River Flooding

The causes of river flooding

The causes of river flooding

• When rivers cannot contain all the water they are transporting, a flood will occur

• Water flows out of the channel onto the floodplain

• Causes of flooding can be physical or human

Physical causes of river flooding

Precipitation type and intensity

• Intensive rainfall produces increased overland flow

• Intense rain compacts the ground, reducing infiltration

• Rapid melting of snow can cause high rates of overland flow

Temperature and evapotranspiration

• Colder air can hold less water

• Lower temperatures lead to less evaporation, which means more water in the rivers

Drainage basin size and shape

• Smaller drainage basins respond more quickly to rainfall conditions, increasing the flood risk

• Circular basins respond more quickly to floods than linear basins, increasing the flood risk

Drainage density

• High drainage density is the result of impermeable rock/soil and increases overland flow

Slopes

• Steeper slopes reduce infiltration and increase overland flow

Vegetation types

• Deciduous trees lose their leaves in autumn, reducing the amount of interception in autumn and winter, which increases overland flow

Rocks and soil

• Impermeable rocks and soil decrease infiltration and percolation, which increases overland flow

Human causes of flooding

Urbanisation

• Urbanisation increases the magnitude and frequency of floods in three ways:

  • Creating impermeable surfaces, such as roads

  • Creating gutters and sewers, which increase drainage density

  • Riverside facilities reduce carrying capacity

• Urbanisation tends to have a greater impact on the lower section of the drainage basin due to there being more urbanisation in this section

Deforestation

• Similar impact to urbanisation

• Deforestation can increase overland flow

• Deforestation decreases channel capacity due to an increase in deposition in the channel

• In the Himalayas, there is evidence that changes in flooding and increased silt are due to high monsoon rains and unstable terrain

Agriculture

• Bare soil and ploughing increases the amount of overland flow

The impacts of river flooding

The impacts of river flooding

• There are a range of possible impacts of flooding

• These can be economic, social and environmental

Economic

• The cost of crop destruction will affect commercial farmers and increase food prices

• Destruction of infrastructure such as roads, energy supply and communications which will then need to be repaired

• Increased insurance costs

• Reduction in house prices for those areas affected by the flooding

• Closure of businesses

Social

• Homes destroyed and damaged leading to homelessness

• Increased food shortages and higher food prices caused by the loss of crops

• Deaths and injuries

• Difficulties in moving around the area due to damaged transport

• Loss of jobs

• Lack of clean water leads to disease

• Loss of energy supply

Environmental

• Destruction of animal habitats

• Loss of food supply for animals

• Pollution of the land due to pollutants carried by the water

• Death and injuries of animals

• Destruction of trees and other vegetation

Management of river flooding

Management of river flooding

Flood management

• The key cause of flooding is the amount and duration of precipitation; this cannot be altered

• There are several 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 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