small islands – Coastally Curious

Cracking the Code of Resonant Wave Signals

Coral reefs act as essential flood protection for low-lying tropical coasts, something that is making the news frequently these days. However, as I have explained before on this website, Weird Waves Cause Big Trouble on Small Islands in the Middle of the Big Blue Wet Thing. Essentially, some coral reefs have a tendency to excite normally idyllic swell waves into dangerous resonant low-frequency waves that can act like mini-tsunamis and flood vulnerable low-lying tropical islands. When pushing a child on a swing, you can send them higher and higher with relatively little effort by timing your pushes carefully. In the same manner, waves striking a coral reef can be naturally amplified higher and higher if they are timed at just the right frequency. This can happen even on a sunny day – big storms not necessary! Suffice it to say, this is bad news for islands that are already barely above sea level.

Over the past decade or so, research on this topic by my colleagues and I has focused mostly on how the shape of the coral reef, specific wave conditions, or the combination of both can lead to resonant conditions. But up until now, we have largely stuck to the simplifying assumption that once resonant conditions are met, they stay that way for a while. But is this actually the case? How long do resonant conditions last on coral reefs, when do they occur, and what are the consequences for flooding?

To get to the bottom of this, Bernice van der Kooij came to the rescue! Last week she successfully defended her master’s thesis, Exploring Transient Resonant Behaviour over a Fringing Coral Reef. In Bernice’s thesis (which is simply a joy to say out loud), she dove deep into the mechanics of a complex mathematical technique, the Hilbert-Huang Transform. Bernice did some extremely difficult work that certainly kept her thesis committee on its toes. Armed with this approach, she managed to find that while these intense low-frequency wave conditions typically lasted about 5 minutes, they tended to last for hours during major flooding events.

Bernice’s study used wave measurements from the island of Roi-Namur in the Marshall Islands. Roi Namur is an extremely vulnerable island – several months ago, horrific low-frequency waves like the ones that Bernice studied struck the island. As you can see in this video of the event (which I found quite upsetting and is NOT for the faint of heart), the waves do not resemble the waves you normally see on the beach with crests spaced a few seconds apart. Instead, they smash through the building like tsunamis and then just keep coming for minutes at a time before the crest subsides (this is what “low-frequency” means). Fortunately but remarkably, there were only minor injuries from that incident, but if these events occur more frequently as climate change escalates, not everyone will be so lucky in the future. My colleagues at the US Geological Survey and Deltares argue that most atolls like Roi-Namur will be uninhabitable by the mid-21st century because of sea-level rise exacerbating this sort of wave-driven flooding.

These floods underscore the urgency of the problem Bernice worked on, and we are very proud of her and her research. We wish her all the best in the next steps of her career!

Going Where No Cyclone Has Gone Before

Tropical cyclones or hurricanes threaten the lives of millions and cause billions of dollars in damage every year. To estimate flood risks at a particular location, scientists and engineers typically start by looking at the historical record of all previous storms there. From these records, they can statistically predict how likely a storm of a given size is (e.g., the biggest storm likely to occur there in 100 years).

There are two problems with this approach: (1) What if there isn’t much historical data in the records? This is often the case for Small Island Developing States (SIDS) and in the Global South. If you don’t have enough data points (particularly for rarer, more extreme events), your statistical estimates will be much more uncertain. (2) What if the historical record isn’t representative of the conditions we are likely to see in the present and future? This is also a big problem in light of climate change, which is expected to bring sea level rise and changes in storminess to coasts around the world.

To address these challenges, our team led by Tije Bakker came up with a new approach to estimating tropical cyclone-induced hazards like wind, waves, and storm surge in areas with limited historical data. Our findings are now published open-access in Coastal Engineering here!

Rolling the Dice: Dealing with Uncertainty in Coastal Flood Predictions for Small Island Developing States

Small island developing states around the world are especially vulnerable to the hazards posed by sea level rise and climate change. As engineers, we have a number of tools in our toolbox for reducing the risk posed by coastal flooding and for planning adaptation measures. We often rely on predictive models which combine information about expected wave and sea level conditions, the topography of the coast, and vulnerable buildings and population to estimate potential flooding and expected damage.

However, to use these types of models, we first need to answer a lot of questions: what exactly are the expected wave and sea level conditions? What if detailed topographic measurements are unavailable? What if the population of a given coastal area increases? How are the local buildings constructed, and what are the consequences of that for estimating damage from flooding?

If our information is imperfect (which it almost always is), all is not lost: we can still make educated guesses or test the sensitivity of our models to a range of values. However, these uncertainties can multiply out of control rather quickly, so we need to be able to quantify them. There is no sense in spending the time to develop a detailed hydrodynamic model if your bathymetry data is crap. Can we get a better handle on which variables are the most important to quantify properly? Can we prioritize which data is the most important to collect? This would help us make better predictions, and to make better use of scarce resources (data collection is expensive, especially on remote islands!).

Matteo Parodi investigated these questions in his master’s thesis, and just published his first paper, “Uncertainties in coastal flood risk assessments in small island developing states“. I had the great privilege and joy of co-supervising Matteo during his thesis, and I am immensely proud of him and his work!

Based on a study of the islands of São Tomé and Príncipe, off the coast of Africa, Matteo found that topographic measurements and the relationship between flood depth and damage to buildings were the biggest uncertainties for predicting present-day flood damage. This means that measuring topography of vulnerable coastal areas in high resolution, and performing better post-disaster damage surveys will provide the best “bang for your buck” right now. However, for longer time horizons (i.e. the year 2100), uncertainty in sea level rise estimates become most important.

Matteo’s work will help coastal managers on vulnerable islands to better prioritize limited financial resources, and will improve the trustworthiness of our predictive models. Great job, Matteo!