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 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 2004 Effects 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 engineering e.g. Dams & reservoirs; channel straightening; dredging; levees, embankments, barriers; flood walls; channel diversions & spillways Soft engineering e.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 UK Increasing 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) ISSUES 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.

OS Map work Grid references Scale Map symbols Recognise river landforms Describe rivers and their valleys Land use