Where Categorizing Hurricanes Falls Short

J. Ellis, Harris, M.E., Roman-Rivera, M.A., Ferguson, J.B., Tereszkiewicz, P. A., and McGill, S.P. (2020).  “Application of the Saffir-Simpson Hurricane Wind Scale to Assess Sand Dune Response to Tropical Storms.”  J. Mar. Sci. Eng., 8(9), 670. https://doi.org/10.3390/jmse8090670

Hurricane Katrina was one of the most devastating hurricanes in history, leading to over 1800 fatalities and tying Hurricane Harvey as the costliest hurricane on record.  But at the time Katrina made landfall, it was “only” ranked as Category 3 by the Saffir-Simpson scale, which goes up to Category 5.  So why did Katrina, at only Category 3, cause so much more destruction than most Category 5 storms?  And what does that tell us about what the Saffir-Simpson scale is missing?

What was it about Katrina that the Saffir-Simpson scale couldn’t capture?

Hurricane Katrina was devastating because it caused extreme flooding.  Failing to account for flooding is a major flaw of the Saffir-Simpson scale, which is used to give us a sense of how dangerous hurricanes can be.  We usually think of Category 1 hurricanes as being manageable, and Category 5 hurricanes as being extremely dangerous, but as Katrina demonstrates, an intuition based on the Saffir-Simpson scale alone can lead us astray.

Storm surge after Hurricane Katrina. (Image Credit: Lieut. Commander Mark Moran, NOAA Corps, NMAO/AOC. Retrieved from AccuWeather.)

The Saffir-Simpson scale is based on the maximum sustained wind speed in a hurricane, which is a useful and important metric but doesn’t tell the whole story, especially when it comes to flooding.  A number of other factors matter, too.  Just some of these include:

  • The translation speed of the hurricane. Slow-moving storms, like Harvey and Dorian, can dump a lot more rain in one place.
  • The terrain of the region in which the hurricane makes landfall. The rainiest tropical cyclone on record made landfall over mountains, which increased its total rainfall in a process known as orographic lift.  Meanwhile, overflowing lakes, rivers, and other bodies of water worsened the impacts of both Katrina and Harvey.
  • Storm surge. Storm surge refers to the dangerous sea level rise which accompanies landfalling hurricanes and results in flooding and loss of life. Hurricane Katrina was notable for the severity of its storm surge, which in turn was related to its death toll. As you can see from this video, storm surge looks a lot like a tsunami. Although a hurricane’s storm surge is strongly related to its maximum sustained winds, it also depends on other factors, including (importantly, for Katrina) the size of the hurricane and velocity of its approach.
  • Coastal morphology. Characteristics of the coastline on which the hurricane makes landfall, such as sand dunes and nearby bodies of water, can also affect how severe its storm surge is.

Because of the number of different factors at play, quantifying the flood risk from a hurricane is much more complicated than categorizing it based on its maximum wind speed.  That’s why it’s been such a challenge for researchers to develop a single index that truly captures the risk from flooding.

Hurricane rainfall and flooding in mountainous regions can also lead to deadly landslides, as seen here in Guatemala from Hurricane Eta (2020). (Image Credit: Foto Prensa Libre: Courtesy of Mayor Ovidio Choc Pop. Retrieved from AGU Blogs.)

In a 2020 study, Jean Ellis and her co-authors narrowed in on one of these factors, namely the interplay between storm surge, coastal morphology, and the effect of hurricanes on sand dunes.  Ellis and her co-authors found that the impacts of hurricanes on sand dunes depend on more than just their wind speed, which further emphasizes our need to look beyond the Saffir-Simpson scale when we think about hurricane risk.

Why are sand dunes important?

Sand dunes help shelter inland areas from storm surge, which mitigates some of the risk from flooding.  However, sand dunes are also subject to erosion, or wearing away, by very powerful winds and waves, which means that they both impact our risk from hurricanes and are affected by hurricanes themselves.  The more hurricanes damage sand dunes, the less sand dunes can protect us from flooding.

Sand dunes form a natural barrier between us and storm surge as it comes in from the ocean. (Image credit: Isle of Palms Facebook page.)

Ellis and her co-authors looked at how two different hurricanes affected the protective sand dunes on the Isle of Palms (IOP), South Carolina.  IOP experienced similar wind speeds from both Hurricane Irma in 2017 and Hurricane Florence in 2018, so the researchers decided to compare the damage that both hurricanes caused to IOP’s sand dunes.  They took topographic measurements to assess how much the height and volume of the dunes changed after each hurricane, indicating how much erosion each hurricane caused, and then compared the two.  They found that IOP’s dunes were much more severely eroded by Irma than by Florence, suggesting that it is incorrect to use wind speed alone when estimating how much a hurricane might damage a coastline’s protective sand dunes.

Why did Irma cause more damage than Florence?

The researchers found that Irma did more water damage to the dunes to Florence did.  Irma made landfall at high tide, it had a stronger storm surge to begin with, and it caused much more precipitation than Florence.

I think this study is interesting because, although it’s a deceptively simple case study between two hurricanes, it reveals how complicated and interconnected this problem really is.  In order to evaluate how much sand dunes can shield us, the researchers had to think about many different processes, including the tides, the rainfall, the shapes of the dunes, and each hurricane’s wind speed, storm surge, and direction from which it made landfall.  Maybe most importantly, this study shows us that the flood risk from a hurricane can’t be entirely understood just by looking at a hurricane in isolation, as the Saffir-Simpson scale does, but must also be studied with an eye towards specific characteristics (such as sand dunes) of the region in which the hurricane makes landfall. I think it’s a great reminder for all of us that, if we want to tackle problems like hurricane flooding, we can’t restrict ourselves to thinking about a single number, but instead must prioritize local considerations and work together across a whole range of different processes and backgrounds.

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity. Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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