Lighthouse Fever

I love lighthouses, and am borderline obsessed with them. I have lucky lighthouse socks (I’m wearing them today), I own books about them, and in high school I made a papier-mâché lighthouse lamp. I have spent hours in the rain trying to photograph them during storms. Furthermore, they are most often found in places where the sea is at its most violent, and from a technical perspective, most interesting.

Naturally, when a bright master’s student from our lab asked if I was interested in joining his thesis committee to research the impact of breaking waves on lighthouses, I could not help but say yes. My research on coral reefs focuses mainly on the complex ways in which waves change as they break across the reef. In many ways, this is a similar process to what happens when waves hit the rocky shoals that many lighthouses are built on. This makes for a compelling intersection between my professional interests and private obsessions!

I am extremely proud to announce that Jan van Gorsel has successfully defended his MSc thesis, “Numerical analysis of broken regular wave forces on a shoal-mounted cylinder“. I was extremely privileged to sit on his graduation committee and work with him for the past 9 months.

Simulation of a wave breaking on a rocky shoal and then colliding with a lighthouse, from Jan van Gorsel’s thesis. The blue represents air and the red water. It is really hard to predict something like this, because of the complex bouncing and splashing motion of the broken wave. Simulating this process is a bit like trying to predict the exact spatial distribution of spilled beer after a drunk guy stumbles and trips in a crowded pub. Sigh. Remember crowded pubs?
Continue reading Lighthouse Fever

Super Speedy Reef Modelling

Coral reefs and the islands that they protect from flooding are in big trouble. This is a recurring theme on this blog, and now it’s time for the latest update. We are currently building towards the development of an early flood warning system for low-lying tropical islands fronted by coral reefs. Our previous work on this topic has focused on finding ways to do this accurately for a wide variety of coral reef shapes and sizes, as well as different wave and sea level conditions. However, it’s not enough to be accurate- to deliver timely early warnings, you also need to be fast.

That’s where the latest research of Vesna Bertoncelj comes in.

I am extremely proud to announce that Vesna Bertoncelj has successfully defended her MSc thesis, “Efficient and accurate modeling of wave-driven flooding on coral reef-lined coasts: On the interpolation of parameterized boundary conditions“. I had the great privilege of sitting on her graduation committee and working with her over the past year or so.

Vesna’s research provides us with new approaches for making highly accurate predictions of coastal flooding, at limited computational expense. The numerical models that we use to estimate flooding often take a long time to simulate, since they resolve many complex physical processes at high resolution in space and time. However, by paring down these models to only the most essential components for the task at hand, we can do this much faster. My colleagues at Deltares recently developed the SFINCS model, which has been successfully used to predict flooding in a fraction of the time that our standard models take. But how do we put all these different pieces together?

A schematic overview of Vesna’s research methodology. [Source].

First, Vesna established a baseline for model performance by running a computationally intensive XBeach Non-Hydrostatic model (XB-NH+), and a much faster SFINCS model. These models provide an estimate for runup (R2%), which can be taken as a proxy for coastal flooding. In the second step, she used a lookup table (LUT) of pre-computed XBeach model output and to derive the input for the SFINCS model. The crucial task is doing this quickly and accurately, so she experimented with different interpolation techniques for deriving that input. She then compared her new approach with the standard models to find the fastest and most accurate combination.

Her research gives us a useful methodology that we can implement to speed up our early flood warning system, saving time and hopefully someday saving lives.

Vesna’s quality of work is excellent and she has a fantastic attitude towards research and collaboration. Her curiosity, professionalism, and diligence will undoubtedly serve her well in the years to come. I hope that we will have other opportunities to collaborate in the future. If anybody out there needs a bright young coastal researcher and/or modeller, hire her!

Restoring Coral Reefs for Flood Protection

We frequently hear in the news about dying coral reefs, and also about the threats of sea level rise and climate change. But there is a key gap: what if we can hit two birds with one stone, and restore damaged ecosystems while providing vital protection against flooding? Our latest research demonstrates how coastal managers and ecologists can join forces to achieve both goals, which may help stretch limited funding further.

I am extremely proud to share Floortje Roelvink’s first scientific paper, “Coral reef restorations can be optimized to reduce coastal flooding hazards“, published in Frontiers of Marine Science. I had the great privilege of sitting on her MSc thesis committee two years ago, and she has been our colleague at Deltares ever since.

Continue reading Restoring Coral Reefs for Flood Protection

Ebb-tidal deltas: Badass Morphological Features (BAMFs)

At TU Delft, a requirement for our PhD defense is to make ten propositions based on what we have learned during the previous years. Claims posed by my friends and colleagues deal with the nitty gritty details (“All diffusive processes can be derived from an advective one, and failing to do so yields incorrect modelling[1]) but also the bigger picture of how we do what we do (“The way morphological models are presented and interpreted has a lot in common with predictions of snow depth in five years on December 26th at 4pm. The knowledge in these models deserves a better presentation[2]).

The propositions must be both defendable and opposable, so as to stimulate an interesting debate during the defense. Some of the propositions should reflect the findings of our research, but it is also traditional to include statements that have nothing to do with it. One colleague[3] even suggested (tongue in cheek) that the increase in the height of Dutch men over time could be explained by sea level rise. I couldn’t resist analyzing the data myself, and the results were surprisingly good:

Spurious correlation of the week! As John von Neumann said, “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”

These propositions are a chance to inject a bit of last-minute philosophizing into our Doctorates of Philosophy, and range from the wise (“No wind is favourable if a person does not know to what port (s)he is steering – Seneca[4]) to the downright cheeky (“This proposition is not opposable[1]).

What I Learned by Counting Sand for 5 Years

As the clock is ticking on my own PhD (259 days, 13 hours, 39 minutes, and 51 seconds, but who’s counting? *eye twitches*), I started preparing some propositions of my own (obviously in a fit of procrastination on my dissertation). After nearly five years of scrutinizing sand and contemplating connectivity, my research has led me to an inescapable conclusion:

Ebb-tidal deltas are badass morphological features (BAMFs),
(c.f. Phillips [2015]).

What, pray tell, is an ebb-tidal delta, and why is it so badass? Ebb-tidal deltas are large underwater piles of sand at the mouth of estuaries and tidal inlets, deposited by outflowing tides and reshaped by waves. I spend my days studying how waves and tides move sand around on the Ameland ebb-tidal delta in the northern part of the Netherlands (see below). We need to know this in order to plan ecologically-sustainable flood protection measures for the Dutch coast. A morphological feature is just a fancy name for some part of a landscape, like a hill or a valley or a beach.

An ebb-tidal delta is the giant pile of sand located at the mouth of a tidal inlet like this one. This may not sound glamorous, but understanding the way it moves around is extremely important for ensuring safe navigation and for predicting erosion and flooding of adjacent coasts.

What makes a badass “badass”?

Phillips[5] defines the archetypal badass as “individualistic, non-conformist, and able to produce disproportionate results”, and applies this concept to geomorphology (the study of how landscapes evolve, at the crossroads of geology and physical geography). Ebb-tidal deltas meet these three criteria, which makes them badass morphological features (BAMFs):

  1. Ebb-tidal deltas are each unique (in shape, location, composition, and in terms of the environmental forces shaping them (like waves and tides)), and hence individualistic.
  2. Ebb-tidal deltas are chaotic systems which defy accurate prediction using physics-based numerical models[6], and hence are non-conformist or “naughty”[7]. This numerical naughtiness is a serious problem for coastal engineers and scientists, since a failure to accurately predict ebb-tidal delta evolution can threaten public safety and lead to costly property or infrastructure damage. They do not “play by the rules” of our existing physics-based deterministic models. 
  3. Ebb-tidal deltas are highly nonlinear systems which can greatly amplify small instabilities[8], and hence produce disproportionate results.

In addition to the strict definitions of Phillips[5], ebb-tidal deltas are also “belligerent or intimidating, ruthless, and tough”, other traits reflective of badassery [Oxford Engish Dictionary]. The Columbia River ebb-tidal delta alone is responsible for dozens shipwrecks in the past century, and Ameland ebb-tidal delta has also featured numerous wrecks throughout its history.

Quoting Thomas Pynchon[9], Phillips also notes that badasses are “able to work mischief on a large scale”. Ameland ebb-tidal delta covers an area of approximately 100 square kilometers, roughly the size of The Hague. Many ebb-tidal deltas around the world are even larger!


Now admittedly, ebb-tidal deltas are just big piles of sand. A big pile of sand is probably not the first thing that comes to mind when you hear the word “badass”, unless you are Ralph Bagnold or a Sarlacc. This could probably also be considered gratuitous personification or anthropomorphization.

I’m sure that many of my friends and family have been scratching their heads as to why I would sacrifice the latter half of my 20s to understand them better. A critical reader might ask, “is it possible that you have only convinced yourself that ebb-tidal deltas are cool out of self-preservation?” And the answer is yes. Yes, I have. Nonetheless, I remain steadfast in my assertion that ebb-tidal deltas exhibit major symptoms of geomorphological badassery.

Although the concept of geomorphological badassery may seem silly at first, it illuminates several important truths of our (mis)understanding of these complex bathymetric features. Ebb-tidal deltas are important to study for reasons of coastal flood protection, navigational safety, and ecological value, but we are bad at predicting how they will evolve. This is because each ebb-tidal delta is unique, making it challenging to generalize their behavior.  Furthermore, their chaotic, non-conformist behavior renders many of our usual deterministic prediction techniques ineffective.  Lastly, the amplifying effect of highly nonlinear physical processes means that small physical changes (e.g., the development of a tiny shoal) could have disproportionately large consequences (e.g., relocation of a channel several kilometers wide).  As such, badassery provides a useful conceptual framework for describing the challenges presented by ebb-tidal deltas to coastal engineers and scientists. 



[1] Chavarrias Borras, V. [2019]. Obtaining well-posedness in mathematical modelling of fluvial morphodynamics. PhD Thesis, Delft University of Technology.
[2] de Schipper, M.A. [2014]. Alongshore variability of nourished and natural beaches. PhD Dissertation, Delft University of Technology.
[3] Meirelles Nunes da Rocha, S. Effects of a stratified tidal flow on the morphodynamics. PhD Dissertation, Delft University of Technology.
[4] de Vet, P.L.M. [2020]. Intertidal Flats in Engineered Estuaries: On the Hydrodynamics, Morphodynamics, and Implications for Ecology and System Management. PhD Dissertation, Delft University of Technology.
[5] Phillips, J. D. (2015). Badass geomorphologyEarth Surface Processes and Landforms40(1), 22-33. (Yes, this is a legitimate scientific publication!)
[6] Elias, E., Teske, R., van der Spek, A., & Lazar, M. [2015]. Modelling tidal-inlet morphodynamics on medium time scales. In The Proceedings of the Coastal Sediments 2015.
[7] Kennedy, B. A. [1979]. A naughty world. Transactions of the Institute of British Geographers, 550-558.
[8] Elias, E. P., Van der Spek, A. J., Pearson, S. G., & Cleveringa, J. [2019]. Understanding sediment bypassing processes through analysis of high-frequency observations of Ameland Inlet, the Netherlands. Marine Geology, 415, 105956.
[9] Pynchon T. [1984]. Is it O.K. to be a Luddite? The New York Times Book Review (October 28): 40–41.

Keeping it Connected Around the World

It has been a crazy year, but work-wise I am on the final stretch, at least. Tonight at the ungodly hour of 12am CET, I will present my poster at the American Geophysical Union conference. It is at a much more reasonable 3pm PST in California where the conference organizers are located. If you have registered for the conference, you can see the poster via this link. Otherwise, I will try to put you in the loop here.

Estuaries are complex environments shaped by the interaction of waves, tides, rivers, and humans. Understanding how sand and mud move through estuaries is essential for their effective management. In an approach known as connectivity, the pathways taken by sand and mud through estuaries can be represented as a connected network of nodes and links, similarly to a subway map. Connectivity provides numerous mathematical techniques and metrics that are well-suited to describing and comparing these pathways in estuaries.

Network diagrams depict the sediment transport pathways of each estuary as a series of nodes and connecting links. The Mouth of the Columbia River (1) and San Francisco Bay (2) are on the west coast of the United States, while Ameland Inlet (3) and the Western Scheldt (4) are in the Netherlands. Red arrows indicate the 90th percentile of all connections in terms of sediment fluxes, superimposed on greyscale bathymetry of each estuary.

We use connectivity to map out and analyze sand and mud pathways in four estuaries around the world: the Wadden Sea (the Netherlands), Western Scheldt (NL), San Francisco Bay (US), and Columbia River (US). Our analysis is based on the outcome of numerical simulations, and we explore the benefits of different simulation techniques. We conclude that connectivity is a useful approach for visualizing and comparing the pathways that sand and mud takes through different estuaries. We can use this method to plan and predict the impact of human interventions in these environments, such as dredging.

However, a comparison of connectivity metrics suggests a dependency not just on sediment transport processes, but also on the choices made in schematizing networks from underlying models.  Essentially, we’re not comparing apples to apples yet, so if we are going to make comparisons between different estuaries, we need to make sure that we set up our models in an equivalent way. Our ongoing research will focus on optimizing these numerical models to make more meaningful quantitative comparisons of different estuaries.

Keeping our feet dry and safe from the big water with lots of tiny rocks!

Some ideas are really hard to understand, but it helps if we can talk about them using simple words. One of my favourite books of all time is the Thing Explainer by Randall Munroe, the cartoonist behind XKCD. In it, Munroe tries to describe scientific concepts using only the 1000 most common words in the English language. With the help of a text editor to flag any illegal words, I prepared the following summary of my PhD research, and am presenting it tomorrow in a special science education session at the online American Geophysical Union conference. If you are not attending, you can also check out my presentation here:

Here are the words that go with my pictures:

There is a very low land next to the big water. It has a lot of wind and it rains there most of the time. It is so low that it would be under water now if people didn’t build big walls around it and suck all the water out. The big water is going up and up and up, and we want to keep everyone’s feet dry so that they stay safe for a long time to come. The plan is to put lots and lots of very tiny rocks along the edge between the big water and the very low land. When there is too much wind, the big water will make huge waves. These will hurt the wall of very tiny rocks, but if we have enough very tiny rocks, the big water won’t get inside the very low land and the people will be safe. It is hard to guess where these very tiny rocks will go when we put them on the edge of the big water, because the waves move them around. We use water-counters, rock-counters, and computers to learn more about how the very tiny rocks move through the water and make better guesses about what they will do. People in many other lands are also worried about huge waves and the big water going up, so we hope that the things we learn in the very low land can help them too.

This was one of the most fun presentations I have ever made, and it changed the way I think about my research. After all, if I can’t explain what I’m doing, the dissertation I have spent five years writing will just collect dust on a bookshelf instead of contributing something useful to the world.

But perhaps even more importantly for the fulfillment of my muppet-loving childhood dreams, it meant being able to legitimately refer to “the big blue wet thing” at a Serious Scientific Conference.

The Side Effects of Trying to Keep Our Feet Dry

In an era of rising sea levels, ambitious plans for coastal protection works are emerging around the world. One such plan is the Delta21 project, proposed by group of Dutch coastal engineers and entrepreneurs. Their goal is to improve flood protection at the mouth of the Haringvliet estuary and develop a tidal power facility, all in one integrated project.

However, the law of unintended consequences often looms large in these sorts of massive infrastructure projects, particularly for environments as complex as estuaries. After a massive flood in 1953, the Dutch constructed the Delta Works, damming most of the estuaries in the southern half of the Netherlands. Prior to that, the Afsluitdijk was constructed across the Zuiderzee in the northern part of the country. These protection works have had dramatic consequences on the physical and ecological development of the Dutch coast, and many of my colleagues here have devoted their careers to analyzing the impact of these interventions.

But instead of just looking back and dissecting the successes and failures of 50 or 100 years past, what if we could also use our latest diagnostic tools for predicting the potential impact of bold future interventions? If the Delta21 plan goes ahead, how will the mouth and ebb-tidal delta of the Haringvliet estuary and surrounding coastline evolve? Will existing habitats (particularly in vital intertidal areas) be preserved, disappear, or even expand?

Today, Mayra Zaldivar Piña tackled these questions head on, and successfully defended her master’s thesis, “Stability of intertidal and subtidal areas after Delta21 plan“. I had the pleasure of co-supervising Mayra’s work throughout the last eight or so months, and am very proud of her. She embarked on a challenging modelling project and showed an exemplary critical scientific attitude. I was also so impressed with the persistence and tenacity she showed in doing nearly her entire project during the pandemic. Writing your thesis is a difficult and isolating experience at the best of times, and these are not the best of times. Nonetheless, she kept at it and delivered an impressive thesis in the end!

Congratulations Mayra, and best of luck in the next steps of your career!

Sediment Connectivity: Where does all the sand go?

I am thrilled beyond measure to announce that the first paper of my forthcoming PhD dissertation, Sediment Connectivity: A Framework for Analyzing Coastal Sediment Transport Pathways, has finally been published (open-access) in the Journal of Geophysical Research: Earth Surface!

What’s it about? As sand moves along coasts and through estuaries, the pathways it takes are determined by a complex combination of waves, tides, geology, and other environmental or human factors. These pathways are hard to analyze and predict using existing approaches, so we turn to the concept of connectivity.

What is connectivity? Connectivity represents the pathways that sediment takes as a series of nodes and links, much like in a subway or metro map (see here for a primer). This approach is well used in other scientific fields like neurology, oceanography, and fluvial geomorphology, but in our study we apply these techniques to coastal sediment dynamics.

So what? To show how the sediment connectivity approach can be used in practice, we map sediment pathways with it at Ameland Inlet, an estuary in the Netherlands. The statistics we compute using connectivity let us quantify and visualize these sediment pathways, which tells us new things about the coastal system. We can also use this approach to answer practical engineering questions, such as where to place sand nourishments for coastal protection. We hope to use sediment connectivity to predict the response of our coasts to climate change, and the human adaptations that these changes provoke.

A conceptual summary of our paper on sediment connectivity at Ameland Inlet in the Netherlands. (a,c) We can schematize the sediment transport pathways in an estuary as a series of nodes (A,B,C,…) and the links connecting them. (b) This network can in turn be represented by a matrix, showing where the sediment is coming from (a source) and where it’s going to (a receptor). In this form, we can investigate questions like, “where does the sand in node F go?” (d), or “where does the sediment reaching node D come from?”(f). In my research, we are most interested in answering questions like, “what is the main pathway between the two islands, node A and G?” (e). We can also learn how the system is organized into “communities” that share sand (g), and how that changes when pathways are added or removed (h).

It has been a long journey, beginning with The Magical Figure that Changed My Entire PhD and now culminating in this publication. I am especially indebted to my supervisors, Bram van Prooijen and Zheng Bing Wang, for their constant support. I am also grateful to my other co-authors, Edwin Elias from Deltares and Sean Vitousek at USGS, for seeing potential in this approach and really strengthening and clarifying my story. I was also blessed with curious and constructive reviewers who provided a much-needed non-engineering perspective on our work.

Now that this paper is out, the fun can really begin! I have lots of fun ideas for applying connectivity to other estuaries around the world including the Mouth of the Columbia River and San Francisco Bay. We are also in the process of developing new modelling tools that well help us better unravel how estuaries and coasts are connected. Stay tuned!


Pearson, S.G., van Prooijen, B.C., Elias, E.P.L, Vitousek, S., & Wang, Z.B. (2020). Sediment Connectivity: A Framework for Analyzing Coastal Sediment Transport Pathways. Journal of Geophysical Research: Earth Surface. [Link]

Ameland Datapalooza!

Check out our new paper here!

Three years ago, I experienced one of the highlights of my professional career so far. Alongside researchers from 3 universities, the Dutch government, and several other institutions, we carried out a 40-day field measurement campaign at Ameland Inlet in the north of the Netherlands. We deployed several frames loaded up like Christmas trees with every instrument imaginable: ADVs and ADCPs to measure waves and currents, LISSTs and OBSs to measure suspended sediment, a YSI multiprobe to measure salinity and other water quality indicators, and even a 3D sonar to track the migration of ripples along the seabed.

One of the five measurement frames that we deployed in Ameland back in Fall 2017. It was stacked full of instruments to measure the waves, currents, suspended sand and mud, salinity, ripples, and more.

Four of our five frames survived the relentless ebb and flow of the tide, and even two major storms (one of which left me stranded in Germany after the wind blew down all the overhead train power lines between Berlin and Amsterdam!). In the end, we obtained enough data to keep me busy for probably 3 PhDs, if not the rest of my career. This is just as well, since that last frame was buried in the storm, and based on our understanding of the local dynamics, it will likely re-emerge in another few decades, just in time for my retirement! I look forward to sharing my other findings with you here in the next few months!

Although it used to be the norm for scientists to squirrel away their data, there is an increasing movement towards open accessibility of research data. This improves transparency and accountability in the scientific process, and opens up new opportunities for collaboration. The data we collected is now available in its entirety here on the 4TU web portal or on Rijkswaterstaat’s interactive web viewer.

However, there is a lot of data – I mean A LOT! To help researchers interpret the contents of this database, we prepared an overview paper, which was finally published in the journal of Earth System Science Data! It is also accompanied by a more detailed report, which gets into the nitty-gritty details we didn’t have room to describe in the paper. Nobody likes to read a phonebook-sized report, but it’s nice to have the information there for the few brave souls who do want to comb through our dataset.

Sailing across the Vlie ebb-tidal delta on our way to retrieve the measurement frames at the end of the field campaign.

It was all a huge team effort, as evidenced by the 20+ co-authors. My contribution to this paper focused on the processing of the LISST and YSI multiprobe data, which tell us about the size of particles floating through the water, and how salty that water is. I also designed the maps. As a kid, I loved to read and draw maps, and I think that 7-year-old Stuart would have been tickled to know that he would still be dabbling in cartography all these years later.

As the research in the rest of my PhD (and beyond!) will continue to focus on the fruits of this measurement campaign, I am very keen to work together and collaborate with other researchers who have an interest in this dataset. Please get in touch if you are interested!

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!