Sand castles
Last weekend I went surfing with my son to a beach called Hodevik on the west coast of Norway. The waves weren't so big so we spent quite a bit of time sat on the beach waiting for the tide to change in the hope that when the water went out the waves would pick up, they didn't but as we sat on the beach we noticed something interesting in a little stream running across the sand. The sand on the stream bed had been formed into ripples and as we watched we could see the sand grains being moved across the bottom. At other places holes had been carved in the sand and grains were being sucked out. The process of ripple building is initiated when the speed of the water is enough to pick up grains from the bottom, these are taken downstream and dumped, as they are dumped more sand is disturbed and the process goes on. One thing that didn't seem to make sense was that since the peaks stick up they would be most likely to be knocked down so how come they remain. I found the answer in Wikipedia, the peaks are continually being blown down as the sand is shifted downstream, the ripples are not static. The wavelength is related to the water speed, faster water will carry the sand further resulting in a longer wavelength. I also found some evidence that the water can become turbulent as it runs over the peak, this causes sand to be picked up on the downstream side of the peak, This is what I was observing in the hole.