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Thoughts From Engineers: Who’s at Risk for Extreme Storms?

Chris Maeder on August 3, 2023 - in Articles, Column

The use of data and computer models to simulate complex physical processes, make predictions and more-effectively plan for the future has never been more important. Precipitation data, for example, have been key to the design and construction of our nation’s critical infrastructure for years. In the United States, the National Oceanic and Atmospheric Administration (NOAA) is the agency responsible for the analysis of these data, which are compiled in the publication known as Atlas 14, the authority for current design standards for all types of new development, ranging from highways to storm sewers to utilities and hospitals. Due to budget shortfalls and other issues, however, these estimates haven’t been updated in years.

Another important benchmark design standard has been the 100-year flood, which describes a flood that has a 1 percent chance of occurring in any particular year. Obviously, the question of which properties are vulnerable when extreme storms hit—with the potential to unleash 3 inches of rain or more an hour—is an important one with costly and potentially life-changing implications for individual property owners, communities, government and private industry. In many respects, the stakes relating to the 100-year flood—and its projected increase in frequency in coming years for many regions within the United States—have never been higher.

Setting the Scene

Late in 2022, Congress provided funding through passage of the PRECIP Act for NOAA to revisit and update its analysis of precipitation data with climate-change considerations. NOAA, long aware of the need for updates, has already started working on this momentous task. Atlas 15 will contain this revised precipitation data, anticipated to be released in 2027.

First Street Foundation is a nonprofit organization that leverages the knowledge of a scientifically diverse team of experts to conduct research and participate in climate discussions at the highest levels. It has provided testimony at Congressional hearings and produced cutting-edge research, but also made its findings regarding a specific property’s risk—from flood, fire, heat and wind—accessible to the general public (bit.ly/FloodRiskFactor) through specialized “Risk Factor” analyses.

The work of First Street has earned the respect of private and public agencies as well as organizations such as Realtor.com; the Departments of Interior, Treasury and Commerce; and NASA. Its research is about to get an even broader audience. The nonprofit released findings from its latest risk-assessment publication, “The 8th National Risk Assessment, The Precipitation Problem, June 26, 2023” (bit.ly/PrecipProblem), just weeks before this column went to press. The findings of First Street’s Precipitation Model (FSF-PM) are notable because they conflict with estimates set by NOAA’s Atlas 14. First Street believes Atlas 14 overestimates the flood risk in certain parts of the country, mainly in central and southwestern states. However, in many other parts of the country, First Street believes the flood risk has been significantly underestimated. For example, First Street claims that up to 51 percent of the U.S. population could be living in areas where the risk of experiencing a 100-year flood is actually two times more likely than indicated in Atlas 14.

Main Points of Difference

According to First Street’s Risk Assessment, the data on which Atlas 14 is based upon are incomplete because the estimates are driven by precipitation data from the years between 1973 and 2018 and don’t factor in data from subsequent years, which are clearly important in view of the intensity and frequency of recorded events in recent years. For example, in the last 20 years, 30 locations in the United States have recorded 100-year events, while 13 have experienced the equivalent of 500-year events.

Another point of difference: Analyses for different regions of the country were generated at different times, largely due to funding gaps, which First Street notes creates inconsistencies from one region to another.

Finally—and most importantly—unlike Atlas 14, the FSF-PM model integrates climate-change metrics, including giving the last 20 years of precipitation data more weight in the model in anticipation of a climate trajectory that includes atmospheric water levels increasing with rising air temperatures.

Rethinking the 100-Year Flood

If there’s one principal finding of First Street’s Risk Assessment that rocks age-old presumptions, it’s this: The standard 100-year flood is projected to occur much more frequently in specific parts of the country in future years than indicated in Atlas 14. According to the FSF-PM, roughly 13 percent of the U.S. population, which makes up about 43 million people, reside in areas that are five times more likely than in the past to experience a 100-year storm. These residents can expect storms capable of producing severe flooding roughly every 20 years. In other high-risk areas identified by First Street, which include parts of Indiana, Pennsylvania and Texas (as well as other areas), the 100-year storm may be experienced as often as once every 10 years.

The historic Infrastructure Investment and Jobs Act made available $1.2 trillion in 2022 to rebuild important infrastructure, well before NOAA’s revisions to precipitation estimates via Atlas 15 are released in 2027. The report points out that countless projects have already been funded and are at various stages of implementation. Many of these projects have been designed using standards that don’t reflect current or future conditions.

First Street points out that some of this infrastructure may already be obsolete in terms of climate resilience, having been built to standards that no longer are relevant. The report’s findings are disruptive, but also undeniably important, functioning as the figurative “bull in the china shop” as the United States grapples with the matter of responsible flood-risk assessment—while in the thick of one of the most-active periods in U.S. history in terms of infrastructure investment. At the very least, these findings give us an opportunity to revisit designs and make adjustments if needed—provided the budget permits. Ten years from now, critical infrastructure—stormwater systems for densely populated metropolitan areas and the like—need to be up to the task.

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About Chris Maeder

Chris Maeder, P.E., M.S., CFM, is engineering director at CivilGEO Inc.; email: [email protected].

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