When you think of coastal climate change impacts, what do you think of? Probably sea level rise, changes in wave climate or storminess, or loss of coastal habitat. But a silent intruder lurks: salty seawater, sneaking into estuaries and rendering our precious freshwater supplies undrinkable. The threat of estuarine salinity on deltas and coastal regions is one that I greatly underestimated, even as someone working in this field for over a decade. That is, until Gijs Hendrickx came along.
Large populations near the sea are vulnerable to coastal floods, making coastal safety and sustainability an urgent societal priority. This is especially true in the Netherlands, where over a quarter of the country lies below sea level, and the main protection from deadly coastal floods is a barrier of wide, high sandy beaches and dunes. However, this sandy buffer is constantly moving and chronically eroding. To plan effective future coastal adaptations, we need to know where that sand is coming from, going to, and which paths it takes to get there. I am delighted to share that I have just received a Veni grant from the Dutch Research Council (NWO) to investigate this!
Where is the sand on beaches going, and how does it get there?
My overarching goal is to enable effective sediment-based climate adaptation strategies for vulnerable coasts. To approach this, I consider coasts as an interconnected network of sediment pathways, like a subway map showing how stations are linked. This connectivity reveals the hidden structure underlying chaotic sediment pathways through coastal systems. These pathways are immensely difficult to identify on real coasts due to the challenge of tracking individual sand grains from multiple sources in such a dynamic environment.
Proof-of-concept connectivity analysis of a beach and harbour. (a) Map of tracer particles in example SedTRAILS model from 7 different source patches at a snapshot in time. (b) The number of particles (np) from a given source in each receptor is counted to yield a connectivity matrix, graphically represented by a connectivity network diagram (c).
To deal with this challenge, this grant will enable me to develop both a scale model in a physical laboratory and a numerical model in a digital laboratory. In a wave tank the size of an Olympic swimming pool, I will construct a beach from multi-coloured sand. As waves disperse the sand, the resulting rainbow of sediment will reveal their pathways, which I will then quantify as a network in the digital laboratory. The resulting open datasets and numerical models will serve as a benchmark for the coastal research community, generating new theories and improved tools. My collaborators in the Netherlands, US, and New Zealand will help me to implement these findings in research, engineering practice, and coastal management policy. In this way I hope to enable more effective management of sediment for coastal adaptation and a more holistic understanding of our coastal systems.
Stay tuned for more updates once the project begins!
Coral reefs act as essential flood protection for low-lying tropical coasts, something that is making the newsfrequentlythese 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.
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!
Mangrove forests provide valuable coastal habitats but also provide a natural form of coastal flood protection and a host of other services. However, many of these mangrove forests are threatened by coastal development and groundwater pumping-induced subsidence, among other natural and human changes. Part of the challenge is that mangroves are extremely choosy about their habitat, and need just the right combination of tidal submergence and mud to take root. If these habitats are thrown out of balance by people or natural causes, it becomes hard for new mangrove seedlings to grow there and sustain the forest.
To make happier places for the mangroves to develop, different kinds of coastal fences/dams have been proposed. The general principle is that waves and currents are attenuated or blocked by the fences, which makes a nice quiet area behind them for mud to accumulate and mangrove propagules to take root. What impact do these structures have on the coastal “conveyor belt” transporting mud and propagules? Enter Nirubha Raghavi Thillaigovindarasu!
Just before Christmas, Raghavi successfully defended her thesis, “Mangrove-Sediment Connectivity in the Presence of Structures Used to Aid Restoration“. Beginning with a numerical model of a site in Indonesia to simulate the motion of rivers and tides, she then applied the SedTRAILS model to visualize and interpret the pathways of sediment and mangrove propagules as they journeyed along the coast. By adding structures to her model, she was able to demonstrate how this trapping behaviour has an influence in the vicinity of a structure but also up to a kilometer away.
Have you ever walked along a windy beach and noticed shells sitting atop small peaks of sand, like a miniature mountain range? Does a shell “protect” the sand underneath, or does the sand pile up behind it? Does the shell actually cause more erosion around it? How does a shell affect the way sand moves along beaches? What happens when you have millions of shells along a beach? Does that affect the way the beach as a whole erodes? Now what about the parts of a beach that we can’t see, below the water?
Shells emerging from the beach on little pedestals of sand
These might seem like the thoughts of an idle beachgoer, but are actually essential to helping us understand how to sustainably protect our coasts. As part of the TRAILS project, Tjitske Kooistra is investigating how sand nourishments influence sensitive ecosystems on the Dutch coast. In order to do that, we need to understand how shells and sand interact at the bottom of the sea, since there are many locations along the Dutch coast where shells make up a significant portion of the beach material. This is really difficult to understand in the field, so to figure this out in a more controlled setting, Tjitske planned a series of lab experiments and we recruited Steven Haarbosch to carry them out.
Do you ever think about the swirling patterns in your cappuccino as you stir your spoon around, the brown coffee folding in past the white foam? And do you ever think about sediment transport as you do it? Just me? Ok, never mind…
I had the great privilege of hanging out in New Orleans this past week, being a sand nerd with four hundred of my fellow sand nerds at the Coastal Sediments conference. In between jazz sets at the Spotted Cat, we shared our latest ideas about coastal dynamics, built new collaborations, and rekindled old pre-pandemic friendships. My contribution this year was an attempt to bring the science behind cappuccino coffee swirls to coastal sediment transport.
Mangrove forests protect tropical coastlines around the world from the effects of waves, in addition to providing valuable habitat for countless species. As such, their preservation and restoration is a key element of many plans for improving coastal resilience against flooding and erosion in the face of climate change. However, you can’t *just plant* a mangrove forest anywhere – mangroves are extremely picky, dancing on the edge of the intertidal zone where they get just wet enough but never too wet for too long. They also need safe, stable shorelines for their seedlings to take root and grow stronger, without too many waves and with just the right sort of muddy conditions to make a comfortable home.
Mangroves drop their seeds (called propagules) in the water, which then float around with the currents for days to weeks until they find a suitable home. But which pathways do these mangrove seedlings take as they float along the coast? Are those the same pathways that sand and mud take? These are questions that we need to answer in order to make better decisions about mangrove restoration. To get to the bottom of this, we recruited Femke Bisschop.
Keeping Dutch feet dry is mainly done by placing piles of sand along the coast as “nourishments”. These nourishments build out the beaches and dunes to act as a protective buffer against storms. However, as was recently pointed out by an official at Rijkswaterstaat, the Dutch water ministry, the Hamvraag or “bacon question” is still “where the heck does all that sand actually go?”
Knowing where nourished sand goes is important for understanding the ecological impact of nourishments, as well as their effectiveness. If you want your sand to reach a certain destination, how much of it actually gets there and how quickly?
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!
How do sand and mud move around on our coasts? This is a question that we need to answer in order to sustainably manage coastlines in the face of sea level rise and climate change. To do so, we use a combination of field measurements and computer simulations at Ameland Inlet in the Netherlands. In the course of my PhD we developed several new methods, including morphodynamic mapping techniques, a sediment composition index (SCI) derived from optical and acoustic measurements, techniques for sediment tracing, the sediment connectivity framework, and a Lagrangian sediment transport model (SedTRAILS). Together, these approaches reveal new knowledge about our coasts which can be used for managing these complex natural systems.
That’s a bit of a mouthful, so let’s break it down and try to explain what I have been doing with sand for the last half-decade…
Coastally Curious 
