What exactly does it mean to live in a 100-year flood plain? Many people misunderstand this to mean the chance of flooding is only once every 100 years, though that’s not the case. This term is actually referring to the 1% chance of a 100-year storm event occurring in any given year. That’s why 100-year floods can happen multiple years in a row, or multiple times in one year. 

But what constitutes a 100-year storm event, and do they always cause a 100-year flood? According to the United States Geological Survey (USGS), 100-year storms are defined by a relationship between rainfall and streamflow, and 100-year storms don’t always cause a 100-year flood. 

For example, the City of Austin says the standard 100-year storm for the city has 24 hours of rainfall and produces a total rainfall of more than 10 inches. 

Importantly, the 100-year flood level can change, says USGS, because it is statistically computed using past and existing data, and scientists re-evaluate the frequency of flooding, particularly after significant storm events. Atlas 14 is exactly that. 

Atlas 14 is the nationwide study of rainfall intensities, led by the National Oceanic and Atmospheric Administration (NOAA) with additional input from other federal, state, and local entities. Historical rainfall data was collected from 3,900 water gauges and is beneficial in understanding rainfall events and flood risk. Atlas 14 assigns the statistical probable occurrence of 100-year storms and other rainfall frequency events. 

Texas REALTOR® magazine explains more about Atlas 14, and how this recent rainfall study will impact further property development in Texas.