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…
As a kid, I was obsessed with maps. Give me an atlas and I would be sucked in for hours. Eventually I wound up in coastal engineering and took a GIS course with the amazing Dr. Kate Parks in Southampton. This reignited my interest in cartography, as I now had the tools I needed to make my own maps. Over the coming years this led to an interest in how we can map our coastal regions to better communicate their morphodynamics. Also (mostly), I just wanted to make pretty maps! Making figures sometimes feels like one of the only avenues for artistic expression that we have in science.
To reach these goals, a good colourmap is a key ingredient. For a map showing the topography/bathymetry of a coast, a colourmap is the range of colours that correspond to a particular elevation. In this post, I will walk you through how I created two of my favourite colourmaps.
Originally presented earlier today at the AGU 2021 Fall Meeting in the “Upgoer Five” Session, this video was inspired by the XKCD comic and book in which scientific concepts are described using only the 1000 most-common words in the English language. I participated in the session last year and had so much fun, I thought I would try it again with my coral reef research.
Unfortunately, ”ocean” and ”sea” were not on the list, so I had to go with ”big blue wet thing” instead. Want to give it a try yourself? Here is a handy tool which checks your writing to see if it meets the list of 1000 most common words: https://splasho.com/upgoer5/ It’s harder than it looks!
Here is a summary of my video:
Some small but beautiful lands in the middle of the big blue wet thing were built by tiny animals that turn into rock when they die. Although these lands might seem perfect and calm most of the time, they are actually in big trouble. The big water is going up and up and up, and the little lands could be completely under it before our kids grow old. However, they are also in trouble right now — waves can hit the little lands and make them go under the water too, even if just for a short while. These waves can hurt people and make the drinking water not-drink-able. It is hard to guess if the waves will cause trouble because they break in different ways than we are used to when they hit the rocks built by animals. The waves become longer and weirder as they move across the rocks, and can hit the land with more power than we would expect. It is even harder to guess what the waves will do because every small land made of rocks built by animals is different, and there are so many of them all around the world. To keep everyone safe, we showed a computer lots of made-up waves so that it could learn how waves look when they hit different sorts of rocks and land. The computer can then make good guesses about what real waves would do if they hit real rocks and land. If the computer thinks that the waves will cause trouble, we can warn people to go somewhere safer until the waves stop. In this way, we hope to keep everyone’s feet dry until long after our kids are old.
You can find more about this stuff in bigger words here:
1. Pearson, S.G., Storlazzi, C.D., van Dongeren, A.R., Tissier, M.F.S., & Reniers, A.J.H.M. (2017). A Bayesian‐based system to assess wave‐driven flooding hazards on coral reef‐lined coasts. Journal of Geophysical Research: Oceans, 122(12), 10099-10117. https://doi.org/10.1002/2017JC013204
In the past few weeks, Vancouver and the BC Lower Mainland have suffered not just one but three record-breaking rainstorms, a succession of ”atmospheric rivers” that dumped several hundred millimetres of rain. Highways washed out and disappeared, and numerous communities were flooded. This resulted in an enormous quantity of sediment reaching the sea via the Fraser and other local rivers. But where exactly does the sediment that’s already in the sea around Vancouver go? How has that changed in the past few hundred years since Europeans colonized the area? To get to the bottom of this, we enlisted Carlijn Meijers.
Last week, Carlijn successfully defended her thesis, ”Sediment transport pathways in Burrard Inlet”. To answer these questions, she created a detailed hydrodynamic and sediment transport model of Burrard Inlet and Georgia Strait in D-Flow FM. She then used the SedTRAILS model that we have developed to visualize sediment transport pathways.
From these models, Carlijn showed that sediment transport is largely controlled by flow through the First and Second Narrows (where the Lion’s Gate and Ironworker’s Memorial bridges cross). As the tide comes in, the water shoots through these narrow passages at speeds of up to 2 m/s and comes out the far side as a jet, spiraling off into eddies. The tide then goes out and the same happens in reverse, with water shooting out the opposite side.
Due to the sheltered nature of the inlet, waves have only a minor role in sediment transport. However, given the intensity of the tides and the great depths of Burrard Inlet (especially the Indian Arm fjord to the north), most sediment liberated by erosion tends to get carried away from shore and is essentially lost from the coastal sediment budget.
Another key point of her project was to investigate how land use changes and other human effects (e.g., damming rivers, port construction) have changed Burrard Inlet. Using the model, Carlijn showed that these changes to the inlet have shrunken its tidal prism, influencing the currents and sediment transport patterns.
These changes are especially evident when we compare satellite photos from the present day with the oldest available images from the 1940s.
Carlijn wrote an excellent report and capped it all off with one of the best master’s thesis defenses that I’ve seen in a long while. She also handled the cultural context of the project with great respect, interest, and sensitivity. If anyone reading this is looking to recruit a new engineer/researcher with heaps of potential, I cannot recommend Carlijn enough.
All in all, this was a fascinating project and one very close to my heart — I was born in the Vancouver area and was excited to see how the SedTRAILS model could be used in my original backyard. Let’s keep the Delft-Vancouvercollaborations going!
Ebb-tidal deltas are notoriously unpredictable. Battered about by waves and tides, their ever-shifting sands can be a royal pain in the arse for everyone from coastal residents to pirates. I have spent most of the past five years trying to identify the pathways that sand takes across these deltas as part of my PhD. However, the holy grail of ebb-tidal delta research is to take that one step further and make accurate morphodynamic predictions of their evolution on timescales of decades.
This past year, Denzel Harlequin took up the challenge, and I am pleased as punch to announce that last week he successfully defended his master’s thesis, ”Morphodynamic Modelling of the Ameland Ebb-Tidal Delta”. This is a really tricky problem to solve because of the complexity of the processes that need to be simulated.
What’s cool about Denzel’s work is that brings us closer to good morphodynamic predictions than we were before. Furthermore, where the predictions deviate from reality, he illuminates the areas where we still need to make improvements — specifically, our representation of wave-driven transports. Denzel also shows how the location of a sand nourishment can have major knock-on effects on the evolution of the ebb-tidal delta.
Denzel is a very talented modeller and I am delighted that he has joined us as a new colleague in the Applied Morphodynamics department at Deltares. I look forward to many more great collaborations to come!
We forgave Bagnold everything for the way he wrote about dunes. “The grooves and the corrugated sand resemble the hollow of the roof of a dog’s mouth.” That was the real Bagnold, a man who would put his inquiring hand into the jaws of a dog. – Michael Ondaatje, The English Patient
Ralph Bagnold, widely considered one of the godfathers of sediment transport, was a soldier in the British army who spent much of the Second World War scouting around in the Libyan desert. In the process, he learned much about the dynamics of sand dunes, and formed the basis for many theories that are still in use today for explaining how sediment is blown around by wind or water.
This week I am proud of our very own up-and-coming Bagnold, Charlotte Uphues, who successfully defended her thesis, Coastal Aeolian Sediment Transport in an Active Bed Surface Layer, on Thursday. Charlotte did a fantastic job of designing and carrying out her own super cool field experiments, using tracer sediment to estimate aeolian (wind-blown) sediment transport on a beach here in Holland. As the dunes of the Netherlands are a key component to Dutch coastal defenses against flooding, it is essential that we understand better how they evolve by improving our abilities to predict aeolian transport.
I would elaborate a bit more about her findings, but Charlotte will be submitting her thesis for publication in a journal soon, so it will remain under wraps for now. Stay tuned, I don’t think we’ve heard the last from Charlotte!
This morning as I was finishing my first coffee, one of my Deltares colleagues shared a video of a house on the beach in El Salvador. It was a surreal, post-apocalyptic, Planet of the Apes scene, made all the more mysterious by the accompanying news text (translated from Dutch): “Abandoned villa washes up entirely on the beach in El Salvador. The ruins of a villa have recently washed up on the beach of La Puntilla in El Salvador. It is a great mystery how the entire house ended up on the beach. Hurricane Elsa may have blown the building away.“
Being a bunch of sediment transport nerds, my colleagues made all kinds of jokes about building a house-transport module into XBeach or SedTRAILS, or estimating the critical shear threshold and porosity of this giant “particle”. However my curiosity was irrevocably piqued- how did the damn thing get there? It must have been there the whole time, with the coast eroding around that, but how or why?
The Dutch news site was not particularly helpful, so I snooped around on the internet until I found a promising lead in the El Salvadorian news. Apparently the house is located near the village of La Puntilla, on the Pacific coast of El Salvador. Apparently it was a former hotel built (unwisely) right next to the beach. Suffering significant damage during Hurricane Mitch in 1998, it later became a church. Numerous people died in the former hotel and more recently among the ruins, so the building is even spookier than it first seemed. Indeed, after looking in Google Earth, it became apparent that the hotel has been there the entire time:
So the hotel didn’t just “wash up on the beach” one day. But why exactly did it erode so much? Was there a particularly devastating storm? Is climate change or some nefarious human meddling in the coastline to blame? By zooming out a little bit, we can put this coastal erosion into a larger context.
Google Earth is nearly always my first go-to tool for investigating a new site as a coastal engineer — you can learn so much by carefully observing how a given beach or estuary has changed through time, and by looking for evidence of the physical processes shaping the landscape. For instance, white foam usually indicates breaking waves, and by watching how sand piles up in certain places, we can estimate in which direction the sand is mostly moving. What can we learn about this site?
It seems that the hotel was built on a type of coastal feature that is notorious for massive erosion world-wide: the beach adjacent to a tidal inlet. Tidal inlets connect lagoons with the ocean, the water rushing in and out of them several times a day. The water moves very fast and is stirred up by waves, so it can carry lots of sand and mud to and from the nearby coast. This means that this tidal inlets and the coastlines next to them have the potential to change shape dramatically over time. The inlet channel and smaller channels connected to it have a tendency to migrate back and forth across the underwater ebb-tidal delta like an out-of-control fire hose. If one of these channels is pushed up against the coast, it can cause massive erosion. This seems to be what happened to the abandoned hotel.
If you examine a timelapse in Google Earth (here), the incredibly dynamic nature of this system is apparent:
By a remarkable coincidence, this is the same process that I am investigating in my PhD: how does sediment move around at tidal inlets like this? It is not only the El Salvadorians who need to worry about this process — the topic is also a vital matter of investigation for Dutch coastal management authorities trying to protect their coast. Lesson learned: don’t mess with ebb-tidal deltas!
Ebb-tidal deltas are gigantic piles of sand that form at the seaward mouth of tidal inlets. They are constantly on the move, shifting shape and size in response to the waves and tides. Where exactly is that sand going? This is a question I have been struggling with for the past 5 years during my PhD, and we have recently made great strides in part due to the efforts of Paula Lambregts.
Yesterday, Paula Lambregts successfully defended her master’s thesis, “Sediment bypassing at Ameland inlet“. I had the great honour of co-supervising Paula’s research throughout the last ten or so months, and I am enormously proud of her. Her project encompassed a range of approaches, including bathymetric analysis and numerical modelling, to solve the mystery of the sediment pathways.
First, Paula’s detective work led her to examine detailed measurements of the seabed bathymetry at Ameland Inlet in the Netherlands, taken over the past fifteen years. These measurements give snapshots of the underwater delta landscape. By comparing the bathymetry from different months or years, we can track the delta’s evolution. In the image below, we see four snapshots of the ebb-tidal delta before and after the construction of a sand nourishment (i.e., the large pile of sand that appears in panel B). This nourishment was a large-scale pilot test to determine if creating sand deposits like this is a viable strategy for strengthening the coast of nearby islands.
After describing how the delta has evolved in the past, Paula developed hypotheses about the physics underlying this behaviour- how do waves and tides move the sand around to create the patterns we observe? To answer this question, she used a combination of computer models to estimate sand transport pathways. This allows us to “connect the dots” and explain how the sand moved from one place to another. The first component was a D-Flow FM model, which is used to simulate the hydrodynamics (waves and tides) and sediment transport (where and how much sand moves). The second component of her modelling approach was to apply SedTRAILS, a brand-new tool developed by my colleagues and I at Deltares for visualizing predicting sediment transport pathways. Using SedTRAILS, she was able to create some really cool maps that indicate where the sand goes.
Drawing on her prior expertise in geology, Paula combined those two lines of evidence (the measurements of the seabed and the modelled sediment pathways), to come up with a series of fantastic conceptual diagrams. These diagrams distill the mysterious piles of sand and complex spaghetti of the images above into a more easily understandable picture:
The work that she did is extremely valuable for coastal management, since it gives more insight into where (and where not!) to construct sand nourishments. It also brings new insights to science about how these complex systems work. Last of all, it is enormously helpful for the research that we are continuing to do at TU Delft and Deltares. In September I will continue on with the work on sediment transport pathways at tidal inlets begun during my PhD, and build on the work that Paula has carried out in her thesis project. I am extremely proud of her and hope that we can continue to collaborate in the future!