River Management (AQA GCSE Geography)

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Bridgette

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Geography Lead

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

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.

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

flood-hydrograph

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.

screenshot-2022-11-21-at-14-32-09

'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	Multi-purpose can be used for energy production (HEP) and water storage as well as flood control Provides opportunities for recreation: Water sports, fishing	Expensive Loss of farmland and homes due to reservoir creation Displacement of people Affects on ecosystems: can affect fish breeding Reservoir silts up over time
Embankments / levées	Artificially raise the banks of the river, increasing channel capacity	Increased river capacity means it is less likely to flood New river bank habitats may be created	Expensive Visually unattractive particularly if made from concrete May fail and lead to more serious flooding
Straightening Channels	Straightening the river channel by removing meanders	Speeds up the movement of water over a short distance Allows easy navigation for boats	May increase flood risk downstream as the discharge reaches those areas more quickly Expensive Affects river ecosystems due to changes in velocity
Flood Relief Channels	Channels built to allow excess water to flow around high value areas	Effective in reducing the flood risk in high value areas New habitats may be created Insurance costs may be reduced for people living nearby	Expensive Regular maintenance is needed Disruption to existing habitats Can be visually unattractive

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	Increases the number of habitats Restores wetland areas Slows down water flow reducing flooding downstream	Expensive to construct the new channels Some areas will flood
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	Low cost Conserves habitats on floodplains and wetlands	Restricts areas where houses can be built and may impact on economic development Can only happen in places where development has not already happened
Afforestation	Planting of trees to increase interception and infiltration. Trees also use up large quantities of water	Inexpensive Absorbs and stores CO₂ Slows down water transfer, increasing lag time	Can increase acidity in the soil Loss of farmland
Flood warnings	Monitoring of rivers to provide people with warnings when flooding may occur	Helps people to prepare and evacuate if needed Less expensive than hard engineering	Expensive to set up monitoring equipment People may not take warnings seriously

Worked example

Study Figure 1, a diagram showing floodplain zoning.

screenshot-2022-11-21-at-14-36-40

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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River Management (AQA GCSE Geography)

Revision Note

Flood prediction

Increased flood risk

Low flood risk

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:

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:

The need for flood management

Flood management

Social issues

Environmental issues

Economic issues

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Author: Bridgette