Sardinia, Shifting Sand Dunes
Size:  (not chosen)
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The movement of coastal dunes can be seen at Sardinia. It can easily be reached with a normal sedan as the access road is tarred. Even with a difficulty of 2.5 younger kids will not have to much trouble getting to ground zero, as it is only a short climb.
Sand Dune basics:
There are two sides to a sand dune. The “windward” side, and the “slip face” side. The windward side is the side of the dune which the wind blows on, and the wind compacts the sand much more on this side than on the slip face side. The area between two dunes is called a “trough”.
How do Sand Dunes move:
Sand can only be moved by strong, steady winds. The air must be moving at least at 24 kilometres per hour (or 15 miles per hour) to be able to pick up the sand grains.
For dunes to move, the winds must blow more or less constantly from the same direction. As the wind blows, it pushes the sand ahead of it.
Sand is not as easy to move as you might think. Even very strong winds can't lift the sand much higher than about a meter of the ground. As the wind blows, it lifts small sand grains a few centimetres off the ground, and then drops them. When the grains hit the ground, they bump into other sand grains and causes them to jump up and be caught by the wind, and the process is repeated. It's almost as if the sand is playing leap-frog, jumping and bumping each other along. This kind of jumping movement is called “saltation”.
As the tiny sand grains slowly work their way up the windward dune face, they finally reach the “crest”, or top of the dune. They fall over the crest and start to pile up because they are protected from the wind. Now gravity takes over in moving the dune. As more and more sand grains pile up, the angle of the slip face becomes steeper and steeper. A pile of loose material, like sand, can only hold a slope of around 32 to 34 degrees. When the slope gets greater than about 34 degrees, gravity pulls the loose sand down and small avalanches will occur. The sand might run down the slip face side like a waterfall, or the whole side might slip at once. When the entire face of the dune slips, it's called “slumping”.
How fast a sand dune moves depends on a number of things where the speed of the wind is a biggest factor. Wind that is blowing at 70 kilometres per hour will move more sand than a wind at 25 kilometres per hour. The size of the dune is also important. Smaller dunes with less sand move much quicker than larger ones.
The vegetation on the dune can also play an important factor. The dunes get caught on the plants that grow in the area, and this can slow the movement of the dune down.
Some info also supplied by environmental management from Nelson Mandela Metro.
Sardinia Bay Marine Reserve
A marine protected area of 55 ha was proclaimed in 1974 and, to provide landward protection for this reserve, two adjacent areas, Sylvic Reserve at 78 ha, and Sardinia Bay Reserve at 320 ha, were consolidated in 1980 to form the Sardinia Bay Nature reserve.
The beach is still relatively undeveloped and uncrowded. Few visitors know about this little haven, but more find out about it with each passing year. While doing this cache you are bound to see people swimming, the older generation walking their dogs, youngsters kite surfing, and the odd person sandboarding down the dunes. If you are really lucky you might see a few horse riders along the beach as well.
In the case of Sardinia bay the wind comes from the ocean and thus the movement of the dunes are in an inland direction, and they move between 2 and 5 meters per year. In about 50 years time the upper car park will be run over by the dune. Signs of the impact the dunes are having can already be seen by the amount of sand on the tar road which goes towards the lower car park. Measures taken, in 2007 and 2008 to halt the movement of the dunes, by clearing with a front end loader have proved expensive and fruitless, in the long term. This intervention is also not in line with recent legislation to maintain the natural attributes of the of coastal landscapes, and not to interfere with the natural movement of the sand dunes.
The height of the dune where I took the measurement, it was measured to be 16 meters above sea level. Over time this will change, as the dune moves along.
To be eligible to log this cache you must do the following:
1) Take a picture of yourself with your GPSr at the cache location where one can see part of the dune, and upload it to the web site. (Optional)
2) Place your GPSr on the sand at ground zero, and take a height measurement. This measurement can be added to the web site as well, so other visitors can see in which direction the height is changing. (If your GPSr can measure height above sea level)
In an email:
3) Estimate the height the wind is lifting the grains of the ground.
4) Explain, which side of the dune it is easier to walk on and why you think it is easier to walk on that side.
5) In your opinion is it good or bad to allow sand boarders on these dunes, and why do you say this.
6) On which side (sea side or car park side) do the avalanches occur? Were you able to create a small avalanche of your own?
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