
Long profile and changing
cross profile.
Landforms resulting from
erosion – waterfalls and
gorges (1); landforms
resulting from erosion and
deposition – meanders and
ox-bow lakes(2); landforms
resulting from deposition –
levees and flood plains(3).
Factors affecting discharge –
amount and type of
rainfall(1), temperature(2),
previous weather
conditions(3), relief(4), rock
type (impermeable,
permeable, porous and
pervious) (5)and land use(6).
Discharge is the amount
of water flowing in the
river
Diagram:
Diagram:
1. Waterfalls happen where a layer of hard rock lies on
top of soft rock. The soft rock underneath is undercut
to form a waterfall with a plunge pool. Eventually the
overhang of the hard rock will collapse and the
waterfall will move upstream towards the source,
leaving a gorge behind.
2. Meanders are deep on the outside
bend (river cliff) because this is where
the current is fastest and has more
energy to erode. The current is gentle
on the inside bend so the river
deposits it’s load making the inside
bend (slip off slope) shallow.
Eventually, the erosion of the neck of
the meander will meet up to form an
ox-bow lake.
3 When a river floods it deposits the heaviest of these
particles first. The larger particles, often pebble-sized,
form the leveés. The sands, silts and clays are similarly
sorted with the sands being deposited next, then the
silts and finally the lightest clays. Every time the river
floods deposition builds up the floodplain.
Diag. of levee and floodplain:
1. Heavy rain or long periods of rain will cause the
discharge to go up a lot.
2. Warm temps will evaporate the
water and vegetation will take up
more water so discharge is lower
3. If it has been very wet beforehand, the surrounding
soil will be full so there will be more runoff causing the
discharge to increase quickly.
If it has been dry it will take longer for the water to
reach the river and the water will be lower.
4. If the valley sides are steep, there
will be more runoff and discharge will
increase quickly.
If the valley has gentle slopes........
5. Impermeable rock doesn’t let water pass through it
so it runs across the surface and straight into the river,
increasing the discharge.
Porous rock lets water through the spaces between the
particles in it so less water goes into the river.
Pervious rock lets water through cracks in the rock so it
takes longer for the water to get into the river
6. Towns and cities have lots of
impermeable surfaces like roofs and
roads so water flows across the
surface and straight into rivers,
increasing the discharge.
If it is countryside with a lot of plants,
these will use up a lot of the water and
reduce the discharge.
Water on the Land revision notes and case studies
The causes of flooding
Physical:
1.
2.
3.
4.
Human
1.
2.
The frequency and location
of flood events – in the UK in
the last 20 years.
A case study of flooding in a
rich part of the world ………
Cockermouth, 2009
…..and one from a poorer area
– the different effects of and
responses to flooding.
.
Bangladesh, 2004.
Hard engineering strategies
– costs and benefits
Prolonged rain-soil fills with water so no more infiltration and more runoff.
Heavy rain-too fast for infiltration so more runoff.
Snowmelt-in spring, all snow melts at once so lots of runoff.
Relief-steep slopes means water gets into rivers quickly.
Deforestation-less water taken up by trees through their roots. Also no roots to hold
soil in place so it gets washed into rivers, filling them up.
Building construction-building on flood plains, eg Keswick, creates impermeable
surfaces so more runoff into rivers and drains take water to rivers very quickly
Floods are more frequent, especially since 1998.
Eg Keswick flooded in 2004 and again in 2009, although floods have been all over the UK
Effects
1.
2.
Responses
1. Emergency services, including
Mountain Rescue, rescue
stranded people; some by
3.
helicopter.
2. Local hotels offer emergency
4.
shelter.
3. Local people have food and
5.
clothing collections.
4. National flood appeal set up.
5. Visit by the prime minister,
6.
Gordon Brown and Prince Charles.
6. All bridges checked for damage
7.
and collapsed bridges repaired.
7. Temporary railway stations set
up.
8. Army brought in to build a
temporary bridge across the
R.Derwent to connect Northside
to the rest of Workington near the
mouth of the Derwent.
9. Insurance companies put up their
prices.
Effects
Responses
1. 60% of the country under water
1. Food, medical supplies clothing
destroying homes and crops.
and blankets were distributed to
2. 600 deaths
accessible areas but some places
3. 30 million homeless
were cut off.
4. Hundreds of thousands suffer
2. Local people began to rebuild
diarrhoea from raw sewage left
their homes.
behind
3. The UN set up an appeal for $74
5. Airport, roads, railways flooded and
million, but didn’t manage to get
bridges collapsed so it was difficult to
this amount.
get help and supplies to people.
4. Water Aid supplied water
6. The rice crop and other food crops
purification tablets
were wiped out in some areas.
5. Embankments built on river
banks.
6. Flood shelters built on high
ground.
Hard engineering: using technology to control
Soft engineering: tries to work with the
rivers.
river’s natural flow.
Dams
Reservoirs
Straightening
Concrete barriers across rivers
Lakes are created in valleys
Meanders are straightened so
hold back the water. Good
to store the flood water.
water flows in a straight line.
points:
Good points:
Good points:
1. Good control of
1. Can be used for
1. Water leaves the
floods.
water supply or
area quicker.
2. Can save water for
recreation.
Problems
times of drought.
2. Good control of
2. Only useful where
3. Often have HEP at
rivers.
there are
the same time.
3. Can save water
meanders
Problems
for times of
3. Places
1. Expensive to build
drought.
downstream get
so LEDCs may have
Problems:
the floodwaters
to borrow money to
1. People living in
quicker too!
build them
the valleys need
2. Silt is trapped
to move.
behind them
2. Farmland lost.
Water up to 6’
People evacuated from businesses
and homes
Businesses close down, some
temporarily and some permanently
People have to live in caravans and
local hotels
Bridges collapse and others are
closed as a precaution causing
disruption to travel.
One person dies, policeman Bill
Barker, when a bridge collapsed.
Farmland flooded and then left
covered in debris after the water
goes down
Water on the Land revision notes and case studies
Soft engineering costs and
benefits.
The UK – increasing
demand for water; areas of
deficit and areas of surplus;
the need for transfer.
A case study of a
dam/reservoir to consider
resulting economic, social
and environmental issues
Flood warning
Telling people in advance eg. Environment
Agency warnings by phone (Floodline), radio,
TV, text message and Internet warnings.
Good points
1. Cheap
2. Helps people save their possessions
3. Helps save peoples’ lives
Problems
1. Doesn’t stop floods
2. Doesn’t stop destruction of crops
and property
3. Can only be done by megaphone in
LEDCs
Preparation
Environment Agency website has information
about how to prepare and what to do eg. get
important papers together eg insurance
documents, turn off the gas, move valuables
upstairs.
Good points
1. Cheap
2. Helps people save their possessions
3. Helps save peoples’ lives
Problems
1. Doesn’t stop floods
2. Doesn’t stop destruction of crops
and property
Flood plain zoning
Controlling what is built on different parts of
the floodplain eg cattle near to river where
flooding will be worst because they can be
moved away, buildings will be further away on
higher ground which is less likely to flood.
Good points
1. Animals can be moved from near
to the river
2. Buildings less likely to flood
Problems
1. No use for areas already built on
2. Doesn’t stop flooding
‘do nothing’
Not effective in stopping flooding but may be
the only thing available in LEDCs.
Should we leave the river to do what comes
naturally?
Good points
1. Cheapest option
Problems
2. Doesn’t stop flooding.
Demand for water is increasing because:
1. Population is increasing
2. We use more water with labour saving devices such as dishwashers
3. We demand more foodstuffs in season so water is used to grow them
Areas of deficit: not enough rain falls to satisfy demand and shortages may occur eg. London in
SE England has a lot of people and low rainfall.
Areas of surplus: have more water than they need Eg. Cumbria has high rainfall and a low
population density.
Water transfer: water is moved from areas of surplus to areas of deficit.
Economic
All the farms and houses of the villages of Mardale and Measand, and the Dun Bull Inn were
pulled down so farmers lost their livelihoods.
Social
Haweswater in the Lake District
exists in its present form because it
was acquired by Manchester
Corporation to provide water for the
city. Haweswater dam was
completed in the late 1930s which
considerably increased the size of
the lake. Water flows via the
pipeline down through Lancashire to
a reservoir north of Manchester
before being distributed to homes
and businesses around the city.
The valley of Mardale was populated by the farming villages of Measand and Mardale Green, and
the construction of the reservoir would mean that these villages would be flooded and lost, and
the population would have to be moved.
Coffins were removed from the graveyard, and buried elsewhere, and Mardale church was
demolished
The need for sustainable
supplies.
Now the focus is on local schemes rather than large scale transfers like Haweswater:
1. New houses are all fitted with a water meter so people use less.
Environmental
A concrete dam, 1550 feet wide and 120 feet high was built, and this raised the lake level by 95
feet.
The valley was considered one of the most picturesque in Westmorland, and many people
thought it should be left alone.
2.
Rainwater can be collected to use on the garden.
3.
Bath water can be recycled to flush toilets.
4.
Devices can be fitted to toilet cisterns to reduce the amount of water used each flush.
5.
More people are taking showers than baths.