Have you ever walked along a windy beach and noticed shells sitting atop small peaks of sand, like a miniature mountain range? Does a shell “protect” the sand underneath, or does the sand pile up behind it? Does the shell actually cause more erosion around it? How does a shell affect the way sand moves along beaches? What happens when you have millions of shells along a beach? Does that affect the way the beach as a whole erodes? Now what about the parts of a beach that we can’t see, below the water?
These might seem like the thoughts of an idle beachgoer, but are actually essential to helping us understand how to sustainably protect our coasts. As part of the TRAILS project, Tjitske Kooistra is investigating how sand nourishments influence sensitive ecosystems on the Dutch coast. In order to do that, we need to understand how shells and sand interact at the bottom of the sea, since there are many locations along the Dutch coast where shells make up a significant portion of the beach material. This is really difficult to understand in the field, so to figure this out in a more controlled setting, Tjitske planned a series of lab experiments and we recruited Steven Haarbosch to carry them out.
After months of hard work shovelling sand and crunching numbers, Steven successfully defended his thesis, “The influence of bivalve shells of different shapes and sizes on current-driven sediment transport“! He conducted excellent, tremendously useful research, and I am really proud of what he accomplished.
To understand the effects of water on sand, we typically use a flume, which is like a big elongated bathtub with a pump or wavemaking paddle to move the water around. This sort of setup allows us to replicate natural physical processes in a more controlled environment than you typically find at a real beach. In the laboratory at NIOZ Yerseke (the Royal Netherlands Institute for Sea Research), Steven and Tjitske focused on the effect of currents (i.e., a relatively steady flow like those caused by tides) on shells and sand, whereas their colleague Jorn Bosma (PhD candidate at Utrecht University) concentrated his efforts on the same sort of waves that you see at the beach.
These experiments involved a lot of careful planning to choose realistic combinations of sand, shells, and water conditions that could then be scaled up to real-world conditions in places like Ameland. Building on previous work conducted in the same facility, they used cameras and miniature sonar probes to measure the currents and track the migration of sand and shells.
They then asked three questions:
- Do shells make it harder for the sand to move? Their first tests examined how strong the currents needed to be to move the sand and whether the shells made it easier or more difficult.
- Does less sand move when shells are present? By placing a bucket at the end of the flume, they could catch the sand and shells moving along the bed and see how much accumulated after an hour.
- Does the species (and shape) of shells make a difference? By testing different species (long skinny ones, short round ones, and mixtures of the two), they could investigate whether shell shape had an important effect.
Steven’s findings suggest that shells have two main effects: stabilizing the seabed, but also making it rougher. That increase of roughness enhances turbulence, which can actually increase erosion to a small degree. The shape and species of the shells indeed also affect the turbulence generated. In general, however, the presence of shells limits the amount of sand transported along the bed.
Now that Steven has graduated, he will join the Environmental Engineering team at Van Oord, where they are lucky to have him. If you’re curious about the details of these experiments, stay tuned for more news from Tjitske Kooistra about the next steps in our shelly research!
Coastally Curious 