Edexcel IGCSE Geography 復習筆記 1.3.3 Flooding - Causes & Control
Edexcel IGCSE Geography 復習筆記 1.3.3 Flooding - Causes & Control
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 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 some human and physical factors
- These lead to increased overland flow which shortens the lag time
Human and Physical Factors which Increase the Risk of Flooding
| Human Factors | ?Impact |
| Deforestation | Lack of trees reduces interception and infiltration, increasing overland flow |
| Urbanisation | Impermeable concrete and tarmac increase overland flow
Water flows into the drains reaching the river rapidly |
| Agriculture | Bare soil and ploughing increase overland flow |
| Climate Change | Rising global temperatures may increase storm frequency and intensity |
| Physical Factors | Impact |
| 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 tend to occur in the autumn and winter when rainfall is more frequent
In areas affected by monsoon much of the annual rainfall occurs in a few weeks saturating the ground and increasing overland flow Higher temperatures in spring leads to snow melt in mountainous areas increasing overland flow |
| Drainage Density | Where drainage density is high there are many tributaries taking water to the main channel causing a rapid increase in discharge |
| Vegetation | Where there is little natural vegetation there is reduced interception leading to increased overland flow |
Exam Tip
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 use of computer modelling has 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 lack of monitoring of rivers and 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
- Soft engineering is an example of?mitigation?where schemes aim to minimise damage rather than trying to prevent the flooding
Examples of Hard and Soft Engineering
| Hard Engineering | Soft Engineering |
| Dams and reservoirs?enable the amount of discharge downstream to be controlled | River restoration,?this supports the river by restoring it back to its original regime - putting meanders back in, stabilising banks and connecting to flood plains |
| Levees?increase the capacity of the river | Wetland conservation?these areas provide somewhere for excess water to go and slow the flow of the flood water |
| Straightened channels?mean that the river flows more quickly pass vulnerable areas, reducing the risk of flooding | Catchment management plans?assess the risk of flooding in an area and outline how this will be managed |
| Flood relief channels?allow some water to flow out of the main channel reducing the discharge | Flood plain zoning?means that only certain land uses are allowed on the flood plain, reducing the risk |
Flood Resistant Buildings
- Increasingly people are using?flood resistant building design?in areas at risk:
- Homes may be raised up on stilts
- Replacement of carpet with tiles
- Power sockets above the level of likely flood
Exam Tip
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
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 requires 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 for 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
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?
- 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
- Aims to transfer?12 trillion gallons?of water a year from the south of China to the area around Beijing
- Central section was completed in 2014
- Final completion estimated to be in 2050
- It will link China's four main rivers - Yangtze, Yellow, Huaihe and Haihe
| Advantages | Disadvantages |
| Water is being supplied to industries and to irrigate large scale farms for food production | By 2014?US$79 billion?had been spent |
| Reduces water insecurity in the north-east | Many people have been relocated for construction of dams, reservoirs, pipes and canals |
| Reduces the abstraction of groundwater | In some areas in the south people's water use has been restricted to ensure that there is sufficient to transfer |
| Improved water quality | Recent droughts in the south have reduced the amount of water available |
Three Gorges Dam
- Constructed between 1994 and 2012
| Advantages | Disadvantages |
| Provides?10-14%?of China's electricity through hydroelectric power | Over?1.3 million?people forced to move due to the flooding of the valleys |
| Allows ships to navigate further up the river improving trade | Many cultural sites were lost |
| Reduces the severity of flooding downstream | Increases the risks of landslides |
| The reservoir provides water for irrigation | Cost?US$37 billion |
| Reduces reliance on fossil fuels | Sediment is building up behind the dam and reduces sediment downstream |
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