Causes & consequences of river flooding (WJEC Eduqas GCSE Geography B): Revision Note

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The notes on this page cover part 2.2. 2 of the WJEC Eduqas GCSE B Geography.

2.2.2 Why do rivers flood and what are the consequences of flooding?

• The physical factors (to include geology and vegetation) and human activities (to include urbanisation and changes to ecosystems) that cause rivers to flood.

• Climatic patterns that cause seasonal floods.

• Extreme weather events that cause flash floods.

• Social and economic consequences of flooding (positive and negative) on different groups of people.

Physical factors leading to flooding

• Flooding occurs when the capacity of the river channel is exceeded

  • The water flows over the banks and onto the surrounding area

• There are two main types of flooding:

  • Flash floods which occur when there is heavy rainfall over a short period of time

    • This causes a rapid increase in river discharge

  • Seasonal floods when there is prolonged rain and the ground becomes saturated, reducing infiltration

• The key factor in flooding is the time taken for the precipitation to reach the river from where it falls – the lag time

  • The shorter the lag time, the higher the risk of flooding because the discharge of the river increases faster

• The lag time depends on both human and physical factors 

• These lead to increased surface run-off, which shortens the lag time

Physical factors which increase flood risk

• Relief

  • Steep slopes reduce infiltration and increase overland flow

• Rock type

  • Impermeable rocks reduce percolation and increase overland flow

• Soil

  • Frozen, saturated or compacted soil reduces infiltration and increases overland flow

  • Some soil types, such as clay reduce infiltration and increase overland flow

• Weather

  • Heavy or prolonged rainfall means that the rate at which water reaches the surface exceeds the infiltration rate, leading to increased overland flow

  • After a period of snow rising temperatures can cause rapid melting which increases overland flow

• Seasonal variations

  • Flooding in the UK tends to occur in the autumn and winter when rainfall is more frequent

  • Higher temperatures in spring lead to snowmelt in mountainous areas, increasing overland flow

• High drainage density

  • Where drainage density is high there are many tributaries taking water to the main channel causing a rapid increase in discharge

• Lack of vegetation

  • Where there is little natural vegetation, there is reduced interception leading to increased overland flow

Human factors leading to flooding

• Deforestation

  • A lack of trees and vegetation reduces interception and infiltration, increasing overland flow

• Urbanisation

  • Impermeable concrete and tarmac increase overland flow

  • Water flows into the drains which are designed to take water to the river rapidly

  • Bridges and supports reduce the capacity of rivers

• Agriculture

  • Bare soil and ploughing increase overland flow

• Human-induced climate change

  • Rising global temperatures may increase storm frequency and intensity, which leads to heavy and prolonged rainfall

Consequences of flooding

• There are a range of possible impacts of flooding

• These can be both social and environmental

Social impacts of flooding

• The social impacts of flooding are the effects it has on the quality of lives of people for example health, well-being and education

• They may include:

  • Damage and destruction of homes leading to homelessness

  • Damage to crops and agricultural land, which can lead to food shortages and increased food prices

  • Deaths and injuries

  • Loss of clean water supply

  • Loss of electricity and gas

  • Damage to schools affecting school attendance

  • Loss of jobs which affects people's incomes

Economic impacts of flooding

• Economic impacts are anything which affects the economy of an area

• They may include:

  • Loss of jobs

  • Damage to businesses which affects the owners due to loss of stock and income

  • Damage to crops and agricultural land which leads to loss of income for farmers

  • Cost of repairs

  • Increased insurance prices due to the large payouts made by insurance companies

  • Damage to roads and railways which disrupts business supplies