My friend Claudia responded with great zeal to my call for sand from different beaches around the world. In addition to her samples from Sword Beach and Dunkirk, she also brought back sand from her holiday to the Greek island of Crete. The sand from Xerokambos Beach is interesting compared to those two French beaches, since it is much more diverse- there are many different colours and likely different mineral origins for the sand grains that we see there.
That being said, when I see pictures of how lovely Crete looks, I have the feeling that I would not be too focused on the finer details of local sand composition if I went on holiday there!
Ga je naar het strand? Mag ik
als je terug komt het zand
uit je schoenen voor
de bodem van mijn aquarium?
—
Are you going to the beach?
when you come back, may I have the sand
from your shoes for
the bottom of my aquarium?
– K. Schippers
I have had that Dutch poem on a postcard on my bedroom wall for a few years now, but it unexpectedly came to life a few weeks ago. I mentioned to some friends that I was taking pictures of sand from different beaches with a microscope and wanted to expand my collection. My colleague Silke enthusiastically responded- she had just returned from a holiday in France and still had sand in her shoes! “Should I bring it to the office tomorrow?” she asked. How could I say no?
Her holidays had taken her to the beautiful Glenans Archipelago off the coast of Brittany, not too far from where I am living right now in Brest. Unlike a lot of the sand I have looked at so far (which was mainly quartz), this beach appears to be quite shelly. The islands are famous for their maerl beds, a sort of coral algae rich in limestone. That may account for some of the interesting shapes and colours we see, but if you look closely, it seems there are also some threads and bits of lint from Silke’s socks! It might not be a scientifically valid sample, but I’ll take it!
It was Christmas 2016, and I felt like I had bitten off more than I could chew. I’m not talking about turkey, though. Four months into my PhD, I was feeling completely overwhelmed and starting to wonder what I had gotten myself into.
The goal of my project is to identify the pathways that sand takes as it moves in and around the Wadden Islands in the northern part of the Netherlands. Since the Dutch coast has a chronic erosion problem, accurately accounting for the whereabouts of their sand is a matter of national security. Right now, the Dutch deal with a deficit in their coastal sediment budget by adding more sand or “nourishing” wherever there is a shortfall.
Knowing when, where, and how much sand to add is especially challenging around these islands. Here, the persistent push and pull of the tide competes with the chaotic brutality of the waves to move sediment in complex patterns. These patterns are hard to predict with our usual box of tools, so we planned to throw everything we had at the problem: state-of-the-art field measurements, sophisticated computer models, reams of historical data, and a support team of experts from across the Netherlands. As PhD students go, I felt [and still feel!] pretty darn lucky to be a part of such a large and well-conceived project.
The Spaghetti Problem
However, as I started reading more and more about my topic, my initial enthusiasm began to wane. I was floored by just how much research had already been done on what I had thought was a fairly specific niche. The Dutch have been scrutinizing their coast for centuries, and to my inexperienced eyes, it seemed like they had already thought of everything.
There was another problem: at the end of almosteverystudyaboutsedimentpathways, there seems to be a diagram summarizing all the paths with lots of curvy arrows flying all over the place. This veritable plate of spaghetti makes for a nice conceptual drawing, but how can you statistically compare two plates of spaghetti with one another? A “past spaghetti” and a “future spaghetti”, to help understand potential responses to climate change? A “Dutch spaghetti” and an “American spaghetti”, to make my findings more general and useful for other places? If I was going to get anywhere with my PhD, I needed a spaghetti system.
Mmm, sediment pathways… Also, to be clear, this is not The Magical Figure That Single-handedly Changed My PhD. [Source: Flickr]By Christmas, I felt like I was in a weird purgatory between “it’s all been done before! I’ll never come up with anything original!” and “this is insurmountably complex and you’re foolish to think you’ll ever figure this out”. And just a dash of “how-did-I-get-here?” imposter syndrome, for good measure. I spent much of my holiday feeling overwhelmed and inadequate, like I couldn’t possibly live up to my own expectations, or (what I thought to be) the expectations of those around me.
But: new year, new start. On January 11th, 2017, my first day back in the Netherlands from holidays, a paper about coral reef hydrodynamics popped up in my Google Scholar alerts. At that time, I was also finishing up a paper about predicting floods on tropical islands, and I liked to keep an eye on the latest developments in that topic.
“A coupled wave-hydrodynamic model of an atoll with high friction: Mechanisms for flow, connectivity, and ecological implications“. Sounds promising, I like wave models.
In this paper, they wanted to understand how waves and ocean currents move water around Palmyra Atoll, a coral island in the middle of the Pacific. Coral reefs all around the world are in big trouble, and to help them we must first understand the physical processes governing the life and death of corals.
Palmyra Atoll, a coral island in the middle of the Pacific. Unfortunately not my PhD study site… [Source: Wikipedia]This was all very interesting stuff, though not particularly relevant to my research about flood prediction, since they seemed more focused on the ecological impact of their results. It was seemingly even less relevant to my PhD topic on Dutch sand- stay focused and stop wasting your time, Stuart! But then I turned the page and there it was:
The Magical Figure That Singlehandedly Changed My PhD…
The Magical Figure That Singlehandedly Changed My PhD, Figure 13 from Rogers et al (2017). “Connectivity between hydrodynamic zones. (a) connectivity matrix showing the probability a water parcel passing through a destination zone came from a given source zone, and (b) geographic connectivity of top 10% of pathways, where shading is relative importance as an overall source, width of line is relative strength of connection.”
Essentially, the authors had summarized the pathways that coral larvae can take around an island in a mathematically elegant way. This was pretty much identical to the goal of my PhD, if you substitute coral larvae for sand, and an idyllic Pacific island for a stormy estuary in Holland. They did it with a concept called “connectivity”, and it became immediately apparent that I had some homework to do.
So what the heck is connectivity?
So what exactly do they mean by connectivity, and how are we meant to interpret that magical diagram? Let’s start at the top. The upper panel is what we call an “adjacency matrix”, but you can think of it just like one of those mileage charts that you sometimes see in the corner of highway maps.
A mileage chart, which you can read in the same way as the adjacency matrix above. If you want to get between two points, just find the intersection between your row and column of choice. 290 miles from Birmingham to Edinburgh doesn’t sound too bad, until you encounter the menace that is Birmingham traffic… [Source: The Open University]Instead of looking at the distance between two points like in a mileage chart, the authors of the reef paper consider the likelihood of water travelling from one point to another. Darker squares show a higher chance of connection, and lighter squares, a lower chance. For instance, if we look at the first column, water is more likely to flow from the point they call “WT FR NW” to point “WT W” than it is to flow to “WT FR SW”.
The second panel shows the same information as the matrix, but this time actually showing the connections on a map – a “network diagram”. The thickness of the blue lines on the network diagram indicate how strong a connection between two points is. If all this seems rather familiar, then that’s probably because you’ve already met our network diagram’s more famous cousin, the transit map:
Harry Beck’s famous map of the London Underground. This map is a network diagram, much like the coral atoll connectivity map shown earlier. It represents the stations as nodes, and the train lines between them as links. It shows the connections symbolically, rather than at their true geographical locations, but this makes it easier for use to focus on the important things, like knowing where we need to switch train lines to get back to our hotel. [Source: Transport for London]
Cool maps. So what?
After seeing the coral reef connectivity diagram, I started googling and soon realized that I had been woefully ignorant of an entire mathematical discipline. Network theory represents complex systems as a series of points and the links between them. Once you’ve done that, interesting patterns start to emerge, such as the “six degrees of separation” or “small-world” phenomenon. It has been used in neurology, sociology, ecology, epidemiology, geomorphology, and basically every kind of “-ology”, except for coastal science and engineering. As far as I can tell, we seem to be the last ones to the party.
The more I read, the more excited I became, and the more vital it seemed for our field to catch up. Connectivity could help us quantify and bring order to the chaotic spaghetti churned out by our models and measurements – if we could figure out how to adapt it.
The course of my PhD was changed instantly with the discovery of that figure. Not only did connectivity provide a potentially useful tool, but it jolted me out of my funk and got me excited about my PhD again. It was an important finding for my research but not a “eureka moment” where everything was suddenly solved- far from it. It has been a long uphill slog since then, but with the help of some very clever people, I think we have almost reached our first milestone. We presented our early findings at a conference in 2017, and right now we’re in the final stages of preparing a scientific article about our ideas. That paper will then have to survive the woodchipper of peer-review, so it may still be many months before my work sees the light of day. But I remain hopeful.
Would I have stumbled upon connectivity eventually, had I not seen The Magic Figure? Probably not if I had only stuck to reading papers about coastal sediment transport. This finding has shaped my attitude towards coastal engineering research- I believe that the next advances in our field will not come from developing a new bedload equation, but from adopting new tools and techniquesfrom other disciplines. Not that we don’t need better bedload equations – I just don’t think I’m the guy to do it, and I think that we could all benefit from looking over the fence at our neighbours in other fields from time to time. As William Zinsser nicely put it:
“Think flexibly about the field you’re writing about. Its frontiers may no longer be where they were the last time you looked.”
Here is another sample brought back by my friend Claudia, from Dunkirk Beach in northern France. Dunkirk is famous from the Second World War, when the Nazis had cornered Allied troops there and forced a major evacuation across the English Channel.
This is where my inner history nerd and my inner sand nerd collided to ask an interesting question: is the sand on that beach now (and in the photograph below) the same sand that was on the beach during the famous evacuation? There’s no easy answer to that question, but as it so closely relates to the main research questions of my PhD, I can’t resist indulging in such a thought experiment. Shall we try together?
To answer this question, let’s ask ourselves a few things:
What kind of sand is on the beach?
The size of the sand grains will determine how easily it is moved around by the waves and tides. Bigger particles require more energy to move, and are thus more likely to stay where they are. In general, smaller sand grains are more likely to get picked up and transported far away*. Based on the photo above, let’s assume that most of the sand grains are about 200 μm in diameter (that’s 0.0002 m).
The sand also seems to be mainly made of clear or white-brownish grains, so we can probably make a safe guess that they are mainly made of quartz. This will come in handy later if we need to make an assumption about how dense the particles are. Most of this sand comes from large sand banks offshore, which is moved to shore by waves during large storms [1].
How do waves and tides shape the coastline here? To predict how sand moves around on a beach, we need to understand the behaviour of the water there. The tidal range on this part of the French coast is quite large, between 5-8 m [1]. That large range means that a correspondingly massive volume of water is moved back and forth past the beach twice a day, which generates powerful tidal currents. Waves here mainly come from the English Channel to the west or the North Sea to the northeast, and are generally at their strongest during occasional winter storms.
In which direction does the sand usually move? There are several possible fates for our 1940 sand: (a) staying where it is, (b) moving offshore into the English channel, (c) moving westward towards Calais, (d) moving eastward to Belgium, or (e) moving onshore to build up the sand dunes there.
At these beaches, the tidal currents moving eastward towards Belgium are slightly stronger than the ones moving westward back towards England [1]. This is eastward motion is reinforced by waves and wind-driven currents, which also tend to move eastward on average [2]. As a result, the sediment effectively takes two steps forward and one step back, gradually moving in an eastward direction (i.e. (d) rather than (c)).
We also know that there is a regular supply of sand from offshore [2], so let’s rule out (b) for simplicity. The dunes in that area are also relatively stable [2], so let’s rule (e) out, too. If most of the sand is then either moving east (d) or staying put (a), what is the likelihood that our 1940 sand is still there?
Have humans intervened with the coast there? In 2014, the French government created the largest sand nourishment in the history of France on the beach at Dunkirk [3]. This is visible in Google Earth as the giant pile of sand near the red pin (below). If there was still 1940 sand on the beach there, it is now likely buried underneath the nourishment. Depending on where my friend collected her sand, there is a good chance that it is made up of this sand that was dredged from the nearby harbour, rather than sand that was on the beach in 1940.
I had a similar issue with my tracer study: several months after our investigation, the Dutch government placed a huge nourishment right on top of our study site. That means that even if some of our tracer sand is still out there, it is likely buried deep beneath a giant pile of sand, which means that we can’t go back there to take more samples.
What is the likelihood of sand leaving the beach?
After placing the nourishment at Dunkirk in 2014, scientists monitored how the beach changed, and found that it lost 9% of its volume in 2 years [3]. Most of this sand appeared to migrate eastward, as predicted by those other studies. If we had similar data about how much the volume of the beach has changed in the past 80 years, we could estimate the rate at which sand is leaving, and hence how likely it is to still be there. From that, we could come up with a sort of “residence time”: how long we expect sand to remain on the beach given the volumes that are coming in from offshore sandbars and leaving down the coast. That would at least give us a ballpark idea of what to expect. We could also use computer simulations to more precisely predict this transport, but that’s a lot of work for our little thought experiment!
Given all of this information, I would guess that most of the sand that was on the beach in 1940 is somewhere on its way to Belgium, or is still there but buried beneath the new nourishment. Based on the assumptions that we made about this being quartz sand about 200 μm in diameter, we can estimate that in a handful of sand (say, 250-300 mL), there will be about 5 million individual grains!** If we scale this up to an entire beach, then I think the odds are good that at least a few grains have stuck around since then.
There are lots of different ways that you could go about this, though- how would you try to tackle it? Am I missing anything important?
* This “smaller-particles are more likely to get picked up by the waves and currents” rule only works for sand grains that are all more-or-less the same size. If your sand has both large and small particles, you can also have “hiding” effects where little grains of sand hide behind big grains and are harder to move. And don’t even get me started on mud! Mud particles (usually 10-100 times smaller than sand) obey a whole other set of complicated rules that are frankly a little absurd sometimes. But these are discussions for another time…
** Even though the grains in that picture are clearly a bit irregular in shape, we can pretend that they are spheres and calculate their volume Vgrain = 4/3π(0.0002/2)3 = 3.3×10-11 m3. The volume of your hand Vhand is 300 mL = 3×10-4 m3, so we can calculate the number of grains as Vhand /Vgrain, which is about 9 million. But wait! We have to account for all the spaces in between the sand grains, since we’re not dealing with a solid block of quartz. This is usually about 40% for sand, so this is how we get our final number of about 5 million.
[1] Sabatier, F., Anthony, E. J., Héquette, A., Suanez, S., Musereau, J., Ruz, M. H., & Régnauld, H. (2009). Morphodynamics of beach/dune systems: examples from the coast of France. Géomorphologie: relief, processus, environnement, 15(1), 3-22.
[2] Anthony, E. J., Vanhee, S., & Ruz, M. H. (2006). Short-term beach–dune sand budgets on the north sea coast of France: Sand supply from shoreface to dunes, and the role of wind and fetch. Geomorphology, 81(3-4), 316-329.
[3] Spodar, A., Héquette, A., Ruz, M. H., Cartier, A., Grégoire, P., Sipka, V., & Forain, N. (2018). Evolution of a beach nourishment project using dredged sand from navigation channel, Dunkirk, northern France. Journal of Coastal Conservation, 22(3), 457-474.
Today I received the sad news that Gerbrant van Vledder, an assistant professor at TU Delft, passed away unexpectedly last week.
Many in our field know him for his work with SWAN, but I would like to shine a light on something else: his research on using wave models to understand how the people of the Marshall Islands have used wave diffraction around islands to navigate their boats for centuries.
Gerbrant had a strong curiosity about using modern tools to find an overlap with more traditional ways of perceiving the world around us; to listen to voices that were not often heard, and find their scientific merit. I think this was unique among engineers, and a really inspiring example. Whether you knew him or not, I encourage you to check out this fascinating article in which his Pacific adventures were featured:
Although I hadn’t spoken to him much recently, Gerbrant was very supportive during my MSc thesis, when I was researching the impact of waves on low-lying tropical coasts like those of the Marshall Islands. He showed a keen interest in my work, actually reading and giving feedback on my whole 232-page report! Given the dread with which I now confront verbose master’s student reports myself, I am especially grateful for the time he gave me. He supported my interest in Marshallese wave piloting, and with his encouragement, I eventually wrote a brief chapter in my thesis about it. I was lucky to have his experienced and critical eye on my side.
This all comes as quite a surprise. I still have a book that he lent me last year, which I haven’t given back yet…
The main protective barrier for the Netherlands against the threat of flooding from the sea is a row of colossal sand dunes and wide beaches that stretch the length of their coast. However, that barrier is not completely natural — since the Dutch coast is in a constant state of erosion, the sand in their coastal zone has to be continually replenished. This replenishment takes the form of nourishments, which are essentially just massive piles of sand placed on beaches, dunes, or just offshore. The Dutch are lucky, since the bottom of the North Sea is covered in sand for hundreds of kilometers in every direction, meaning that there is a ready supply available for this purpose.
Although we still have plenty to learn about how to construct these nourishments effectively and in an environmentally friendly way, we are starting to get the hang of it — at least for long, straight, sandy coastlines like in Holland. However, this all gets a bit trickier when we turn our attention to the Wadden Islands dotting the northern coast of the Netherlands. These little islands sit between the stormy North Sea and the shallow Wadden Sea, a large estuary whose ecological value is unmatched in the Netherlands.
The coast of these islands is punctuated by a series of inlets connecting the two seas. Chaos reigns at these inlets, where strong tidal currents pass in and out, clashing with waves and whisking sandy shoals in and out of existence in unpredictable ways. This makes the inlets treacherous for ships, but also a challenge to simulate with our computer models and design nourishments for.
How, then, are are we meant to nourish the coast of these islands? We want to keep their inhabitants (and those on the nearby mainland) safe from flooding, but also need to be careful about inadvertently disrupting the vital ecological habitat of the Wadden Sea.
To answer that question, the Dutch government initiated the Kustgenese or Coastal Genesis project. In collaboration with several Dutch universities, companies, and research institutes, they set out to better understand how these tidal inlets work, and whether it is possible to effectively nourish them. The project focuses on Ameland Inlet, which is located between the islands of Ameland and Terschelling.
My PhD project is but a very tiny piece of the very large Kustgenese pie. My goal is to figure out specifically how the size of sand grains affects the paths that they take around tidal inlets. It has been the dream job for someone who has loved playing in the sand ever since he was a little kid. As a result, it has entailed a lot of time at my computer and in the laboratory, investigating the characteristics of the sand in Ameland inlet (that’s also why I have so many pictures of sand on this blog- we have a really cool microscope!). It is very fine sand and would be absolutely perfect for squidging your toes through on a hot day — if it weren’t at the bottom of the sea, that is:
Native sediment from Ameland ebb-tidal delta
In the spirit of ‘why not?’, the Dutch government decided that the best way to test whether nourishments would be effective in this environment was to just go ahead and try one out last year. They dredged up 5 million cubic metres of sand (that’s enough to fill 3 Skydomes, for anyone reading this back home in Toronto) and placed them just outside the inlet. A few months ago, one of the Dutch government officials showed up at a meeting with a “present” for me… some sand from the nourishment!
Nourishment sediment dredged from offshore and placed on Ameland ebb-tidal delta.
Needless to say, I was very excited. At first glance, it appears quite similar to the native sediment, so that means it should behave in a similar manner. Time will tell how the nourishment evolves- we are watching very closely!
There are plenty of things that I should probably be working on now instead of writing this (or alternatively, things that I should be doing instead of thinking about my work), but here we are. Why am I blogging? Here are five reasons:
Just for kicks I really enjoy writing for fun, but in academia, writing needs to be very clear, precise, and factual, so the format sometimes feels a bit constraining- there’s not as much room for colour, creativity, or opinions. But this is my website, so I can write whatever the heck I want!
Coastally curious Every day, I encounter so many utterly fascinating things in the course of my work, and this gives me a place to share them. This curiosity and love of learning is one of the main driving forces in my life, and I want to spread the joy! Besides, who doesn’t love beaches?
Practice makes perfect This is also a chance for me to practice my explaining my work to a broader audience. One of the biggest challenges facing scientists is communication: how can we distill the important messages from our work and explain them in a way that is relevant and persuasive? I’m introverted at heart and a little uncomfortable with shameless self-promotion, but it seems that “selling your science” to the public is becoming more and more necessary for researchers to survive these days.
Thinking out loud Writing is the medium in which I feel I can express myself most clearly, but in order to write clearly, you first need to think clearly. As I’ve learned from my adventures as a TA, you really don’t understand something until you can explain it to someone else. I hope that by writing here about my work, I can bring some order to the chaos of my muddled thoughts on muddy water…
The clock is ticking…
I am in the final third of my PhD now, and it’s time for me to write up all of my findings from the past few years into scientific articles. Most of the writing advice I have read suggests that the secret to productivity is developing a habit of writing a little bit every day. I’m hoping that this blog gets me into a groove that will spill over into my professional writing.
I have spent many months holed up in the laboratory counting green grains of sand. Last year we dumped over 1 ton of fluorescent, magnetic tracer sand into the North Sea, where the waves and tides then scattered them along the coast. We then spent the following weeks circling around on a boat to try and find it all again. We scooped up over 200 samples of sand from the seabed, then brought them back to the lab for analysis. We used a super strong magnet and blue UV light to separate the tracer (bright green) from the normal sand (looks grey or purple under UV light). This part is REALLY boring because most samples don’t have any tracer but we still have to look hard for it . But then we get to look at all the sand under a fancy microscope, which is my new favourite toy! Under the UV light, the tracer reminds me of little green constellations of stars in a purple night sky.
The tracers glow bright green under ultraviolet lights. On the top left we have a jar of normal beach sand sitting next to a jar of tracer. The distinction becomes clear once we place them under UV illumination (top right). This is especially important when we analyze samples taken from the seabed, where there may be only a few grains of tracer (bottom left). The fluorescent properties of the tracer help it stand out from normal sand (bottom right), which lets us count the individual grains.
Fortunately, we can use computers to count the individual grains and tell us their size. With this information, we can estimate how the size of a sand grain determines how far and fast it will travel. This is important for planning sand nourishments to protect the coast.
If we can figure out where the green tracer did (or didn’t) go, that will tell us how normal sand moves around on the Dutch coast. And this will hopefully keep our feet dry here in Delft for a long time to come!
Beautiful pink sand from the Gaspé Peninsula in Quebec (near Gignac)! I brought some of my sand collection with me when I came back to NL from Canada this Christmas because I wanted to see how it looked under the fancy microscope in our lab. So far, so cool!
This sand was taken from the little spit across the mouth of the embayment, just off Route 132:
Some really cool sand that my friend Claudia brought back from France. I especially like the purple shell fragments. This image is magnified 40x from the actual size. If anyone else goes to the beach on holiday, please bring me back some sand!
Sword Beach is located on the coast of Normandy in northern France, and is also where the British landed on D-Day in World War II.
Coastally Curious 