A flood occurs when water covers land that is normally not submerged, usually because the volume of water entering or accumulating in an area exceeds the capacity of rivers, drainage systems, soils, coastlines or other pathways to contain or remove it. Flooding has many causes, including intense rainfall, prolonged rainfall, rapid snowmelt, storm surges, coastal storms, river overflow, dam or embankment failure, blocked drainage, groundwater emergence and combinations of these processes. Floods are not all alike.
River floods develop when river discharge exceeds channel capacity, surface water or pluvial floods occur when rainfall overwhelms local drainage, flash floods develop rapidly after intense precipitation or sudden water release, and coastal floods can result from storm surge, unusually high sea levels or wave action. Flooding is a natural part of many river and coastal systems, but human settlement and land use can greatly increase exposure and damage. Understanding floods requires considering the source of water, the rate at which it arrives, the landscape through which it moves, the ability of soil and drainage systems to absorb or transport it and the vulnerability of people and infrastructure in the affected area.
Heavy rainfall is one of the most common causes of flooding. When precipitation falls faster than the ground can absorb it or drainage systems can transport it, water begins to accumulate at the surface. Rainfall intensity, duration and spatial distribution all matter. Short bursts of extremely intense rain can produce flash flooding even when total daily rainfall is not exceptionally large, while prolonged moderate rainfall can saturate soils and eventually cause rivers to overflow.
Soil type, antecedent moisture, vegetation, slope and land cover determine how much rainfall infiltrates and how much becomes surface runoff. Impermeable urban surfaces such as concrete, asphalt and roofs prevent infiltration and can route water rapidly into drains and channels. If drainage infrastructure is designed for smaller rainfall events, extreme precipitation can exceed its capacity. Climate change can influence flood risk by increasing the atmospheric capacity for water vapour and, in many regions, increasing the potential for more intense rainfall, although actual flood changes depend on local precipitation patterns, catchment characteristics and human development.
River flooding occurs when the discharge of a river becomes greater than the capacity of its channel, causing water to spill onto surrounding floodplains. River discharge depends on precipitation, snowmelt, groundwater contributions, upstream storage and catchment characteristics. A large watershed can collect water from an extensive area, so rainfall occurring far upstream may cause flooding downstream hours or days later.
Floodplains are naturally shaped by repeated river overflow and can contain fertile sediments, which historically encouraged settlement and agriculture despite the associated hazard. Levees, embankments, reservoirs and flood barriers can reduce flood risk but can also change river behaviour and create residual risks if they are overtopped or fail. Urbanisation within floodplains increases potential damage because more buildings, roads, utilities and people are exposed. River flooding can therefore be understood as the interaction between hydrological processes and the physical and social characteristics of the floodplain.
Flash floods are especially dangerous because they can develop rapidly, sometimes within minutes to hours, leaving limited time for evacuation. They commonly occur when intense rainfall falls over steep or poorly drained terrain, but can also result from sudden releases of water caused by dam failure, glacial lake outburst floods or the collapse of temporary natural barriers. Steep slopes accelerate overland flow and can cause water levels to rise quickly in narrow valleys.
Urban areas can also experience rapid flooding because impermeable surfaces generate fast runoff and drainage networks concentrate water into channels. Flash floods can transport large quantities of sediment, rocks, debris and vehicles, making the flow mechanically destructive as well as dangerous because of its depth. The speed and unpredictability of such events distinguish them from many slower river floods. Warning systems increasingly combine rainfall radar, stream gauges, hydrological models and automated alerts to identify rapidly developing conditions.
Coastal flooding has different physical causes from rainfall driven river flooding. Storm surge is a major mechanism and occurs when strong winds associated with a storm push seawater towards the coast while atmospheric pressure changes can raise sea level locally through the inverse barometer effect. Waves can then travel on top of the elevated water level and increase inundation.
Tides are also important because a storm surge occurring during a high tide can produce substantially greater coastal flooding than the same surge at low tide. Coastal flooding can also be influenced by long term sea level rise, which raises the baseline from which storm driven water levels occur. Subsidence, erosion and changes in coastal morphology can further alter exposure. Tropical cyclones are particularly capable of producing severe coastal flooding because they can combine storm surge, large waves, heavy rainfall and river discharge. Coastal flood risk therefore depends on both marine and terrestrial processes.
Human activities strongly influence flood impacts. Urbanisation replaces permeable soil and vegetation with impermeable surfaces, increasing rapid runoff. Roads, buildings and drainage networks can alter natural flow pathways and concentrate water. Loss of wetlands can remove natural storage areas that temporarily retain floodwater. River straightening, embankment construction and channel modification can change flow velocities and floodplain connectivity. Deforestation can alter infiltration and runoff, although its effect varies with soil, vegetation and terrain.
Building in floodplains places more people and assets in areas that naturally experience periodic inundation. Climate change adds another layer because changing precipitation extremes, snowmelt, sea levels and storm characteristics can alter hazard patterns. Flood risk is therefore not determined solely by the amount of water. It is a product of hazard, exposure and vulnerability, meaning that the same physical flood can produce very different consequences in different locations.
Flooding can cause drowning, injury, structural collapse, contamination of drinking water, disruption of transport, crop destruction, electrical hazards and long term economic losses. Fast moving water can exert substantial forces on buildings and vehicles, and even relatively shallow moving water can destabilise people or carry debris. Floodwater can contain sewage, chemicals, pathogens and other contaminants, creating health hazards after the water recedes. Saturated ground can destabilise slopes and contribute to landslides.
Flooding can also damage foundations, electrical systems, roads, bridges and telecommunications infrastructure. Nevertheless, floods are not inherently abnormal environmental events. Rivers depend on periodic high flows to transport sediment and nutrients, replenish floodplain soils and sustain some wetland ecosystems. The principal problem arises when human populations and infrastructure are exposed to water levels and flow conditions that exceed the capacity of natural or engineered systems.
Flood management combines structural and non structural approaches. Structural measures include dams, reservoirs, levees, flood walls, drainage systems, retention basins and coastal barriers. Non structural measures include land use planning, floodplain zoning, wetland restoration, property level protection, insurance, forecasting, warning systems and evacuation planning. Natural flood management can involve restoring wetlands, increasing soil infiltration, reconnecting rivers with suitable floodplains and increasing vegetation where appropriate.
No defence provides unlimited protection, so effective risk management considers residual risk and possible failure or overtopping. Flood forecasting uses rainfall observations, river gauges, radar, satellite data and hydrological and hydraulic models to estimate where water is likely to accumulate. A common misconception is that flood risk can be eliminated entirely through engineering. In reality, extreme events can exceed design assumptions, and development patterns, climate conditions and catchment behaviour can change over time.
