Flood is an overflow of water that causes inundation of land or structures. It is a natural phenomenon that can occur anywhere water flows, whether in river and stream channels, ponds and lakes or on the surface of soil. Flooding is caused by rain events, dam failures, snow melt and human activity such as draining wetlands and building impervious surfaces that do not absorb rainfall and runoff.
Floods can carry debris, sediment and toxins that pollute waterways and impact on water quality, habitats and flora and fauna. They also supply nutrients such as nitrogen, phosphorus and organic material to the surrounding land. This is why ancient civilizations grew up along seasonal floodplains of rivers such as the Nile in Egypt.
As floodwaters spread they can threaten lives, damage property and disrupt access to vital infrastructure such as roads, railways and tramlines. Flooding can also be expensive and time consuming to repair for communities. Floods are especially dangerous for people caught on foot or in vehicles, and fast-moving water can sweep them away without warning. People can sustain serious or fatal injuries.
The frequency and intensity of flooding is increasing globally due to climate change. Warmer and drier conditions make soil less able to soak up rainfall, so more of it runs off and accumulates in river and lake basins. Increased rainfall also increases the intensity and duration of storms, adding to the load on flood defences. This is a particularly important issue in low-lying areas such as Bangladesh, where climate change is expected to increase the number of floods.