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