The meaning of a 1 in 100 years flood storm event
The phrase “100-year storm” refers to the estimated probability of a storm event happening in any given year. A 100-year event has a 1 percent chance (or 1-in-a-100 chance) of occurring in any given year. The term “100-year flood” allows us to place a particular weather event in context with other similar events.
A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year.
The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%. For river systems, the 100-year flood is generally expressed as a flowrate. Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain. Estimates of the 100-year flood flowrate and other streamflow statistics for any stream in the United States are available. In the UK The Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood. Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves. Maps of the riverine or coastal 100-year floodplain may figure importantly in building permission, environmental regulations, and flood insurance.
By; Steve Benson Jan 2019