The Cappuccino Effect

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.

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Of Sediment and Seedlings

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.

Last Friday, Femke successfully defended her thesis, “Modelling sediment and propagule pathways to improve mangrove rehabilitation: A case study of the pilot project in Demak, Indonesia“. She developed a numerical model of a site in Indonesia to simulate the motion of rivers and tides there, and then used the SedTRAILS model to visualize and interpret the pathways of sediment and mangrove propagules.

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Ch-ch-ch-ch-changes

Big news to start 2023: I am now an Assistant Professor in Coastal Engineering here at TU Delft! An opening appeared online last summer, and after weeks of preparing applications, several rounds of interviews and a teaching demonstration, and a lot of waiting, I finally got the good news. This has been my dream job for a long time and I can’t believe it came true.

Officially, my new portfolio will focus on “Climate-Robust Deltas”. How does sediment contribute to the strength and adaptability of our coasts and deltas against the effects of sea level rise and climate change? In my research we approach this gigantic problem by quantifying sediment pathways and connectivity for strategic placement of sediment, using a combination of numerical modelling and field measurements. In the coming years, I hope to build up a diverse team of enthusiastic, coastally curious researchers to tackle these challenges. Stay tuned for opportunities to join our group!

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Tracking Sand that Hides from the Sun

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?

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Going Where No Cyclone Has Gone Before

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!

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Sediment Pathways on Ebb-Tidal Deltas

After 5 years of blood, sweat, and tears, I present to you my PhD thesis: Sediment Pathways on Ebb-Tidal Deltas: New Tools and Techniques for Analysis! I will defend my PhD on March 8th.

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…

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Custom Colourmapping

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.

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Weird waves cause big trouble on small lands in the middle of the big blue wet thing

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

2. Pearson, S.G. (2016). Predicting Wave-Induced Flooding on Low-Lying Tropical Islands Using a Bayesian Network. MSc Thesis, Delft University of Technology. http://resolver.tudelft.nl/uuid:c3988f4b-99f8-4936-9504-261b32bb0cd1

3. Roelvink, F.E., Storlazzi, C.D., van Dongeren, A.R., & Pearson, S.G. (2021). Coral reef restorations can be optimized to reduce coastal flooding hazards. Frontiers in Marine Science, 8, 440. https://doi.org/10.3389/fmars.2021.653945

4. Scott, F., Antolinez, J.A., McCall, R., Storlazzi, C.D., Reniers, A.J.H.M., & Pearson, S.G. (2020). Hydro-morphological characterization of coral reefs for wave runup prediction. Frontiers in Marine Science, 7, 361. https://doi.org/10.3389/fmars.2020.00361

Sediment Pathways in Vancouver

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.

Modelled sediment transport pathways in Burrard Inlet. The red arrows highlight key patterns in the SedTRAILS particle trajectories. Burrard inlet is characterized by strong flows through the narrowest points of the fjord, and large eddies in the wider areas. Source: Meijers (2021).

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.

Conceptual diagram showing the dominant sediment pathways in the Inner Harbour. Source: Meijers (2021).

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.

Comparison of the present-day shoreline with the high and low tide lines from 1792, prior to colonization by European settlers. The Second Narrows are so narrow because they were formed by the delta of Seymour River and Lynn Creek. The area has since been dredged and walled off for the construction of the port and to create log booming grounds. Source: Meijers (2021).

These changes are especially evident when we compare satellite photos from the present day with the oldest available images from the 1940s.

Second Narrows in the 1940s and 2021. Please forgive my crappy georeferencing, I eyeballed it. Source: City of Vancouver and Google Earth.

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-Vancouver collaborations going!

Predicting the Unpredictable

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.

Different nourishment designs tested by Denzel. If you place the sand in a more dynamic area like a channel, it can have a much wider effect on the rest 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!