Flooding - Causes & Control (Edexcel IGCSE Geography)

Revision Note

Bridgette Barrett

Written by: Bridgette Barrett

Reviewed by: Jenna Quinn

Updated on

Causes of flooding

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

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

  • Flooding is usually the result of heavy or prolonged rainfall

  • 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 overland flow which shortens the lag time

Human factors which increase flood risk

  • 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

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 Northern Europe tends to occur in the autumn and winter when rainfall is more frequent

    • In areas affected by monsoons, much of the annual rainfall occurs in a few weeks saturating the ground and increasing overland flow

    • Higher temperatures in spring lead to snow melt 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

Examiner Tips and Tricks

In the exam, you may be asked to explain the causes of flooding. Remember this means that you need to give connections between factors such as deforestation and the increased flood risk. Rather than simply stating that deforestation increases flood risk you need to explain the reasons - decreased interception and infiltration, leading to increased overland flow and shorter lag time.

Prediction & warning

  • Improvements in weather forecasting and the use of computer modelling have made flood prediction more accurate, particularly in developed countries

  • The Flood Forecasting Centre in the UK is a partnership between the Met Office and the Environment Agency

  • Forecasting uses the following information:

    • Rainfall amounts

    • Hazard impact mapping to identify the areas most at risk from flooding

    • Past storm hydrographs to forecast the likely rate of discharge change in rivers

    • Drainage basin characteristics - type of rock, amount of vegetation, relief

  • Flood alerts are then issued to inform people of current flood risk and the risk of flooding over the following five days

  • Flood prediction and warnings are limited in emerging and developing countries due to a lack of monitoring of rivers and the cost of setting up an alert system

  • The Bangladesh Water Development Board has focussed on improving flood warnings in recent years and is now able to inform people of the risk of flooding over 3 days

Prevention of Flooding

Hard and soft engineering

  • The two main categories of flood management are hard and soft engineering:

    • Hard engineering involves building structures or changing the river channel

    • Soft engineering works with natural processes of the river and surrounding environment

  • Soft engineering is increasingly popular because it has less impact on the environment

  • Soft engineering is an example of mitigation where schemes aim to minimise damage rather than trying to prevent flooding 

Hard engineering

  • Dams and reservoirs enable the amount of discharge downstream to be controlled

  • Levees increase the capacity of the river 

  • Straightened channels mean that the river flows more quickly past vulnerable areas, reducing the risk of flooding in those areas

    • This may lead to a greater risk of flooding further downstream

  • Flood relief channels allow some water to flow out of the main channel reducing the discharge

Soft engineering

  • River restoration supports the river by restoring it to its original regime

    • This involves putting meanders back in, stabilising banks and connecting to floodplains

  • Wetland conservation these areas provide somewhere for excess water to go and slow the flow of floodwater

  • Catchment management plans assess the risk of flooding in an area and outline how this will be managed

  • Floodplain zoning means that only certain land uses are allowed on the floodplain, reducing the risk

Flood Resistant Buildings

  • Increasingly people are using flood-resistant building designs in areas at risk:

    • Homes may be raised on stilts 

    • Replacement of carpet with tiles

    • Power sockets above the level of likely flood

Examiner Tips and Tricks

Students sometimes confuse hard and soft engineering. Remember hard engineering is when structural changes are made to the river. Soft engineering is working with the natural environment.

Case Studies: Spain and China

  • River management is essential for a range of reasons:

    • To control flooding

    • To store and supply water

    • To generate electricity

    • To improve navigation

  • The following case studies will provide some information about river management in Spain and China

Case Study

River Management in Spain

  • The average precipitation in south-east Spain is approximately 365mm a year

  • Most precipitation occurs in winter

  • The area is one of the driest in Europe

  • Agriculture uses 80% of the water available

    • 147,000 hectares of land require irrigation

  • Regular droughts mean the supply of water is further reduced

  • There is a water deficit as demand exceeds supply

  • Tourism increases the problem:

    • Water parks and golf courses use significant quantities of water 

    • An average tourist uses between 450-800 litres per day

    • An average Spaniard uses 127 litres per day

Tagus-Segura Project

Map showing river basins in Spain, highlighting the Tagus-Segura transfer, reservoirs, and locations like Valencia, Murcia, and Cartagena.
Map of the Tagus-Segura project
  • Completed in 1978

  • 60% of the water flowing into the Tagus is transferred

  • A 286km pipeline which connects four Spanish river basins Tagus, Jucar, Segura and Guadiana

  • The aim was to supply Alicante, Murcia and Cartagena in the south-east to reduce the water deficit

Issues

  • Much of the water transferred went to the tourist and leisure users not to small-scale farmers

  • Water consumption in the south-east increased due to an increase in supply

  • It is estimated that 15% of the transferred water is being illegally used by leisure users such as golf courses

  • Large commercial farms are benefitting more than small-scale farmers

Ebro Project

  • A second water transfer project was proposed in 2001 to transfer water from the River Ebro

  • This project was abandoned due to:

    • The failure and issues with the Tagus-Segura project

    • The cost

    • The threat to the Ebro delta as the scheme would have disrupted sediment flow to the delta

  • Spain has now moved to utilising desalinisation plants to meet the demand for water

Case Study

River Management in China

  • The average precipitation in north-east China is approximately 200-400mm a year

  • Most precipitation occurs during the brief wet season

  • Average rainfall has steadily decreased since the 1950s

  • The main industrial areas and regions of economic growth are on the North China Plain around Tianjin and Beijing

  • The main water source for the area has been groundwater:

    • Demand for water in Beijing alone is 3.6 billion cubic meters 

    • The supply of freshwater in Beijing is 3 billion cubic meters

    • There is a significant water deficit

    • The over-abstraction of groundwater has led to subsidence in some areas of the city of 11cm

South-North Water Transfer Project (SNWTP)

  • The project aims to transfer 12 trillion gallons of water a year from the south of China to the area around Beijing

  • The central section was completed in 2014

  • Completion estimated to be in 2050

  • It will link China's four main rivers - Yangtze, Yellow, Huaihe and Haihe

Advantages of the SNWTP

  • Water is being supplied to industries and to irrigate large-scale farms for food production

  • Reduces water insecurity in the north-east

  • Reduces the abstraction of groundwater

  • Improved water quality

Disadvantages of the SNWTP

  • By 2014 US$79 billion had been spent

  • Many people have been relocated for the construction of dams, reservoirs, pipes and canals

  • In some areas in the south people's water use has been restricted to ensure that there is sufficient to transfer

  • Recent droughts in the south have reduced the amount of water available

Three Gorges Dam

  • The Three Gorges Dam is a separate project in China which was built for several reasons including:

    • flood control

    • hydroelectric power

    • water storage

    • improved navigation

  • Constructed between 1994 and 2012 it is the world's largest multipurpose dam

  • Located on the Yangtze River in South China

  • The dam is over 2km wide and 100m high

  • The reservoir it created is over 600km long

Advantages of the Three Gorges Dam

  • Provides 10-14% of China's electricity through hydroelectric power

  • Allows ships to navigate further up the river improving trade

  • Reduces the severity of flooding downstream

  • The reservoir provides water for irrigation

  • Reduces reliance on fossil fuels

Disadvantages of the Three Gorges Dam

  • Over 1.3 million people were forced to move due to the flooding of the valleys 

  • Many cultural sites were lost

  • Increases the risks of landslides

  • Cost US$37 billion

  • Sediment is building up behind the dam and reduces sediment downstream

  • Impact on wildlife including the Chinese river dolphin

  • Large areas of forest were cleared

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Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.

Jenna Quinn

Author: Jenna Quinn

Expertise: Head of New Subjects

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.