Water on the Land Fluvial Processes, landforms and flooding.
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Transcript of Water on the Land Fluvial Processes, landforms and flooding.
Water on the Land
Fluvial Processes, landforms and flooding
Why do river valleys change shape?
Erosion Transportation Deposition
Hydraulic action – the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away.
Abrasion - rocks carried along by the river wear down the river bed and banks.
Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles.
Solution - soluble particles are dissolved into the river
Vertical
Lateral
Solution - minerals are dissolved in the water and carried along in solution.
Suspension - fine light material is carried along in the water.
Saltation - small pebbles and stones are bounced along the river bed.
Traction - large boulders and rocks are rolled along the river bed.
When a river loses energy, it will drop or deposit some of the material it is carrying.
Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example, after a flood or during times of drought.
Deposition is common towards the end of a river's journey, at the mouth.
Deposition at the mouth of a river can form deltas - for example, the Mississippi Delta.
Long profiles and cross profiles
River landforms
Landforms resulting from erosion
Landforms resulting from erosion and deposition
Landforms resulting from deposition
Waterfalls
Gorges
Meanders
Oxbow lakes
Levees
Floodplain
Factors affecting discharge (amount of water in a river)
• Amount and type of rainfall• Temperature• Previous weather conditions• Relief• Rock type (impermeable,
permeable, porous and pervious)
• Land use
Causes and frequency of flooding
Causes• Physical – prolonged
rain, heavy rain, snowmelt, relief,
• Human – deforestation and building construction
• Increased frequency of floods in UK in last 20 years
• More frequent• Few very big floods• Lots of smaller floods• More flash flooding
2007 River Severn
UK Case Study of flooding – Boscastle 2004
Effects• Destruction of buildings,
bridges, homes, shops, offices in Boscastle
• Damage to homes and businesses
• Loss of revenue from tourism
• Cost of clean up and re-homing people
• Responses• Emergency services and
armed services quickly alerted and on the scene very quickly
• Coordinated response• Fire brigade• Helicopters to winch
people to safety• Use of media – radio, TV• No deaths
Causes: Heavy torrential rain in a very short period; narrow valley of River Valency.
Case Study – Bangladesh 2004Effects• floods occurred July to Sept• July 2004 - 40% Dhaka was under water• 60% of country was flooded• 600 reported deaths• 20 million homeless• 100,000 in Dhaka suffered from diarrhoea as flood waters left
mud and sewage• 35cm of rain fell in 1 day on 13th Sept• Death toll rose to 750Airport, roads and railways flooded• Bridges destroyed• $7billion damage • Rice crop destroyed along with food supplies – vegetables• Cash crops – jute and sugar lost
Responses• Short Term• A heavy reliance is placed upon emergency aid – food,
drinking water medicines, plastic sheets, boats• Assistance is provided from United Nations,
governments, charities• Problem is distribution because so much of the country
is underwater.• As flood water recedes it is easier to set up medical
treatment centres, distribute water purification tablets and provide help with repairing homes and restarting economic activities
• Boats to rescue people • Emergency supplies for food, water, tents and
medicines • Fodder for livestock • Repair and rebuild houses, as well as services such as
sewage etc • Aid from other countries
Long Term Management• Reduce Deforestation in Nepal & Himalayas • Build 7 large dams in Bangladesh to store excess
water $30-$40 million and 40 yrs to complete • Build 5000 flood shelters to accommodate all the
population • Build 350km of embankment - 7 metres high at a cost
of $6 billion to reduce flooding along the main river channels
• Create flood water storage areas • Develop an effective Flood Warning Scheme
Hard and Soft engineering
Hard engineeringe.g. Dams & reservoirs; channel straightening; dredging; levees, embankments, barriers; flood walls; channel diversions & spillways
Soft engineeringe.g. flood shelters, flood plain zoning; ‘do nothing’; flood warning; flood preparation.
Costs Expensive to build and maintain, need technical
skill Floods happen less often – but when they do occur they tend to be more hazardous … imagine if a dam fails
Natural processes are disrupted – crops don’t get fertile silt
Ugly structures
People’s homes do get flooded
Disruption during a flood
River may change course
Expensive to rebuild & repair after a flood
Benefits Large scale
Multi purpose – fish in reservoirs, leisure, HEP
Prestigious
Farmland is protected
Better navigation
Kinder to the environment
Less pollution
Fewer people have to move
River is left in natural state
River Management in the UKIncreasing demand for water in the UK as
living standards increase and population rises. Homes now use more water for washing and cleaning.
Uneven rainfall in the UK – high in the west e.g. Wales and Scotland; low in the east e.g. East Anglia and South-east.
Areas of surplus water: Scotland, Wales, South-west England.
Areas of water shortages (deficit): East Anglia, London, South-east; Midlands
Water transfer
(pipes, tunnels, aqueducts)
Case Study of UK Dam/reservoir: River Derwent in Peak District; 3 dams (Howden, Derwent, Ladybower) provide water for Sheffield, Leeds, Nottingham and Leicester.
Economic Social Environmental
Dams are expensive to build and maintain
Jobs are created in construction, power generation.
Reservoirs cover large areas of land and many people might have to be moved away from their homes. People can use the reservoir for fishing, sailing and as a beauty spot.
Wildlife lose their habitats.
Loss of biodiversity.
Natural systems are disrupted.
Fish affected e.g. salmon.
Reservoir sedimentation.
ISSUES
OS Map work
Grid references
Scale
Map symbols
Recognise river landforms
Describe rivers and their valleys
Land use