According to Study 100-Year Rain Events Have Been Underestimated – How Often do They Actually Happen?

According to Study 100-Year Rain Events Have Been Underestimated – How Often do They Actually Happen?

Atlas 14

Developed by the National Oceanic and Atmospheric Administration (NOAA) to serve as the United States Government’s source for precipitation frequency estimates, Atlas 14 quantifies the expected frequency of extreme precipitation events. The estimates quantify how much precipitation at a location for a given duration qualifies as a certain year event, such as a 100-year rainfall.

The Information and Data

All known sources of historical information were used for this study, including newspaper records from more than 100 years ago. The records are the most accurate that they have ever been by using decades of precipitation data, more rain gauge networks, and improved methods in statistical hydrology. NOAA developed a Comparison Map, which shows the Atlas 14 estimates alongside previous estimates. The Atlas 14 maps are now accessible to the public.

Source: TP 40 National Weather Service National Oceanic and Atmospheric Administration Maps (1961)

The Estimates

Precipitation frequency information is essential when it comes to informing infrastructure design, hydrometeorological analysis and forecasting, and flood risk. Engineers and planners utilized the precipitation frequency estimates to understand expected extreme rain events and bring knowledge of areas susceptible to flooding into land use and development decisions, including managing and designing stormwater infrastructure.

The estimates are also used as input to hydrological models that delineate flood zones and identify flood risks for FEMA’s National Flood Insurance Program. The updated values replace previous federal studies completed till this point.

Source: NOAA Atlas 14, Volume 11, Version 4

The Results

In comparison to previous studies, the Atlas 14 study shows significant increases in rainfall frequency values throughout Texas. Larger cities, like Houston and Austin, show values increasing from 13 inches to 18 inches and 10 inches to 13 inches respectively, in a 24-hour period. What was previously considered a 100-year event in the Houston area is now classified as a 25-year event.

For more information about Atlas 14, read our previous blog post. Please subscribe if you would like to receive our monthly post summary. Follow us on LinkedIn or Contact Us if you would like to speak to an engineer about how Atlas 14 may affect you or your project.