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Floods can occur slowly in large catchment areas, rainfall can build up over hours, days or weeks. The runoff from this rainfall may create significant floods that inundate large areas of land for days, weeks or months.

What are the consequences of floods?
Floods impact on individuals and communities, and have social, economic, and environmental consequences.

Floods can occur suddenly Sudden, heavy, and intense rainfall can cause floods to quickly rise in the minutes or hours that follow. These are known as flash floods and are typically associated with relatively small catchment areas.

Flood consequences can be immediate or long-term and affect: • individuals • communities • businesses • economies • infrastructure • vital services • tourism • agriculture • built and natural environments.

Floods are a natural process that can be caused by several factors and affected by human activities. Floods occur at irregular intervals and no two floods are the same

How floods vary in severity? 

The flood volume (the total amount of water in the flood): this contributes to both the level and duration of flooding. Dams and detention basins are less effective at flood mitigation during large volume floods. • The rate of rise (how fast the flood rises): a flood that rises quickly provides less time for warning and evacuation. • The flow velocity (how fast the water is flowing): faster flow causes a higher risk to human life, a higher risk of erosion, and more damage to infrastructure. • The flood duration: a flood that lasts for a longer time can have a greater impact due to the increased duration of the disruption to transport, business and personal networks. • The areal extent of flooding: flooding that affects a larger area often has a greater impact.

Flood consequences, both negative and positive, vary greatly depending on the location and extent of flooding, and the vulnerability and value of the natural and constructed environments they affect.

We cannot stop floods; there will always be a residual risk of flooding in the future. However, we can manage this risk through a combination of measures including: applying and communicating lessons learned from past floods; improving land use planning and floodplain management; using improved knowledge and technology to enhance flood forecasting and warnings; and adopting smarter urban design and integrated water management.