Flooding (Edexcel A Level Geography)

• 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 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 

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

Environmental and Socio-Economic Impacts

Environmental	Socio-Economic
Loss of crops  Loss of habitats Pollution of waterways Soil and bank erosion Eutrophication Siltation but also soil replenishment Recharge groundwater stores Flooding can be a trigger event for breeding, migration and dispersal  Recharge of wetlands Increase the connectivity between aquatic habitats  Move sediment and nutrients around the landscape	Loss of life, property and infrastructure Road closure and loss of communications Long-term rehoming of people Cost of house insurance increases Loss of income from employment, agriculture and/or tourism Health risks from water contamination Trauma and loss of possessions Psychological impacts can be long lasting

• 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

UK 2007

UK 2012

Wettest summer since records began in 1766, with 414mm of rainfall across England and Wales from May to July In late July, Gloucestershire and Worcestershire flooding, left 48,000 homes without power for two days, and 10,000 motorists were stranded on the M5 overnight  The RAF and Army were called in to help when Tewkesbury  became cut off  Surface water and river flooding affected more than 55,000 homes and businesses across the country 7,000 people were rescued 17,000 families had to leave their homes 13 people died Estimates put the total losses at about £4bn, of which insurable losses were reported to be about £3bn

The 2012 floods were a series of weather events during the course of the year and on through the winter into 2014 At the start of 2012, the UK experienced drought and a March heatwave By April, a series of low pressure systems brought by the jet stream, the wettest month in a 100 years It continued through May and led to the wettest start to June in 150 years, with flooding and extreme events throughout the UK and parts of Europe By end of June and again early July, heavy thunderstorms gathered strength across mainland UK; a product of two fronts colliding over the UK - warm air from the Azores meeting water laden cold air from the west Afternoon of 6 July saw the Met Office issue its highest alert of Take Action Intense low pressure systems in September, November and December brought heavy rains, which overwhelmed the already saturated ground, causing widespread flooding