River Management (AQA GCSE Geography): Revision Note
Exam code: 8035
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 rises more quickly
The lag time depends on some human and physical factors
These lead to increased overland flow which shortens the lag time
Human factors which increase flood risk
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 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 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 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.
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 runoff or overland flow lead to:
Short lag time
Steep rising limb
High discharge
These factors include:
Steep slopes
Impermeable rocks
Deforestation
Urbanisation
Saturated or frozen soil
Heavy or prolonged rainfall
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 runoff or overland lead to;
Longer lag time
Gentle rising limb
Lower discharge
These factors include:
Gentle slopes
Permeable rocks
Lots of vegetation or afforestation
Rural areas
Deep, dry soils
Light rainfall
As a result the river is more able to cope with the water entering the channel and the flood risk is lower
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]
Answer:
Yes, the shape of flood hydrographs is caused by both human and physical factors. [U] In Figure 14, the two drainage basins experience the same levels of precipitation over a 4 hour period [U] but the shape of the hydrographs is very different. Stream A is a flashy river where there is a short lag time, steep rising limb, and high peak flow (from 5 to 25 cumecs). This could be due to the physical features of the drainage basin, such as steep slopes, or the result of impermeable rocks, [Ap] both of which would result in low/no infiltration and increased surface runoff. This means more water will reach the river, and it will do so more rapidly. [Ap] Equally, the shape could be the result of human activity such as urbanisation.[U] The covering of surfaces with tarmac and concrete as well as the installation of artificial drainage will lead the water to reach the river very quickly, reducing lag times and increasing discharge. [Ap] The amount of precipitation is the primary factor in the response of a river, but it is clear from streams A and B that other factors can make a significant difference and may be either human or physical.[Ap]
Marking guidance
This is a 'Level of response' answer.
Each point made in the answer does not equal a mark.
The command is 'explain your answer' which means the answer needs to support whether your decision was 'yes' or 'no' by explaining the causes.
The question asks refers to both physical and human factors. You can focus on one side of the argument, 'the shape is affected more by physical factors,' or you can take a more balanced approach, looking at the impact of both on the shape.
Mark allocation
3 marks for understanding [U]
3 marks for application [Ap]
Common errors
The following errors mean that answers cannot achieve a Level 3 (top marks):
A lack of evaluation regarding what causes the differences in the hydrograph shapes.
Alternative content
The answer above is just one example of a response to this question. Other information which could be used in the answer includes:
Human causes:
Deforestation.
Agriculture.
Physical causes:
Precipitation.
Vegetation.
Hard engineering
The main 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
Hard engineering strategies
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
Embankments and 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
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
Afforestation is the 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]
Answer:
Soft engineering strategies work with the natural processes along the river to reduce flooding risk, such as floodplain zoning shown in figure 17. [U] In the area closest to the river, the land is used for animal grazing, and houses and industry are built on the land furthest away from the river. [Ap] This is a low cost strategy and does not impact on river processes. [U] It reduces the chances of houses and industry being flooded and therefore the amount of damage. [Ap]
Marking guidance
This is a 'Level of response' answer.
Each point made in the answer does not equal a mark.
The command word is 'explain' which means the answer needs to outline how and why soft engineering reduces the impact of flooding.
Mark allocation
2 marks for understanding [U]
2 marks for application [Ap]
Common errors
The following errors mean that answers cannot achieve a Level 2 (top marks):
Explaining hard engineering strategies rather than soft engineering strategies.
No reference to Figure 17 (maximum 2 marks, level 1).
Alternative content
The answer above is just one example of a response to this question. Other information that could be used in the answer includes:
Planting trees.
Reduction in discharge and flooding risk.
New habitats.
River restoration.
Flood warnings.
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
Over 540 properties were flooded and 320 were at serious risk
Approximately, 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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