A flood is an overflow of water onto normally dry land. It can come from heavy rain, melting snow, storm surges, or dam failures. Floods are the most common natural disaster. They kill thousands and displace millions every year.

Types vary. River floods build slowly over days. Flash floods strike in minutes, often in canyons or urban areas. Coastal floods come with storm surges and rising seas. Urban floods happen when pavement blocks infiltration and drainage systems overflow. Each type demands a different response.

Flood damage has risen, not because floods are more frequent everywhere, but because more people and property sit in harm's way. Climate change intensifies heavy rainfall in many regions. Sea-level rise makes coastal flooding more common. Solutions include levees, floodplain restoration, permeable pavement, and better warning systems. Retreat from high-risk areas is the hardest option and increasingly necessary.

Common types

• River floods
• Flash floods
• Coastal and storm surge floods
• Urban floods
• Ice jam floods