Causes & Impacts of Flooding (Cambridge International IGCSE Environmental Management)

Causes of flooding

• Flooding is one of the most common natural hazards on Earth

• Flooding occurs when the volume of water is greater than the river's capacity

• Floods can be triggered by:

  • Natural features of the land, like steep hills or heavy rain
  • Human activities that change how water moves, such as cutting down forests or building lots of concrete surfaces

• Climate change is also increasing the risk of floods by bringing more intense storms and rising sea levels

Land relief

• On steep slopes, rainwater flows quickly downhill because the water does not have time to infiltrate

  • Water levels in rivers rise sharply when water rushes down slopes faster than the channel can handle, increasing flood risk

• Flat, low-lying land near rivers or coasts is easily flooded because water spreads and has nowhere to drain

  • Water flows into low-lying areas and collects quickly during storms, making them vulnerable

• Funnel-shaped valleys or narrow river basins channel water into a small space, causing the river to rise quickly during heavy rain

Saturated soil, compacted soil and soil erosion

• Soil that is already saturated cannot hold any more water

  • Any extra rain will run off the surface into the rivers

• Compacted soil from footsteps, vehicles or farm machinery reduces the tiny air spaces that normally store water, so even light rain becomes surface run-off

• Soil erosion removes the top layer of soil that normally absorbs water, leaving hard or bare ground that increases surface run-off

• When run-off increases, rivers receive more water in a shorter amount of time, raising the risk of them overflowing

• Areas with poor soil structure struggle to absorb water during storms, making flash flooding more likely

Deforestation, agricultural practices and urbanisation

• Deforestation removes trees that normally intercept rainfall on leaves and roots that help water soak into the ground, so more water runs quickly into rivers

  • Tree roots normally bind the soil together; when trees are removed, soil washes away more easily, reducing water storage and increasing run-off

• Some agricultural practices, like overgrazing, can strip land of vegetation, leaving bare soil that cannot absorb water well

  • Ploughing across slopes can create channels that guide water downhill faster, speeding up run-off into rivers

• Urbanisation covers large areas with impermeable surfaces like roads, pavements and rooftops, which stop water from soaking into the ground

  • Rainwater on these surfaces moves quickly into drains and rivers, overwhelming drainage systems and raising water levels sharply

Climate change: rise in sea level and increase in extreme weather events

• Rising sea levels caused by melting ice and thermal expansion push more seawater onto coastal land, increasing the chance of coastal flooding during storms

  • Higher sea levels mean storm surges start from a higher point, so they can travel further inland and cause worse damage

• Climate change leads to more frequent, intense rainfall events, meaning large amounts of rain fall in a short time; this overwhelms river channels and drains

  • Warmer air holds more moisture, so storms carry extra water, which then falls as heavier rain
  • Increased storm intensity brings stronger winds, heavier rainfall and larger waves, all of which raise flood risk

Storm surges and tsunamis

• These are coastal flooding causes, affecting coastlines rather than inland river systems

• Storm surges form when powerful winds from tropical cyclones or strong storms push seawater towards the coast, creating a temporary rise in sea level

  • This wall of water can flood coastal towns, break seawalls and travel far inland, especially in low-lying areas

• Tsunamis are giant waves caused by underwater earthquakes, volcanic eruptions or landslides that displace huge volumes of water

  • These waves move extremely fast and can flood coastal regions within minutes, destroying buildings, roads and natural coastal habitats

• Both storm surges and tsunamis can remove beaches, damage mangroves and destroy protective coastal features, making future floods more likely

Impacts of flooding

• Flooding can affect almost every part of life
• Flooding damages buildings, disrupts transport, destroys crops, harms wildlife and affects people’s health
• Some impacts are immediate (short-term), like homes being filled with water
• Others are long-term and can last for months or years, such as soil becoming too damaged to grow food.
• Floods also affect natural ecosystems, changing habitats and putting plants and animals at risk

Damage to buildings and infrastructure

• Fast-moving floodwater can penetrate homes, shops, roads and bridges, weakening structures or destroying them completely

  • Roads can crack, lift or wash away because moving water is powerful enough to break apart the surface
  • Bridges may collapse when floodwater hits them or when debris becomes trapped, increasing pressure on the structure

• Power lines, water pipes and communication networks can be damaged, making it harder for communities to access basic services

• Transport routes can become blocked or washed away, slowing down emergency help and preventing people from moving safely

Loss of crops

• Floodwater can drown crops by covering them for too long, preventing them from getting the oxygen they need
• Silt and debris left after a flood can bury fields and make it difficult for farmers to plant new crops
• Floods can wash away seeds, soil nutrients and young plants, reducing food supply and increasing food prices

Impact on livestock and habitats

• Animals can die if they are trapped by rising water or swept away by strong currents

• Floods can destroy natural habitats like riverbanks, wetlands and forests, forcing wildlife to move or reducing biodiversity

  • Aquatic ecosystems can be harmed when sudden changes in water speed or depth disrupt fish populations
  • Floods can wipe out shelters and nesting sites, affecting breeding and reducing biodiversity

• Farm buildings and grazing land can be damaged or contaminated, making it harder for farmers to care for their animals

Evacuation of people and livestock

• People may need to leave their homes suddenly when water levels rise, leading to stress and disruption to daily life

• Livestock often must be moved to higher ground, which can be challenging and risky, especially for large herds

• Evacuation centres can become crowded, increasing pressure on emergency services and local resources such as food, water and medical supplies

  • Long stays in temporary accommodation can disrupt school, work and community life

Landslides

• Heavy rain soaks into hillside soil, making it heavier and more likely to slip downhill
• When the soil gives way, it can create a landslide that buries roads, houses and farmland under mud and rocks
• Landslides can destroy vegetation, which increases future flood risk because the soil loses root support
• Blocked rivers behind landslides can create temporary lakes, which may later burst and cause further downstream flooding

Contamination of drinking-water supplies

• Sewage systems overflow during floods, mixing human waste with floodwater
• Chemicals from factories, petrol stations and farms can leak into the floodwater, making it dangerous
• Wells, reservoirs and water storage tanks can become polluted, making the drinking water unsafe
• Cleaning and repairing water supplies take time, leaving communities dependent on bottled or delivered water

Water-related disease

• Dirty floodwater spreads diseases like cholera and dysentery because harmful bacteria thrive in warm, wet conditions

  • People may have to wash, cook or drink using contaminated water if clean supplies are limited

• Stagnant water after a flood creates breeding sites for mosquitoes, increasing diseases such as malaria or dengue fever

• Poor hygiene in crowded shelters can raise the chance of infections spreading among evacuees

Financial losses

• Families face high costs for damaged homes, loss of belongings and vehicles

• Businesses can lose stock, equipment and income, especially if they have to close for repairs

  • Tourism can struggle if attractions, hotels or beaches are damaged, reducing visitor numbers

• Governments may have to spend large amounts on rebuilding roads, bridges and public services, placing strain on national budgets

  • Long-term costs can reduce funding for schools, hospitals or other community services

Human health

• Physical injuries can occur during rescue efforts or when people try to escape rising water

• Stress, anxiety and trauma can affect mental health long after the flood has ended

  • Respiratory problems can develop when damp homes grow mould, making the air unhealthy to breathe

• Loss of homes, income and security can create long-term emotional pressure for families and communities

  • Mental health can be affected for months or years, especially for children who experience severe flooding
  • Lack of clean water, safe shelter or food also increases the risk of overall poor health

Opportunities provided by flooding

• In many parts of the world, some types of flooding can be necessary and beneficial, not harmful
• These opportunities support ecosystems, give communities access to fresh water, and enable them to grow crops
• For thousands of years, farmers in river valleys have depended on these benefits

Deposition of nutrient-rich silt on farmland

• When floodwater spreads across fields, it slows down and drops silt carried from upstream

  • Silt is full of nutrients like nitrogen and phosphorus, which help crops grow more strongly

• Soil becomes more fertile, reducing the need for expensive fertilisers

  • Farmers in floodplains, such as the Nile or Ganges valleys, rely on this natural soil improvement each year

• Over time, fertile soils increase food production and support healthy rural economies

Recharge of surface water and groundwater stores

• Floodwater fills rivers, lakes and reservoirs, helping communities cope with dry seasons
• Some floodwater soaks through the soil and into aquifers, which are natural underground water stores
• Recharge helps maintain water supplies for drinking, irrigation and industry
• Wetlands and floodplains also refill, supporting wildlife and increasing ecosystem health
• This recharge is especially important in dry regions that depend on seasonal floods for fresh water, such as the Indian monsoons

Worked Example State one physical cause of river flooding.

[1 mark]

Answer

Heavy rainfall can cause rivers to overflow because the channel fills faster than it can drain. [1 mark]

Marking guidance

The command word 'state' requires you to give a simple fact with no explanation.

Mark allocation

• 1 mark for any single correct physical cause

Alternative content

• Steep slopes. [1 mark]
• Saturated soil. [1 mark]
• Storm surges. [1 mark]
• Tsunamis. [1 mark]
• Snowmelt. [1 mark]

Common mistakes

• Giving a human cause instead of a physical one (e.g. urbanisation).