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Wednesday, May 23, 2012

Wind and sand

The wind has been blowing at 20-30 knots out of the northwest since yesterday afternoon.  Not atypical for Bodega Bay in the spring!

The strong winds created some intriguing sand patterns at Doran Beach this morning (23 May 2012).

Harrow marks on the upper beach 

Although I'm not a geomorphologist, I've read that harrow marks often form with a combination of wet sand and strong winds.  With differential drying, the wet sand resists the wind and the drier sand accumulates in the lee.  To my eye, the resulting raised structures look similar to comets with streaming tails.

Here's a close-up.  Can you tell which way the wind was blowing?

Above, the wind was blowing from left to right. 

Some alternative names for these structures include sand shadows, sand tails, wind drifts, and wind shadows.

Here's another photo, taken from the opposite direction.

Sometimes harrow marks start with a small object like a shell fragment, a stone, or a piece of wood.  The object causes turbulence, and the wind velocity is reduced on the downwind side causing sand deposition.

When I zoomed in to one picture, I realized that you could see the sand streaming by.  Look for the fine lines set off by the darker backgrounds on the sides of the harrow marks. 

Several lines look like they're arching upward, a clue that some of the sand grains are being transported across the surface by saltation (leaping).

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