Understanding coastal erosion

Ever wondered what the erosion of protective coastal dunes means? As sea levels continue to rise and storms intensify, countries such as the Netherlands, where a third of land is below sea level, are becoming increasingly concerned about coastal erosion.

Words by Nane Steinhoff & Nortek
Photographs by TU Delft, Thomas Horig & Kimberly Jeffries

We are on a wide, sandy artificial beach called De Zandmotor beach at the northern shores of the Netherlands near The Hague. The wind is blowing and, in the distance, one’s eyes can just about make out what looks like natural dunes. Upon closer inspection, however, it becomes clear that these sandy structures are man-made. De Zandmotor beach is not only an interesting pilot project placing 25.5 million m3 of sand on and in front of an existing beach to create a new natural landscape and research the sand’s movements. The beach is also a location for an important project that is carried out by researchers of Delft University of Technology (TU Delft). To quantify the erosional impact of waves, the research team constructed two artificial dunes here as part of the RealDune/REFLEX project.

While RealDune will focus on studying the erosion process of the artificial dune and its development with time, Reflex will focus on the wave transformation from deep water to the waterline – an important boundary condition of the dune erosion process. The installation of acoustic experiments at different water depths and measuring systems on the dune itself will be important pillars of the research. 

This project is important because dune erosion during storm surges can lead to excessive damage to the dune system with devastating floods as a potential consequence.

With around 254km of the Dutch coast consisting of areas where dunes act as the primary sea defence, and sea levels steadily rising due to climate change, the safety of the hinterland depends on understanding and alleviating dune erosion.

The Dutch have embraced numerous techniques to keep water at bay, but their dune systems are the primary defense against the sea. Understanding how resilient these systems are to coastal erosion is essential. The Dutch government “uses dune erosion models as the basis of their management,“ Dr Jantien Rutten, a post-doctoral researcher at TU Delft, explains.

However, these models need to be validated. For example, Rutten and her colleagues at the RealDune/REFLEX project are particularly interested in understanding how infragravity waves play a role in eroding these protective dunes.

Most waves are generated by the wind blowing across the sea surface. When wind-generated waves reach the coast, they tend to break on the shore, dissipating their energy quickly. Infragravity waves arise as ocean swell interacts with the wind-generated waves. They tend to start very small but grow as they move toward the shore. “The energy is transferred from the wind-generated waves into long waves,” explains Dr Marion Tissier, assistant professor of Ocean Waves at TU Delft.

“When these longer waves reach the beach, they don’t lose as much energy as short, wind-generated waves, so they travel much further up the beach,” adds Rutten. Sometimes, infragravity waves travel so far up the beach they reach the dunes. The backwash then takes sand from the dune with it, causing erosion.

To measure the erosional impact of infragravity waves, the RealDune/REFLEX researchers needed to capture data from land and water across an entire storm season. That’s why the team constructed two artificial dunes at De Zandmotor beach. They placed a series of Acoustic Doppler current profilers, in short ADCPs, designed by NORTEK, in deeper waters a few kilometres offshore from the dunes, along with velocimetres in the shallower waters. The ADCP is an instrument that measures how fast water currents move across the water column with sound, using sound waves.

“This is important because we typically neglect the remotely generated infragravity waves when assessing coastal safety,” says Tissier, noting that this omission could mean models are underpredicting dune erosion. The team has begun the processing of their data. “We are starting to look at separating the different types of infragravity waves and quantifying their direction and energy,” Tissier says.

As climate change exacerbates coastal erosion around the world, researchers in the Netherlands and beyond will certainly be awaiting to see the results of the RealDune/REFLEX project.

Photographs by TU Delft, Thomas Horig & Kimberly Jeffries

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