The term “100-year flood” means a flood event might occur only once every 100 years, “500-year flood” once every 500 years, and a “1000-year flood” once every 1000 years.
FALSE. These terms, from the 1960s, are rather outdated and misleading misnomers. In the first place, we do not have 100 years of data much less 1,000 years’ worth of data. Consider:
From August 2015 to August 2016, there were eight “500 year” floods in the US.
From 2010 to 2014, there were six “1,000 year” floods in the US.
Houston had three “1,000 year” floods in back to back years (2015, 2016, and 2017)!
According to USGS (US Geological Survey), over the course of a 30-year mortgage, a house in a floodplain has a 26% chance of being inundated at least once.
Recent studies indicate over 60 million Americans may be vulnerable to flooding by 2050. This trend of more flooding is due to climate changes, over development of land, and more numerous and more intense storms.