River Management (AQA GCSE Geography)
Revision Note
Written by: Bridgette Barrett
Reviewed by: Jenna Quinn
Factors Affecting Flood Risk
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, especially where ploughing is downslope |
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 |
Examiner 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.
Hydrographs
Flood prediction
Prediction of flooding means that steps can be taken to manage flooding
Hydrographs can be used to understand the risk of flooding because it shows the changes in discharge which result from a rainfall event
A flood hydrograph shows the changes in river discharge after a storm event
The graph shows a short period of time, usually 24 hours
The flood hydrograph has a number of features:
Base flow
Peak rainfall
Rising limb
Peak discharge
Lag time
Recessional limb or falling limb
Increased flood risk
Factors which increase surface run off or overland flow lead to:
Short lag time
Steep rising limb
High discharge
As a result the river may not have the capacity to contain the water and so flood risk is higher
Low flood risk
Factors which cause lower surface run off or overland lead to;
Longer lag time
Gentle rising limb
Lower discharge
As a result the river is more able to cope with the water entering the channel and the flood risk is lower
High Flood Risk | Low Flood Risk |
|---|---|
Steep slopes | Gentle slopes |
Impermeable rocks | Permeable rocks |
Deforestation | Lots of vegetation or afforestation |
Urbanisation | Rural areas |
Saturated or frozen soil | Deep, dry soils |
Heavy or prolonged rainfall | Light rainfall |
Worked Example
Study Figure 1, flood hydrographs for two different streams after the same storm.
'Differences in the shape of flood hydrographs are caused by both human and physical factors.'
Do you agree?
Use Figure 1 and your own understanding to explain your answer
[6 marks]
The command word in this question is 'Explain'
You need to give both human and physical factors and then explain them
There is no correct answer as long as you support your answer with evidence
It is not enough to say 'steep slopes', you need to explain why this leads to a steep rising limb
This is a levelled answer, so marks are awarded for the answer overall rather than for individual points
In a level 2 and 3 answer you are expected to use geographical terms and include examples of both human and physical factors
An example of an answer for each level can be seen below
These are just examples and there are many others which can be found in the mark scheme: See topic question 6
Answer:
Levels | Marks | Exemplar |
|---|---|---|
1 (Basic) | 1-2 | Austwick Beck has much more discharge than Clapham Beck this may be due to impermeable surfaces, steep slopes or lack of vegetation. Clapham Beck may have more vegetation and permeable surfaces. |
2 (Clear) | 3-4 | Austwick Beck has a steeper rising limb and shorter lag time than Clapham Beck reaching a peak discharge of 25 cumecs. Physical factors which may cause this are steep slopes and impermeable surfaces which lead to increased surface run off. Human factors which may lead to this are the building of houses and roads and deforestation which also increase surface run off and mean that the water reaches the river more quickly. |
3 (Detailed) | 5-6 | Both Austwick Beck and Clapham Beck experience the same amount of rainfall over the same period of time but the effect on river discharge in the two becks shown by the shape of the hydrographs is very different. This suggests that there are factors affecting the shape other than just the amount and duration of rainfall. Austwick Beck has a steep rising limb with an increase from 5 cumecs to a peak discharge of 25 cumecs and a lag time of only 6 hours. This may be due to steep slopes or impermeable rock which are natural features and increase surface run off reducing lag time. However, this could also be due to human factors urbanisation and impermeable concrete/tarmac or deforestation which reduces in interception and infiltration. It is most likely that the difference in shape is a combination of both human and physical factors. |
Hard Engineering
The key cause of the flooding is the amount and duration of precipitation: This cannot be altered
There are a number of methods of managing floods and reducing the severity and/or impact
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
Table Comparing Advantages and Disadvantages of Hard Engineered Flood Management Methods
Method | Purpose | Advantages | Disadvantages |
|---|---|---|---|
Dams and Reservoirs | Controls the river flow by blocking the river and letting water in a controlled way. This creates a reservoir behind the dam |
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Embankments / levées | Artificially raise the banks of the river, increasing channel capacity |
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Straightening Channels | Straightening the river channel by removing meanders |
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Flood Relief Channels | Channels built to allow excess water to flow around high value areas |
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Soft Engineering
Table Comparing Advantages and Disadvantages of Soft Engineered Flood Management Methods
Method | Purpose | Advantages | Disadvantages |
|---|---|---|---|
River Restoration | Restores the river to its natural state with meanders and wetland areas |
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Floodplain Zoning | Restricts land use in areas that are at high risk of flooding and ensures high value buildings are not in flood prone areas |
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Afforestation | Planting of trees to increase interception and infiltration. Trees also use up large quantities of water |
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Flood warnings | Monitoring of rivers to provide people with warnings when flooding may occur |
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Worked Example
Study Figure 1, a diagram showing floodplain zoning.
Explain how soft engineering strategies can help reduce the impact of river flooding. Use Figure 1 and your own understanding.
[4 marks]
The command is “explain”
You need to outline how and why soft engineering strategies help to protect against river flooding
It is not enough to say 'afforestation' you need to explain how afforestation helps to reduce the impact of river flooding
Answer:
Flood plain zoning aims to work with the environment [1] the type of land use is based on closeness to the river [1] Land nearest to the river is used for grazing and land furthest away is used for housing [1] this means that when the river floods there is less damage to the 'expensive' land use [1]
Planting trees or afforestation [1] increases interception and storage [1] this reduces surface run off [1] which means that the river discharge is lower and flooding is less likely [1]
River restoration [1] this removes any hard engineering restoring the river to its natural state [1] the natural processes of the river such as meanders slow the river flow [1] this reduces the risk of flooding downstream [1]
Case Study: York
York is a city located in the north of England
The city's population is just over 200,000
It is located at the confluence of the River Ouse and the River Foss
The need for flood management
York regularly floods: Serious flooding has occurred in 1947, 1978, 1991, 1995, 2000 and 2015
The floods in 2000 were the worst on record
The Ouse rose 5.5 meters above its normal level
540 properties were flooded and 320 were at serious risk
18,700 hectares of farmland were affected
The overall cost of the floods was estimated to be over £12 million including lost income from tourism
Flood management
Flood management has been in place since 1978 over time the strategies have been increased including;
The Foss Barrier: This was built to prevent water from the River Ouse backing up the River Foss. This barrier failed in 2015 and led to the flooding of many areas which had not been flooded since 1978. It has since been upgraded at a cost of £38 million
Embankments at Leeman Road: These increase the capacity of the river channel
Floodplain zoning: Clifton Ings has been left as an open, undeveloped floodplain. It can store 2.3 million m3 of water
Flood relief channels around Acomb to divert water directly into the Ouse downstream of York
Flood gates such as those on Marygate: These can be removed when there is no flood risk, flood gates are also on individual properties
Social issues
The flood management strategies have reduced the risk of flooding for many properties in York
Transport disruption during times of flood has been reduced
Sustain cycleway over Clifton Ings made wider and resurfaced
Environmental issues
Some habitat destruction in the creation of the embankments and flood relief channels
Two ponds created connected to the River Foss which have created new wetland habitats
Economic issues
Foss Barrier upgrade cost £38 million rather than the estimated £17 million
Total cost of flood management strategies estimated at £83 million
Increased the protection for over 2000 homes and businesses
Possibly lower insurance premiums
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