“Too much work, hiking in sand”
Sunday, 17. February 2008, 22:56:55
That's what a friend in the US said when I told him my friend Carla and I were going to do a hike along the beach from Zandvoort to IJmuiden. I explained that you can just walk along the waterline, where the sand is hard.
Kentish Plover (Charadrius alexandrinus)
We did go, on Friday, and the weather was cold, but glorious: a cold wind from the east, getting gradually stronger (so much so that my Aussie hat took a dip!), but the sky was clear and the still-low sun made the patterns in the sand come out beautifully. When we started out, there was an upcoming tide, and there were lots of birds, mainly herring gulls, along the water line, feeding on the shellfish being washed ashore. There were even a few crows doing the same. Running in-between them here and there was a Kentish Plover (Dutch: Strandplevier): I love those little guys, running along with their body strictly at the same height, while their legs are running, outstretched, like a little wind-up toy.
The beach is wide and flat here, meaning “along the water line” is actually more like at least half the width of the beach where the sand is firm and hiking is easy. There are a few little creeks, forming temporary islands, but you can find your way across them and stay close to the sea that way. The sand is firm, but almost springy here, making hiking not just easy, but actually quite comfortable. But if dry sand is hard to walk in, because the grains shift, and we cannot even walk on water, why is wet sand firm?
The grains are linked by liquid bridges shaped like a double-ended trumpet. When enough of these bridges form, the mixture is able to hold its shape.
By one of those happy coincidences, yesterday I found the latest (16 Feb) issue of the New Scientist in my mailbox. And on page 18, I found a short news item that explains just why wet sand is firm, and retains its shape so nicely in sandcastles: How a sprinkle of water helps sandcastles stand firm. The researchers at the Max Plack Institute for Dynamics and Self-Organization also found that once there are enough liquid bridges to make the sand-water mixture hold its form, unless the mixture becomes saturated, adding more liquid does not make much difference.
Even where the water washes over the sand, it's still firm enough to walk on
And while we did not see (or make) any sand castles — it was much too cold for that — we found that even where the upcoming tide washed over the sand (so it would have been at the point of becoming saturated) it was still firm enough to walk on, and would only begin to “give” when you stand still. But when more and more water washes over it, it starts to become somewhat “fluid” itself, and even a seagull will leave its footprints in the wet sand.
Once in IJmuiden we took the bus only as far as the ferry: we love the trip over the water between IJmuiden and Amsterdam — much nicer than taking the bus! A tasty meal at an Indian restaurant was the perfect conclusion of a perfect day.
