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.

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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!

Sand: Sword Beach, France

Some really cool sand that my friend Claudia brought back from France. I especially like the purple shell fragments. This image is magnified 40x from the actual size. If anyone else goes to the beach on holiday, please bring me back some sand!

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Sword Beach is located on the coast of Normandy in northern France, and is also where the British landed on D-Day in World War II.

An Ode to the LISST

This is a poem about the Laser In-Situ Scattering and Transmissometry or LISST instrument, which we use for measuring sand and mud floating through the water.  I wrote it in response to a challenge to rap about what we learned during a workshop on estuaries last summer.  I had some fun with it so I thought I’d share…

And now a poem about the LISST
It is a great solution
To measure stuff that’s floating
And its grain size distribution

When processing your measurements
You must beware the floc!
Since if you don’t account for it
You’re in for quite a shock

If there seems like too much mud
We should have some suspicion
Before all else, we have to check
The optical transmission

“We have an awful lot of sand!”
Is this hallucination?
First thing’s first: we should have checked
Our background concentration

We sometimes see before our eyes
Large particles appearin’
When gradients of salt are high
It is the fault of Schlieren

So from the depths of Ameland,
A lesson that does matter:
When working with a fancy LISST
Don’t blindly trust your data!

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The mighty LISST: a Laser In-Situ Scattering and Transmissometry instrument, which shines a small laser through the water.  When the beam hits a particle floating by, it scatters and makes a unique pattern of light and shadow, depending on how large the particle is.  We can then interpret these patterns to estimate the size of the sand or mud that are floating through the water.