Sand: Nourishment at Ameland

The main protective barrier for the Netherlands against the threat of flooding from the sea is a row of colossal sand dunes and wide beaches that stretch the length of their coast.  However, that barrier is not completely natural —  since the Dutch coast is in a constant state of erosion, the sand in their coastal zone has to be continually replenished.  This replenishment takes the form of nourishments, which are essentially just massive piles of sand placed on beaches, dunes, or just offshore.  The Dutch are lucky, since the bottom of the North Sea is covered in sand for hundreds of kilometers in every direction, meaning that there is a ready supply available for this purpose.

Although we still have plenty to learn about how to construct these nourishments effectively and in an environmentally friendly way, we are starting to get the hang of it — at least for long, straight, sandy coastlines like in Holland.  However, this all gets a bit trickier when we turn our attention to the Wadden Islands dotting the northern coast of the Netherlands.  These little islands sit between the stormy North Sea and the shallow Wadden Sea, a large estuary whose ecological value is unmatched in the Netherlands.

The coast of these islands is punctuated by a series of inlets connecting the two seas.  Chaos reigns at these inlets, where strong tidal currents pass in and out, clashing with waves and whisking sandy shoals in and out of existence in unpredictable ways.  This makes the inlets treacherous for ships, but also a challenge to simulate with our computer models and design nourishments for.

How, then, are are we meant to nourish the coast of these islands?  We want to keep their inhabitants (and those on the nearby mainland) safe from flooding, but also need to be careful about inadvertently disrupting the vital ecological habitat of the Wadden Sea.

To answer that question, the Dutch government initiated the Kustgenese or Coastal Genesis project. In collaboration with several Dutch universities, companies, and research institutes, they set out to better understand how these tidal inlets work, and whether it is possible to effectively nourish them.  The project focuses on Ameland Inlet, which is located between the islands of Ameland and Terschelling.

 

My PhD project is but a very tiny piece of the very large Kustgenese pie.  My goal is to figure out specifically how the size of sand grains affects the paths that they take around tidal inlets.  It has been the dream job for someone who has loved playing in the sand ever since he was a little kid.  As a result, it has entailed a lot of time at my computer and in the laboratory, investigating the characteristics of the sand in Ameland inlet (that’s also why I have so many pictures of sand on this blog- we have a really cool microscope!).  It is very fine sand and would be absolutely perfect for squidging your toes through on a hot day — if it weren’t at the bottom of the sea, that is:

NativeSediment
Native sediment from Ameland ebb-tidal delta

In the spirit of ‘why not?’, the Dutch government decided that the best way to test whether nourishments would be effective in this environment was to just go ahead and try one out last year.  They dredged up 5 million cubic metres of sand (that’s enough to fill 3 Skydomes, for anyone reading this back home in Toronto) and placed them just outside the inlet.  A few months ago, one of the Dutch government officials showed up at a meeting with a “present” for me… some sand from the nourishment!

NourishmentSediment
Nourishment sediment dredged from offshore and placed on Ameland ebb-tidal delta.

Needless to say, I was very excited.  At first glance, it appears quite similar to the native sediment, so that means it should behave in a similar manner.  Time will tell how the nourishment evolves- we are watching very closely!

Sand: Tracer Sediment

I have spent many months holed up in the laboratory counting green grains of sand. Last year we dumped over 1 ton of fluorescent, magnetic tracer sand into the North Sea, where the waves and tides then scattered them along the coast. We then spent the following weeks circling around on a boat to try and find it all again. We scooped up over 200 samples of sand from the seabed, then brought them back to the lab for analysis. We used a super strong magnet and blue UV light to separate the tracer (bright green) from the normal sand (looks grey or purple under UV light). This part is REALLY boring because most samples don’t have any tracer but we still have to look hard for it . But then we get to look at all the sand under a fancy microscope, which is my new favourite toy! Under the UV light, the tracer reminds me of little green constellations of stars in a purple night sky.

TracerSediment

The tracers glow bright green under ultraviolet lights.  On the top left we have a jar of normal beach sand sitting next to a jar of tracer.  The distinction becomes clear once we place them under UV illumination (top right). This is especially important when we analyze samples taken from the seabed, where there may be only a few grains of tracer (bottom left). The fluorescent properties of the tracer help it stand out from normal sand (bottom right), which lets us count the individual grains.

Fortunately, we can use computers to count the individual grains and tell us their size.  With this information, we can estimate how the size of a sand grain determines how far and fast it will travel.  This is important for planning sand nourishments to protect the coast.

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If we can figure out where the green tracer did (or didn’t) go, that will tell us how normal sand moves around on the Dutch coast. And this will hopefully keep our feet dry here in Delft for a long time to come!