Flooding (Edexcel A Level Geography)

Meteorological Causes of Flooding

• Flooding is the result of surplus water within the hydrological system 

• Causes of flooding can be physical (meteorological) or human

Intense storms

• These lead to flash flooding (short-lag time)

• The precipitation rate exceeds the infiltration rate increasing surface runoff

• Rivers capacity is rapidly exceeded

• Common in mountainous and semi-arid areas

Prolonged rainfall

• Can occur during monsoon conditions such as in south-east Asia 

  • In parts of India 75% of the annual rainfall occurs during the monsoon

  • In 2019 severe monsoon rainfall in India led to flooding and over 1,600 deaths

  • There was 10% more rain than the 50 year average

• Leads to the saturation of the soil

• This means that rainfall cannot infiltrate 

• Increases surface run off which means the water reaches the rivers more rapidly increasing the risk of flood

• May also occur during depression (low pressure system) conditions over the UK

  • In 2014 the flooding of the Somerset levels was the result of several depressions passing over south-west England

  • There was 350mm in January and February which was 100mm more than the average

Snowmelt

• Rapid snowmelt can lead to flooding

• If the snow melts faster than the water can infiltrate then surface runoff increases

  • This may be due to soil which remains frozen

• In March 2018 following heavy snowfall 'Beast from the east' there was significant flooding in areas of Cumbria, Lancashire and Yorkshire

Other physical causes

• Geology of the underlying rock - impermeable clay and granite

• Tectonic activity results in dam failure or landslides which block river flows 

• Volcanic activity releases meltwater beneath ice sheets

• Ice dams melt releasing glacial lake water

• Impervious surfaces in urban areas

Human Actions & Increased Flooding Risk

• Human activity can exacerbate flood risk as a result of:

  • Changing land use in the drainage basin

  • Mismanagement of the river using hard engineering

Changing land use

• Humans have made numerous changes to the natural landscape for agricultural, industrial and urban uses

• Some of these changes include:

  • Deforestation

  • Agricultural practices

  • Urbanisation 

• The changes made increase surface run off which reduces the lag time, creates a steep rising limb and increases the peak discharge

• Deforestation reduces the amount of interception and infiltration 

• Overgrazing removes the vegetation and so increases bare soil, which animals compact under hoof, reducing infiltration rates and increasing runoff

• Ploughing compacts the soil reducing soil porosity and less storage capacity

• Drainage ditches increase overland flow and change a river's annual regime

• Streams are channelled into culverts to aid rapid drainage of farmland

• Farming often reduces vegetation cover and increase soil compaction from tractors and machinery, this reduces the amount of water infiltration and increases surface runoff

• Interception is greatly reduced on harvested bare fields, meaning any subsequent precipitation will exceed soil infiltration capacity, resulting in increased overland flow

• Urbanisation increases impermeable surfaces

• Bridge supports are built into the river beds and ramps built on floodplains

• Dams are built to supply towns with water

• Sewers feed into river channels

Mismanagement of a river channel

• Hard engineered intervention is designed to 'push back' against nature

• Channelisation adds a liner to a straightened river channel (usually concrete) to reduce friction which aims to improve flow rate and therefore, reduce silting

  • However, the method displaces the river flow downstream (increased silting or flooding) overwhelming locations during peak discharge

• Dams, floodwalls and reservoirs are built to prevent flooding and ensure a regular supply of water

  • Unfortunately, the flow of sediment is halted and the reservoir gradually fills with silt

  • Downstream, erosion is increased in the river bed 

• Embankments and levees can be built or increased in height to improve bankfull capacity, using concrete or sustainable materials (usually dredged from the channel) 

  • However, if these are breached, then the scale of the flooding is much greater

• River straightening increases the flow of water by increasing the gradient of a river's channel and removing natural meanders (bends)

  • Increasing flow velocity, increases risks of flooding downstream if blockages occur or narrowing of the channel through an urban area

• Floodplains are natural natural stores of flood water and self-regulating defences

  • Building on these increase the risks of flooding and damage to homes and livelihoods 

Damage from Flooding

• There are environmental and socio-economic impacts as a result of flooding and the damage it causes

Environmental and Socio-Economic Impacts

• Natural systems have built-in ecological resilience and can accommodate moderate flooding

• However, when human activities degrade the environment, the negative impacts are more pronounced

UK flooding

• The risk of floods are increasing in the UK

• In 2020, England received 141% of its average February rainfall in less than 2 weeks during a series of storms

• Flood defences were overrun in many places (i.e. Nottinghamshire, Leicestershire, and Derbyshire)

• Heavy rain led to drains quickly over filling and back spilling water onto streets

• Extreme weather events are four times more likely than in 1970

• Since 2000 there have 9 record breaking months of rainfall, twice the amount in the previous 80 years

UK Flooding of 2007 and 2012