Flood

A flood occurs when water flows across normally dry land in large quantities. It may result from heavy rainfall, river overflow or rapid snowmelt, dam or levee failure, melting glaciers, or storm surge from the ocean. Flooding is a natural event that forms part of the water cycle, but it becomes dangerous when it affects areas where people and property are located.

Flood severity is measured by water level (height above normal), flow rate (cubic meters per second), land surface topography, and duration. Water authorities use records of measurements to estimate how often a flood of a given magnitude is likely to occur in any year.

Types

River flooding occurs when water entering a river exceeds the channel's capacity, usually days after heavy rainfall. Flash floods develop within hours, particularly in steep valleys and narrow gorges far from settlement; water flow is highly dangerous. Urban flooding results when concrete and pavement obstruct water infiltration into soil, while drainage systems prove inadequate. Coastal flooding is associated with storms and rising sea level.

Aggravating factors

Deforestation and overgrazing reduce the soil's water absorption capacity, increasing surface runoff. Construction in flood-prone areas raises the value of assets at risk. Global warming is linked to more intense rainfall events and rising sea levels, both of which increase the likelihood of certain floods.

Management and prevention

Measures in use include dams and weirs built across rivers, irrigation canals, water storage reservoirs, and restoration of natural wetlands for water absorption. Administratively, land-use regulations restrict building in high-risk zones. Early warning systems combine weather forecasting, rainfall radar, and river gauges, giving people hours or days to prepare.

Somalia

The Shabelle and Juba rivers sometimes overflow during the Gu and Deyr rainy seasons, especially when heavy rains fall on the Ethiopian highlands where these rivers originate. Flooding affects pastoral and riverine settlements, and can cause displacement. Intense rainfall also triggers flash floods in gullies and towns lacking drainage systems.