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.
What is the fate of nourished sand? What are the pathways of sediment on an ebb-tidal delta or in a tidal basin? What role does sediment play in the UNESCO-world heritage area of the Wadden Sea? We are looking for a curious and motivated postdoc to work with us on an exciting project here at TU Delft in the Netherlands. The main goal of this position is to develop and test novel simulation approaches to trace pathways of different sediment types, and to predict sediment dispersal and morphodynamic responses to different nourishment strategies.
I also worked on this project as a postdoc until recently starting a new position, and I really enjoyed both the topic and teammates. Now you have the opportunity to join our team and continue developing this research! A summary of previous work on the project can be found here: https://coastallycurious.com/2022/12/15/tracking-sand-that-hides-from-the-sun/
Come join our TRAILS team! Feel free to get in touch with us if you have any questions. Applications close March 12th, 2023!
Mangrove forests protect tropical coastlines around the world from the effects of waves, in addition to providing valuable habitat for countless species. As such, their preservation and restoration is a key element of many plans for improving coastal resilience against flooding and erosion in the face of climate change. However, you can’t *just plant* a mangrove forest anywhere – mangroves are extremely picky, dancing on the edge of the intertidal zone where they get just wet enough but never too wet for too long. They also need safe, stable shorelines for their seedlings to take root and grow stronger, without too many waves and with just the right sort of muddy conditions to make a comfortable home.
Mangroves drop their seeds (called propagules) in the water, which then float around with the currents for days to weeks until they find a suitable home. But which pathways do these mangrove seedlings take as they float along the coast? Are those the same pathways that sand and mud take? These are questions that we need to answer in order to make better decisions about mangrove restoration. To get to the bottom of this, we recruited Femke Bisschop.
Keeping Dutch feet dry is mainly done by placing piles of sand along the coast as “nourishments”. These nourishments build out the beaches and dunes to act as a protective buffer against storms. However, as was recently pointed out by an official at Rijkswaterstaat, the Dutch water ministry, the Hamvraag or “bacon question” is still “where the heck does all that sand actually go?”
Knowing where nourished sand goes is important for understanding the ecological impact of nourishments, as well as their effectiveness. If you want your sand to reach a certain destination, how much of it actually gets there and how quickly?
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…
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.
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!
Sources:
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]
What pathways does sediment take as it travels through an estuary? Yesterday, Laurie van Gijzen defended her thesis, entitled “Sediment Pathways and Connectivity in San Francisco South Bay“. Laurie is one of the master’s students that I supervise, and she has done a great job on this project.
San Francisco Bay is a massive estuary, with over six million people living nearby. In addition to San Francisco, Silicon Valley sits on its shores. Some of the biggest tech companies in the world like Google and Facebook have their head offices right next to the Bay. For over 150 years, the ecological health of the bay has deteriorated, in part due to land reclamations and contaminated sediment from gold mining. The dynamics of San Francisco thus have a huge economic, social, and environmental impact.
Laurie’s work focused on calibrating and improving a sediment transport model of the bay, in order to track the pathways of fine sediment (i.e., mud). She worked with a notoriously fickle model (DELWAQ) and succeeded in greatly improving its calibration.
Laurie’s thesis summarized into a single diagram (Figure 6.1 from her report). She shows the dominant sediment pathways as dark arrows, and the net accumulation (import, in orange) or depletion (export, in blue). Also indicated are the dominant physical processes responsible for sediment transport in the different parts of the bay. The baroclinic processes mentioned here are currents resulting from density differences in seawater due to changes in salinity and temperature.
Another cool thing about her work is that Laurie was the first person to apply the coastal sediment connectivity framework that I have been developing! She was able to use this to identify key transport pathways and critical locations in the bay. It was extremely helpful for my research, as it gives us a proof of concept that our framework is applicable to multiple sites and can tell us something useful.
Her work was also accepted for a presentation at the NCK Days conference, which was meant to be held this week in Den Helder, but was cancelled due to ongoing societal chaos. Great job, Laurie!
Coastally Curious 
