What floods are and when they occur

Floods happen when water overflows onto land that is normally dry, often because a drainage basin cannot handle excess water. This excess can come from heavy or prolonged rainfall, leading to quick surface runoff. Rivers may reach bankfull capacity, spilling over onto the surrounding low-lying land known as the floodplain. Lakes and reservoirs can also expand, covering new areas. While flooding is a natural process that can benefit farmland by depositing nutrient-rich silt, it becomes a problem when it affects human health, homes, and livelihoods, especially if people have altered natural river flows or built on floodplains.

Key features of floodplains

Floodplains are the flat areas beside rivers that act as natural storage zones for excess water, slowing its movement through the drainage basin. This helps prevent rapid flooding downstream, but human interference can disrupt this function.

Meteorological causes of floods

Floods often result from weather patterns that deliver more water than the land can absorb or drain away. These causes can lead to either slow-building floods or sudden, intense ones, depending on the type of rainfall or other atmospheric conditions.

Prolonged rainfall

When heavy rain falls over many days or weeks, it can cause slow-onset flooding. This happens as a series of low-pressure systems, called depressions, pass over an area like the UK with little break between them. The ground becomes saturated, reducing infiltration and increasing runoff into rivers.

Intense storms

Intense storms bring fast-onset flooding, known as flash flooding, where large amounts of rain fall quickly. This can combine with storm surges from the sea, leaving standing water on the land. Storm surges occur when strong winds push seawater inland, worsening the flooding.

Extreme monsoonal rainfall

In subtropical regions, monsoons — seasonal wind shifts that bring wet weather — can cause prolonged heavy rain. As global climate changes, warmer seas may intensify these rains, especially during La Niña events. This leads to higher runoff and saturated soils, overwhelming rivers and causing floods.

Snowmelt

In upland areas with glaciers or snow, melting ice and snow in spring can increase overland flow, swelling rivers and reservoirs. This might happen due to unusually high temperatures, sudden warmth, or volcanic activity heating the ice. A specific type of flood from glacial meltwater is called a jökulhlaup.

Physical causes of floods

Beyond weather, the natural features of the landscape influence how likely flooding is. These factors affect how water moves and is stored in a drainage basin.

Key physical factors influencing flood risk

• Shape of the land - Flat areas flood more easily because water cannot drain downhill quickly, while steep valleys can funnel water into narrow channels, increasing flood speed and volume.
• Land elevation - Low-lying coastal zones are prone to flooding from storm surges, as seawater can easily overflow onto the land.
• Geology and soil - Impermeable rocks prevent water from soaking in, leading to more surface runoff. Deeper soils can store more water before becoming saturated, delaying floods.
• Drainage density - Areas with many rivers and streams have higher flood risk, as water collects from multiple sources quickly.

How human activities increase flood risk

People can make floods more likely or severe by changing how water flows through landscapes. These changes often speed up runoff or reduce natural water storage.

Building on floodplains

As populations grow, more floodplains are developed for housing and industry. This covers the land with impermeable surfaces like concrete and tarmac, which block infiltration. Water then runs off quickly into rivers, raising flood risk instead of being absorbed slowly.

Altering vegetation or land cover affects water movement

• Deforestation removes trees that intercept rain, exposing soil to erosion. Eroded soil can fill river channels, reducing their capacity and causing overflows.
• Urban construction adds impermeable layers, preventing infiltration and directing water to other areas.
• Farming practices, such as leaving fields bare, reduce interception, allowing faster runoff into rivers.

Hard engineering interventions

• Human efforts to control rivers can backfire.
• Straightening channels or removing meanders speeds up water flow, reducing flooding in one spot but increasing it downstream.
• Dredging to deepen rivers helps water pass quickly through urban areas but can overwhelm sections further along.

Socio-economic impacts of flooding

Floods disrupt economies, infrastructure, and communities, often with long-lasting effects. These impacts highlight why managing flood risk is crucial for society.

Effects on economic activity

• Businesses may close temporarily due to unsafe conditions or inaccessibility, reducing trade and income.
• Flooded farmland can drown crops from lack of oxygen or spread diseases, cutting farmers' yields and earnings.

Effects on infrastructure

• Transport networks suffer as roads, bridges, and tunnels become damaged or unusable, halting vehicles and trains.
• Communication systems like telephone lines can fail from water damage.
• Water supplies may be contaminated by sewage or pollutants, requiring shutdowns for cleaning.

Effects on settlements

• Homes and buildings can become unstable as water soaks into structures, leading to collapses or the need for major repairs to remove sediment, pollutants, and mould.
• Walls, fences, and trees may be washed away.
• Many residents on floodplains struggle to get insurance, bearing full rebuilding costs.
• Human lives are at risk from drowning, injuries, or waterborne diseases like cholera.

Environmental impacts of flooding

Floods can harm or help ecosystems, depending on the context. While they may cause pollution and erosion, they also replenish water sources.

Negative environmental impacts

• Groundwater near cities can be polluted by sewage and road oils.
• Eutrophication occurs when fertiliser runoff causes algal blooms in lakes and ponds, depleting oxygen and killing aquatic life.
• Soil erosion strips away topsoil as water rushes over land.
• Contaminants in floodwater can kill soil microorganisms.

Positive environmental impacts

• Wetlands, lakes, and groundwater recharge, supporting more plant and animal species and boosting biodiversity.
• Farmland gains fertility from deposited silt after waters recede.

Case study: UK floods in 2007

The 2007 UK floods were a major event caused by extreme weather, affecting many regions and leading to widespread criticism of flood management.

Causes of the floods

Unusually heavy summer rainfall, more than double the average in some areas, came from low-pressure systems influenced by the polar jet stream shifting south. The polar jet stream is a fast-moving air current high in the atmosphere that steers weather systems; its southern position brought heavier rain to the UK. This overwhelmed rivers like the Severn, Thames, and Don.

Key impacts

• Human and economic costs - More than a dozen people died, over forty-five thousand homes were flooded, and the total damage cost approximately £3.5 billion.
• Local effects in Upton-upon-Severn - The village was largely isolated by floodwater, with few defences in place, making rescue difficult. Waterfront businesses like pubs and cafes closed for months.

Responses and evaluation

Approximately £4.8 million was invested in new defences, such as flood walls, to protect Upton-upon-Severn. Government management was criticised for inadequate preparation, though some argue the rainfall was so extreme that defences might not have helped much. This event highlighted the need for better river management and flood protection across the UK.