WHAT YOU NEED TO KNOW TO UNDERSTAND FLOODING

People often misunderstand when rainfall is described as “100-year” or “500-year” storms.

For the general public, one underlying question to ask is pretty straightforward: How many inches of rain are we preparing for?

For example, the recently completed Project Bray sin Meyerland was built to withstand a 100-year storm, or 13.2 inches of rain.

That, however, is the previous 100-year storm standard.

The National Oceanic and Atmospheric Administration, which defines the standard, recently increased the rainfall amount based on the storms Houston has experienced the last couple decades.

For example, Tropical Storm Allison in 2001 dropped 25 inches of rain in some parts of town. The 2015 Memorial Day flood resulted from 11 inches of rain in 3 hours. The next year’s Tax Day flood averaged 12 to 16 inches of rain across the city in 12 hours. Tropical Storm Imelda in 2019 exceeded 30 inches over three days. And Hurricane Harvey in 2017 dumped 47 inches over parts of Harris County over four days.

So now, in Harris County, NOAA says local governments should plan for 17 inches of rain to protect against a 100-year storm. A 500-year storm is now 25 inches of rain in 24 hours, instead of 18.9 inches.

Not all local governments have agreed to the changes, but many have, including Harris County, Fort Bend County and the city of Houston.

The new formula does not include projections for how climate change will increase future storms.