Three common types of flood explained

A flood is an overflow of a large amount of water beyond Pluvial floods (flash floods and surface water) its normal limits, especially over what is normally dry land.

EXTREME RAINFALL To plan for floods, you need to understand the type – or OVER URBAN AREA E ID types – of flood you may face. Why? There are several LS IL H N W different kinds of flood, and each one bears a different O S D LOW RIS F EB D impact in terms of how it occurs, how it is forecast, the ND WATER A damage it causes, and type of protection you need. Shown here are the three main types of flood. STRAIN ON URBAN DRAINAGE SYSTEM

Fluvial floods (river floods) Coastal flood (storm surge)

HIGH WINDS EXCESSIVE RAIN SNOWFALL

STORM SURGE

OVERFLOWING WATER WATER IS FORCED ASHORE NORMAL RIVER LEVEL Fluvial floods (river floods)

EXCESSIVE RAIN SNOWFALL

OVERFLOWING WATER

NORMAL RIVER LEVEL

A fluvial, or river flood, occurs when the water level catchment area of the river. Other factors include soil in a river, lake or stream rises and overflows onto the water saturation due to previous rainfall, and the terrain surrounding banks, shores and neighboring land. surrounding the river system. In flatter areas, floodwater The water level rise could be due to excessive rain tends to rise more slowly and be shallower, and it often or snowmelt. remains for days. In hilly or mountainous areas, floods can occur within minutes after a heavy rain, drain very The damage from a river flood can be widespread as the quickly, and cause damage due to debris flow. overflow affects smaller rivers downstream, which can cause dams and dikes to break and swamp nearby areas. To determine the probability of river flooding, models consider past precipitation, forecasted precipitation, The severity of a river flood is determined by the current river levels, and well as soil and terrain conditions. duration and intensity (volume) of rainfall in the Pluvial floods (flash floods and surface water)

EXTREME RAINFALL OVER URBAN AREA

E EXTREME RAINFALLID ON TOP OFL SHILL IL H N W O S D LOW RIS F EB D ND WATER A

STRAIN ON URBAN DRAINAGE SYSTEM

A pluvial flood occurs when an extreme rainfall event the level of water is usually shallow (rarely more than 1 creates a flood independent of an overflowing water meter deep). It creates no immediate threat to lives but body. A common misconception about flood is that you may cause significant economic damage. must be located near a body of water to be at risk. Yet • Flash floods are characterized by an intense, high pluvial flooding can happen in any location, urban or velocity torrent of water triggered by torrential rain rural; even in areas with no water bodies in the vicinity. falling within a short amount of time within the vicinity There are two common types of pluvial flooding: or on nearby elevated terrain. They can also occur via • Surface water floods occur when an urban drainage sudden release of water from an upstream levee or a system is overwhelmed and water flows out into dam. Flash floods are very dangerous and destructive streets and nearby structures. It occurs gradually, which not only because of the force of the water, but also the provides people time to move to safe locations, and hurtling debris that is often swept up in the flow. Coastal flood (storm surge)

HIGH WINDS

STORM SURGE

WATER IS FORCED ASHORE

Coastal flooding is the inundation of land areas along In this type of flood, water overwhelms low-lying land and the coast by seawater. Common causes of coastal often causes devastating loss of life and property. flooding are intense windstorm events occurring at the The severity of a coastal flood is determined by several same time as high tide (storm surge), and tsunamis. other factors, including the strength, size, speed, and Storm surge is created when high winds from a direction of the windstorm. The onshore and offshore windstorm force water onshore – this is the leading topography also plays an important role. To determine the cause of coastal flooding and often the greatest threat probability and magnitude of a storm surge, coastal flood associated with a windstorm. The effects increase models consider this information in addition to data from depending on the tide – windstorms that occur historical storms that have affected the area. during high tide result in devastating storm surge floods.