Tag: hydrodynamics

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!

Can 3D-Printed Corals Help Us Prevent Flooding?

Coral reefs around the world are dying; that much is clear from the headlines we see in the news that grow increasingly distressed with each passing year. This is an ecological catastrophe, but are we also losing another key benefit of reefs? Coral reefs provide a form of natural protection against wave-driven flooding on tropical coastlines. This is partly because the physical form of the reef (often a big rocky shelf) serves as a sort of natural breakwater, but is also due to the frictional effects of the corals themselves.

Many species of coral have complex shapes that disrupt the flow of water across reefs, generating turbulence and dissipating energy. This has the effect of reducing the height of waves as they travel across the reef towards the shore. However, these effects are incredibly complex and poorly understood, so we usually just simplify them in our predictive models by considering a reef to be more “hydraulically rough” than a sandy beach, for example. But we need to do better: these models are used to forecast flooding and estimate the impact of future climate change on vulnerable coasts.

How can we improve this? In coastal engineering, we often conduct experiments in the laboratory to test our theories and understand the chaos of natural systems in more controlled settings. What if we could make a scale model of a coral reef and measure exactly how waves are dissipated?

I am extremely proud to announce the graduation of Paul van Wiechen, one of the Master’s students whom I have had the pleasure of supervising. Yesterday, he defended his thesis, “Wave dissipation on a complex coral reef: An experimental study“, where he built a tiny coral reef in the TU Delft wave flume (a 30-m long bathtub with a wave-making paddle at one end) using hundreds of 3D-printed coral models.

Paul’s thesis, “Wave dissipation on a complex coral reef: An experimental study“.

It was one of the coolest projects I have ever seen, and his research provides us with valuable measurements that give us a deeper understanding of the vital role that corals play in protecting our coasts.

He also did all of this in the middle of a global pandemic, and somehow managed to stay completely on schedule. We are very lucky, because Paul will be joining the Coastal Engineering department here at TU Delft to start a PhD on dune erosion this fall. We are all glad to have him on the team and eager to see what his research unveils next!