Nature-Based Solutions for Salt Intrusion

When you think of coastal climate change impacts, what do you think of? Probably sea level rise, changes in wave climate or storminess, or loss of coastal habitat. But a silent intruder lurks: salty seawater, sneaking into estuaries and rendering our precious freshwater supplies undrinkable. The threat of estuarine salinity on deltas and coastal regions is one that I greatly underestimated, even as someone working in this field for over a decade. That is, until Gijs Hendrickx came along.

Last week Gijs Hendrickx successfully defended his PhD on Nature-Based Solutions to Mitigate Salt Intrusion! I think I speak for everyone involved when I say that we are SO proud of him.

What a journey it has been! Gijs started working with Peter Herman and Stefan Aarninkhof in November 2020 – the middle of the pandemic, no less. In one of their first meetings, Peter gave Gijs a bit of a shock when he put forth the null hypothesis:

“Nature-based solutions for salt intrusion DO NOT EXIST.”

This didn’t stop Gijs from trying to find them, and most importantly, it forced him to stay clear and critical about what can and cannot work. We are very pleased with the way he brought this into practice, coming up with a thoughtful and nuanced dissertation on possible answers to this problem.

To even begin contemplating solutions, he first needed to understand the problem: how does salt intrusion work in all different kinds of estuary? EVERY shape and size. Nobody could ever finish a PhD if they had to run simulations of them all. Peter suggested Latin Hypercube Sampling as a starting point, and that sent Gijs down a rabbit hole of possibilities.

Working closely with Jose Antolinez, he found a way to ensure that you didn’t need to simulate every estuary – only the ones that added useful information. I wish we had an approach like this when I had to simulate 130,000 XBeach models as part of my research on coral reefs. With this fantastic machine constructed, he then used his virtual laboratory to test an unprecedented range of possible estuaries. Armed with these tools, he could then answer all kinds of questions.

Halfway through Gijs’s PhD, Peter retired, and he gave me the great honour and responsibility of taking over his role as Gijs’s supervisor. Enormous shoes to fill! Gijs and I waded in with cautious optimism, but soon plunged in together with scientific glee.

Now to get cracking on this nature-based solution thing: what happens if you put a sill (a sort of speedbump on the riverbed)? Gijs went to New Orleans to work with Ehab Meselhe and his team at Tulane University on this. The resulting paper, An Earthen Sill as a Measure to Mitigate Salt Intrusion in Estuaries was published in Estuaries & Coasts and recently selected to be highlighted in the US Coastal Engineering Research Foundation newsletter, Coastal and Estuarine Science News (CESN). The relevance of this topic and its importance for society was demonstrated several months later, when New Orleans was hit by an acute salinity-driven drinking water shortage of exactly the kind Gijs studied.

As a big fan of mudflats, one of the most interesting parts of Gijs’s research was our exploration of the role that such intertidal flats play on salt intrusion. Gijs found that it really depends on how stratified or well-mixed an estuary is (i.e., is it layered like a cappuccino with fresh river water on top and salty seawater below, or is everything stirred together into a brackish mix by the tides?). Expanding the tidal flats could indeed be a useful solution for reducing salinity intrusion if the estuary is stratified, but could have the opposite effect if the estuary is well-mixed. Who ever said that implementing nature-based solutions for salt intrusion would be easy?

Not content to limit himself to theoretical explorations of his topic and eager to make real-world impacts, Gijs investigated the effect of salinity intrusion measures on port logistics (Chapter 6) and ecosystems (Chapter 7). In doing so, he addressed pressing challenges facing the Dutch economy and environment. Gijs has also given careful consideration to the implications of his research for science and society, with direct input to the Dutch government and industry.

But he didn’t stop there: at the end of his PhD he was determined to pull together the different threads of his research into a coherent philosophy for engineering practice in uncertain times. Gijs argued that future engineering solutions need to be diverse, adaptable, and robust to withstand what lies ahead with climate change. The result is a thesis that “DAREd” to contemplate more “Ph” than most PhDs in our field (see Chapter 8).

Very much in the driver’s seat, he juggled 4 different supervisors, actively managing our agendas,  and making it crystal clear what was needed when, so that we could meet all of his goals. It took a superb combination of technical, organizational, and social skills to make this happen. Gijs had a whole village worth of collaborators, here in Delft, and also internationally. Of note, he mentored students as they started their own research careers, and made important educational contributions to the Building with Nature course at here TU Delft.

Gijs also collaborated intensely within the larger SALTIsolutions consortium. We would like to thank NWO for funding this project, and particularly to thank Julie Pietrzak for her efforts in setting up and coordinating SALTIsolutions. This project offered the perfect framework for Gijs to excel in his research.

Scientific excellence is one thing, but Gijs also tried to help build a better research community. Within our department, he led coastal section meetings on a variety of topics and was a consistent social presence – practically part of the furniture in PSOR! Moreover, Gijs has always been vocal in his concern for his colleagues’ well-being and PhD mental health, and has shown his integrity when confronted by ethical dilemmas.  

Of course, I would be remiss if I didn’t mention just how much FUN we had along the way. We all really enjoyed our scientific sparring sessions with Gijs – he always kept us on our toes.

All in all, Gijs exemplifies TU Delft’s vision of “highly qualified engineers who are creative, innovative, and responsible, pushing the boundaries of technical sciences to solve complex and urgent societal challenges”.

I wish Gijs the best of luck in the next steps of his career as a consultant and researcher at HKV.