What are the types of floods?

Floods occur on areas of low-lying land in the proximity to watercourses or due to rising groundwater levels. The types of floods can be classified as fluvial, pluvial, flash or coastal. Due to their flood risk characteristics, each type of floods has particular consequences.

• Fluvial floods occur when the river flow exceeds the capacity of the river channel. This type of flood tends to occur over long periods of time.

• Pluvial flooding occurs in urban environments when heavy downpours overwhelm drainage systems.

• Flash floods are caused in areas with steep slopes where short duration high-intensity rainfalls or water flows occur, e.g. thunderstorms or dam failures. Flash floods carry heavy sediments due to the high velocity and can also increase the risk of landslides.

• Coastal floods are the consequence of sea surges or the combination of high tides and high river levels leading to backwater effects. Coastal type of floods are usually severe and in case of tsunamis leave little time for evacuations.

The level of damage associated with particular flood events varies significantly according to the types of floods, the land use, depth of floodwater, duration of the flood, velocity of water and sediment load.

For example, the duration of these types of floods can impact the period of disruption, high-velocity flood tends to carry finer sediment which is more difficult to clean and increase the costs of clean up operations. In the UK the level of damage can be estimated from depth/damage graphs depending on the types of floods and type of properties.

Floods are not totally detrimental, many types of floods maintain fragile ecosystems and highly productive soils by bringing sediments.