The National Weather Service called the rain dropped in September 2018 during Hurricane Florence in the Carolinas and Virginia a “1000-year” event. Upward of 50 inches of rain fell in some areas of the Carolinas, equivalent to 8 trillion gallons of rain. The classification of 1000 years refers to probability, not history, because it has a 0.1% chance of occurring in any given year.
Add to that the fact that rainfall and flood data generally go back only 100 years or so, so scientists must extrapolate available data back in time to determine what 500-year and 1000-year events actually represent.
In fact, due to climate changes, such events are occurring with increasing frequency. This is taxing resources and emergency response. Some people affected by Hurricane Matthew (some of whom are in the same area as that affected by Florence) are still waiting for disaster relief money two years after that storm flooded parts of eastern North Carolina.
But in fact, North Carolina passed a law in 2012 that banned the use of, climate science to plan for the consequences of rising sea levels, arguing that the science wasn’t solid enough to justify laws that could change property values and that projections for rising sea levels could only be based on historical data rather than newer studies predicting problems for the future.
Federal flood maps have not kept up with the current actual risk of flooding, many officials say. An area of West Houston called Memorial City, for example, was outside Houston’s 500-year floodplain but flooded three times in the past decade: in 2009, 2015, and 2016. Another example is Cedar Rapids, Iowa, which got hit with a 500-year flood in 2008, followed by a 100-year flood in
“The fact is, the math behind these numbers also tells us a 500-year event at a given place has about a 10 percent chance of happening over a 50-year period,” wrote Brian Bledsoe, a professor of civil and environmental engineering at the University of Georgia in a commentary for the Capital Weather Gang.
— Vox.com, CBS News, Washington Post, and CNN