River Management (Cambridge (CIE) O Level Geography)

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 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

Flood Hydrograph in an Urban Area

• 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 limb 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 limb 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 and Physical Factors which Increase the Risk of Flooding

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 

Examples of Hard and Soft Engineering

• 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 which 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 this often leads to the need for concrete reinforcement of the banks

Case Study: Ganges/Brahmaputra

• The Ganges is 2,510km 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 651,334km²

• Both rivers;

  • Source is in the Himalayan Mountains

  • Waters enter the sea in the Bay of Bengal

The Ganges and Brahmaputra Basin

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

• The causes of flooding are both human and natural - see the table below

Management

• 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 flood water 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 constructions

  • 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