A month ago, a team of researchers from different universities set out to investigate how the coast of the wadden sea will change and what can be done to protect it. In this article, the team describes their experiences and findings from the last month!
For more information, see the previous article
Current meters and sediment tracers
On Monday of the first week, the team placed current meters on the tidal watershed halfway between Holwerd and Ameland. This allows them to "close the loop" and see how much water and sediment is coming in and out of the basin, not just through the main inlet but also around the sides from other basins. Nobody has measured this before so the results will be interesting!
On Tuesday, they began a tracer study in partnership with the UK firm Partrac. How do individual grains of sand move along beaches or the seabed? This is a challenging problem to solve using traditional measurement techniques, so they turned to particle tracking for answers.
Within the white buckets is glow-in-the-dark, magnetic sand (“tracer”), which was placed on the seabed. The team then dug up samples of sand off the seabed every day for two weeks to see where the tracer had travelled, but it was like looking for a needle in a 50 km2 haystack!
In the last day of the week the team scooped up 92 sediment samples off the seabed. Some of them contained the tracer which was easy to notice due to the nice constellation of glowing green particles in the sand when lit by UV light.
The tracer is also magnetic and the team had placed buoys around the dumping site to see how much sediment was also travelling in the water column with the current instead of over the bed.
After a few days, they retrieved the magnets and found out that it had worked; They had tracers on them. The experiment was going well and the result will hopefully tell us more about how sediment moves through Ameland Inlet now and how it will respond to changes in the future.
The Fast and the Furious Tidal Currents: Ameland Drifters
Twice a day, the tides push and pull 480,000,000 cubic metres of water through Ameland Inlet. The violent waves of the North Sea also batter the inlet, changing in shape and size as they encounter the rapid tidal currents. This wave-current interaction makes it really difficult to predict how sediment will be transported around the inlet.
To get to the bottom of this, TU Delft PhD student Floris de Wit and his team are deploying an armada of small buoys or drifters that will float with the tides and give a sense of how currents vary in space and time.
Surviving the storm
A storm was going over the Netherlands in the third week, including the team’s site!
The picture above shows the maximum wave height over a period of 10 minutes. The frames were already hit by waves over 6m. Would they find them again? The good news was that they were all there and later on carefully placed back at their positions again.
Seabed Boxcores; sediment, shells and shrimps
In the 4th week, SEAWAD PhD students Harriëtte Meijer-Holzhauer and Stuart Pearson joined on board the Dutch Coast Guard vessel, the Terschelling, to take boxcores over the entire ebb tidal delta. These boxcores are undisturbed "cakes" of sand extracted from the seabed. In this way, they can learn more about the different creatures living on and within the seabed ("benthic" species). They found many different kinds of shells, algae, eels, shrimp, and sea urchins! Which ones live where? And does that have a relation with the waves and currents or type of sand that they find there?
They can also use these cores to see how the size of sand or mud grains varies across Ameland Inlet. In this way, they can better understand how sand is moving in and out of the Wadden Sea today, and make predictions about how that could change in the future with climate change and sea level rise.
The SEAWAD team
This is the SEAWAD team! From left to right: Harriëtte Meijer-Holzhauer (University of Twente), Laura Brakenhoff (Utrecht University), Floris de Wit (TU Delft) and Stuart Pearson (TU Delft), Together, they hope to combine their findings and build a more complete picture of the processes at work on ebb-tidal deltas.