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.
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.
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]
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.
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!