River Management (Cambridge (CIE) IGCSE Geography)
Revision Note
Written by: Bridgette Barrett
Reviewed by: Jenna Quinn
Managing the Impacts of River Flooding
Flood prediction
Prediction of flooding means that steps can be taken to manage flooding
Flood hydrographs are used to predict the reaction of a river's discharge to 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
The key factor in assessing the flood risk is time taken for the precipitation to reach the river from where it falls—the lag time
Rivers with a short lag time and steep rising limbs have a much greater risk of flooding
The water reaches the river rapidly and the river may not have the capacity to cope with the influx of water
Rivers with a long lag time and gentle rising limbs have a lower flood risk
The water reaches the river more slowly, causing a gradual increase in discharge
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 the risk of flooding
Deforestation
Lack of trees reduces interception and infiltration, increasing overland flow
Urbanisation
Impermeable concrete and tarmac increase the 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 which increase the risk of flooding
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
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
Worked Example
Study Fig 1, which is a hydrograph of a river after a storm.
How long after the start of the rainstorm is the peak flow?
..................................................... hours [1 mark]
Answer
Between 26-28 hours
Examiner Tips and Tricks
In the exam, you may be asked to explain the causes of flooding. Remember, this means that you need to make 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.
Flood management
The key cause of 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
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
Types of hard engineering
Dams and reservoirs which enable the amount of discharge downstream to be controlled
Embankments or levées increase the capacity of the river
Straightened channels mean that the river flows more quickly past vulnerable areas, reducing the risk of flooding
Flood relief channels allow some water to flow out of the main channel, reducing the discharge
Spillways or overflow channels: these take excess water away from the main channel
Types of soft engineering
River restoration supports the river by restoring it to its original regime—putting meanders back in, stabilising banks and connecting to flood plains
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
Afforestation involves the planting of vegetation and trees to increase interception and infiltration
Additional flood control methods
Other methods that can be used to decrease the risk of flooding are:
Leaving the stubble on the fields after the crop is harvested helps to stabilise the soil and increase infiltration
Contour ploughing involves ploughing fields across the slope rather than up and down. This gives the water more time to infiltrate and stops the ploughed furrows becoming channels for water
Improved forecasting and flood warnings
Dredging the rivers to increase capacity, however, often leads to the need for concrete reinforcement of the banks
Case Study: Ganges/Brahmaputra
Case Study
The Ganges is 2,510 km long
It flows through India and Bangladesh, where it becomes the River Padma and joins with the Brahmaputra River
The drainage basin covers 1.2 million km2
The population living within the area drained by the Ganges is over 650 million people
The Brahmaputra River is 3,969 km long
It flows through Tibet, India and Bangladesh, where it joins with the River Padma
The drainage basin covers 651,334 km2
Both rivers;
Have their source in the Himalayan Mountains
Waters enter the sea in the Bay of Bengal
Opportunities
Water supply
There are many cities, including New Delhi and Kolkata, along the Ganges/Brahmaputra drainage basins which take their water supplies from the river
Agriculture and fishing
The regular flooding in the drainage basin leaves deposits of alluvium, which are rich in nutrients and ideal for growing crops such as rice and jute
The rivers provide water for irrigation and for the flooding of rice fields
Fish from the river provide food and jobs for local people
Culture
The Ganges is sacred to Hindus and is worshipped as the goddess Ganga
Tourism
Three sites which are holy to Hindus lie on the banks of the Ganges (Haridwar, Allahabad and Varansi). This leads millions of pilgrims to visit each year
Rafting and river cruises are also increasingly popular
Flat land
The flat floodplains mean that construction is easy, and the floodplains have one of the highest density populations in the world
Energy
There are a number of dams along both rivers, including the Tehri Dam on the Ganges, which is the biggest hydroelectric power plant in India
Hazards
The Ganges/Brahmaputra drainage basin regularly experiences floods, including most recently in May 2022
In 1998, 75% of Bangladesh was flooded over 30 million people were made homeless
Over 1000 people died
700,000 hectares of crops were destroyed
'Normal' floods are vital to provide fertile soil and irrigation, but increasingly flooding is becoming more unpredictable and extensive
Human causes of flooding
Deforestation particularly in the upland areas
This leads to less interception and infiltration, increasing overland flow
Human-induced climate change has led to increased melting of Himalayan snow and ice, which increases discharge
It may also have affected climate patterns, leading to increased frequency and severity of tropical cyclones
Urbanisation, as the population increases and there is more rural-urban migration, leads to increased overland flow due to impermeable surfaces
Agriculture increases overland flow and soil erosion, which reduces the capacity of the rivers
Natural causes of flooding
Low-lying land in the Ganges delta in Bangladesh which is at or just above sea level, meaning that it floods more easily
Monsoon climate which means that there are heavy and prolonged rains for some months
Tropical cyclones bring heavy rainfall
Melting snow and ice from the Himalayas in spring leads to a rapid increase in river discharge
Management of flooding
Bangladesh is an LEDC
The country lacks the money for large schemes to reduce the impact of flooding
Flood Action Plan (FAP) was funded by the World Bank and a number of MEDCs. Measures that were proposed include:
Monitoring of flood levels
Construction of levées/embankments
Building 5000 flood shelters
Creating floodwater storage systems
A more effective flood warning system
Building of dams to store water
Reducing deforestation
The FAP was not considered a success because;
Many parts of the project were never completed, including the dams and floodwater storage areas, due to inadequate funding and corruption
There was later a recognition that some flooding was necessary to maintain agriculture in many areas
8 million people were forced to move to accommodate the FAP construction
Changing the channel upstream meant that areas downstream suffered more
The government cannot afford the maintenance costs
New management suggestions include;
Better flood forecasting and warning systems
More well-stocked flood shelters
These are less damaging to the environment and cheaper to maintain than hard engineering such as embankments, dams and floodwater storage areas
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