Coastal Flood Risks Amplified by Extreme Sea Level Variability

  • 1 February 2021
Coastal Flood Risks Amplified by Extreme Sea Level Variability

Researchers from the University of Central Florida and the University of South Florida recently published a study that shows how incorporating decade-long variations in extreme sea level (ESL) events, such as storm surge, in coastal flooding risk assessments can dramatically increase the severity of expected flooding and the associated economic losses. Their work, funded by CPO’s Climate Observations and Monitoring (COM) program, is showcased in the journal Environmental Research Letters.

Extreme Sea Level & Extreme Floods 

100-year flood return events are extreme flooding events that have a 1 in 100, or 1%, chance of occurring in a given year. Using extreme sea level (ESL) indicators developed in their previous work, Dr. Mamunur Rashid, Dr. Thomas Wahl, and Dr. Don Chambers show how decadal variations in the characteristics of historical ESL events, such as storm surges, ‘amplify’ the chances of a 100-year-return coastal flood event such that the chances of the event occurring in a given year grow from 1% to as high as 78%. 

In simplified terms, the ESL indicators represent a combination of the variations in mean sea level rise year-to-year, the variations in storm surge climatology across decades, and the fluctuations of multi-year tidal cycles. The authors used changes in the ESL indicators over time to determine how much of an impact ESL variability has on controlling extreme flooding events (i.e., 100-year flood return events) and related coastal flooding risks. Sea level rise is the main driver for changes in coastal flood risk; however, these shorter-term variations in sea level described as ESL variability are often left out of coastal risk assessments. Omitting information related to ESL variability can lead to underestimating flooding risks, leaving people, coastal ecosystems, and infrastructure vulnerable to increased costs and damages. 

Amplification Factor

The researchers used the “amplification factor” method to quantify how the probability of the 100-year return flood event changes due to observed ESL variability. In other words, amplification factor lets them put a number to how much more or less likely the average occurrence of a 100-year return flood event becomes as a result of changes in ESL. For example, an amplification factor of 100 indicates that a 100-year event that had a 1% chance of happening now has essentially a 100% chance of happening in an average year.

Two different versions of ESL indicators were used, in order to allow for different assumptions about storm surge behavior. Both versions showed periods of time where ESL variability alone amplified 100-year return flood events. The magnitude of amplification varied across regions and seasons, but was, in general, larger for the US Pacific coast (up to almost a factor of 80). Amplification factors for the US Gulf and East coasts ranged between approximately 1 to 12. There were also time periods where variations in ESL resulted in an even slower time scale such that a 100-year event decreased from a 1% chance to a 0.2% chance. 

The highest amplification factor values for the Pacific coast appear in the 1980s, which coincides with the major 1982-83 El Niño event. “On one hand,” Dr. Wahl notes, “the erosion/flood impacts the West Coast experienced during that time are not surprising because of the high amplification factors. On the other, it’s also not surprising that we find high amplification during that time because there was a strong El Niño, which is known to lead to more storm surge and wave activity on the West Coast. It’s a little bit of the chicken and the egg problem.” 


Future Sea Level Rise

Drs. Rashid, Wahl, and Chambers then compared the historical ESL variability to future relative sea level rise across the US coastal regions. They showed how ESL events can essentially cause a region to temporarily ‘catch-up’ to future increased sea levels. Put another way, the range of ESL is equivalent to the sea level rise expected to occur for a region within the next few years up to several decades in the future. 

“Interestingly,” Dr. Wahl notes, “while the West Coast doesn’t really see higher ESL fluctuations in terms of actual magnitude compared to the other coastlines, it sees higher amplification factors because the general variability in extremes is smaller. In terms of future sea level rise, the projected changes are also relatively smaller, which means that it takes longer before sea level rise amounts are reached that match the ESL variations.” 

As part of this analysis, the coastal researchers separated out the components that make up ELS (mean sea level, storm surge climatology, and long period tides) in order to determine the importance of each to overall ESL variability. Mean sea level and storm surge climatology played more dominant roles, with variations in storm surge climatology equivalent to the amount of relative sea level rise not expected to occur until the 2050s-80s. 

Economic Costs 

Finally, the team used a socio-economic impact model to calculate the extra economic losses ESL variability is costing US coastal cities during certain periods. Using a model built to calculate the economic losses due to a 100-year flood event, they ran the model under two scenarios. The first was the original version of the model, which did not take ESL variability into account, while the second model did include it. The total loss increased by approximately 7% to 28% compared with the original estimated losses calculated without considering ESL variability. 

In their latest work, Drs. Rashid, Wahl, and Chambers have demonstrated that variations in ESL can amplify (or reduce) the conditions leading to coastal flooding and should be taken into account for flood risk assessments, policy discussions, and decision-making. Their team is already exploring if and how these high and low periods could be forecast. “We have already made progress in forecasting these periods using climate model output,” adds Dr. Wahl, “though it will require more research before we can have enough confidence in the forecasts to make them operational.” 

Read the study »

For more on Dr. Rashid and Dr. Wahl’s work, please visit the Coastal Risks & Engineering Lab webpage. For Dr. Chambers please visit the Satellite Oceanography Laboratory webpage

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