flood

Flood is an overflow of water onto land that is normally dry. It can result from rivers overflowing their banks due to heavy rainfall, snowmelt, dam breaks, or storm surges. Floods are among the most common and costly natural disasters globally, affecting people, infrastructure, agriculture, and ecosystems. They are classified by cause (river floods, flash floods, coastal floods, urban floods) and by duration (slow-developing vs flash). River floods develop over days, while flash floods are rapid, often in steep terrain or small basins. Coastal floods are driven by storm surges and high tides; urban floods result when drainage systems are overwhelmed. They can be measured by water level relative to a reference point, river discharge, or flood stage.

Hazards include drowning, contamination of water, landslides, and infrastructure failure. Impacts vary with flood depth, duration, speed, and land use. Risk management includes forecasting and early warning, land-use planning to avoid development in floodplains, and structural measures such as levees, dams, floodwalls, culverts, and retention basins. Non-structural approaches emphasize insurance, community preparedness, and ecosystem-based adaptation like restored wetlands and floodplains to slow and store floodwaters. History includes major events such as floods in the 14th-century Netherlands, the 1931 China floods, the 1998 Mozambique floods, and the 2010 Pakistan floods, illustrating wide-reaching effects. Ongoing climate variability and urbanization are expected to influence flood frequency and severity.