flashflood

A flash flood is a rapid rise in water level that occurs within minutes to a few hours, usually in response to intense rainfall, a dam or levee failure, or rapid snowmelt. It can strike quickly and with little to no warning, crossing terrain from small streams to urban streets and basins.

Causes and mechanisms include extreme, concentrated rainfall that exceeds the infiltration capacity of the ground, rapid runoff from steep or poorly drained terrain, and blockage of channels by debris. Urban areas are particularly vulnerable because impervious surfaces limit infiltration and funnel water into drainage systems, while shallow, narrow channels in canyons or valleys can constrict flow and boost flood velocity. Snowmelt combined with rain can also trigger quick floods in mountainous regions.

Impacts of flash floods are sudden and dangerous. They can sweep people and vehicles away, damage infrastructure and homes, erode banks, undermine roadways, and deposit debris and hazardous materials downstream. Damming or flooding of culverts may create localized hazards as water surges through drainage networks.

Forecasting and warnings rely on radar rainfall estimates, stream gauges, and hydrological models. Meteorological agencies issue watches and warnings when heavy rainfall and runoff are expected. Safety guidance emphasizes avoidance of driving through flooded roads, seeking higher ground, and heeding local authorities. Preparedness includes having an emergency plan, a weather radio, and awareness of vulnerable locations such as canyons, floodplains, and urban drainage systems.