Flooding - Causes & Control (Edexcel IGCSE Geography): Revision Note
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
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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