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  • The meaning of a 1 in 100 years flood storm event

    By Steve Benson
    In Climate Change
    Jan 20th, 2019

    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%.[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.[2] In the UK The Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood.[3] Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.[4] 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

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