What is a flash flood?
As their name suggests, flash floods happen very suddenly and with little warning. They come within six hours of a heavy rainfall. These downpours are from thunderstorms, hurricanes and tropical storms. Heavy rainfall causes rivers and streams to swell rapidly and overflow their banks.Flash floods can occur along rivers, on coastlines, in urban areas and dry creek beds. River floods generally happen when river basins fill too quickly and water pours over the banks. Coastal flooding occurs when tropical storms or hurricanes force ocean water inland. Tsunamis can also drive water surging onto shore.
The pavement that covers urban areas prevents the natural soil from absorbing rainfall. Urbanization increases runoff by two to six times over what would naturally occur. Streets lined with tall buildings can quickly be transformed into fast-moving rivers.
What causes flash floods?
Intense and ongoing rainfall can cause flash floods. These floods are often associated with slow-moving thunderstorms. These thunderstorms spend long periods over an area causing fast water accumulation. The National Oceanic and Atmospheric Administration (NOAA) also blames tropical storms and hurricanes for flash flooding.
Flash floods often begin with the ground becoming heavily moisture saturated. It can no longer absorb the rainwater that has fallen in a short period of time. Since water will run downhill, it collects in low-lying areas. Buildings in the low-lying area will likely be flooded.
Rainfall is not the only cause. The sudden breaking of an ice jam or a collapsing dam can unleash terrible damage. Complete towns have been washed away by failing levees or dams.
Floods are one of nature’s most destructive forces. The damage caused by water’s “wetness” is secondary to the crushing pressures of its currents. A torrent can topple buildings. A flash flood can move quickly beyond the original site of the storm. Unwary motorists and pedestrians can be hit without warning.
‘Super Soaker’ flash floods
A flash flood that completely envelops an area is referred to as a super soaker flash flood. This type of flood is likely to cover entire structures. Depending on the water current, the flooded area may begin to subside after the rain stops. If there is a strong downstream current, the soaker flood may uproot trees. Even buildings can be swept away from their original location.
Casualties due to flash floods
In the US flash floods cause more deaths than any other weather condition, killing an average of 150 Americans annually. This is a higher incidence than hurricanes or tornadoes.
Nearly half of flood-related deaths occur in automobiles. A mere 18 inches (46cm) of water is enough to float a car and an two feet (60cm) will wash a car away.
Deaths due to flash floods have actually increased in the past few decades because the populations of flood-prone areas have continued to grow.