Movement of Coastal Sand by Waves

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How Waves Move Coastal Sand
Beach Sand Movement
Determine if the sand is moving.


As the essentials of this experiment are such that may be easily carried out by a 4th grader, please excuse the author for starting with some of the fundamentals of doing a scientific project. If you are an older student with more math skills you can carry this project much, much further.

There are a number of steps in doing science. These are often given fancy names like "hypothesis" and "theory," but for rapid understanding, only common words will be used here. For the first step, we have to start with a "wonder" before we can set up an experiment. Did you ever go to the beach and wonder how it is that all the little grains of sand all around you seem to be the same size. Afterall, were you to take a hammer to a rock, you'd get all sorts of sizes from chunks all the way down to rock dust. But beach sand is composed of little pieces of rock all the same size. Amazing! But how did it get that way? Where are the big chunks, and where is the dust?

THE HYPOTHESIS. Next we must have an hypothesis - a guess as to how it might have come about. When you look at the beach sand it is as if someone sifted the sand through a sieve to let all the dust go through the screen, and then picked away all the chunks leaving uniform sand sitting on the sifting screen. Of course, this is not how it was done, so how did nature do it and perhaps you can do it that way also. Let us guess that some sort of sifting or sorting was actually done by nature. So you have guess or "hypothesis" number one!

Now how would you guess that nature would do all that sorting? Here comes hypothesis number two! As you sit on the beach wiggling your toes in the sand, you watch the waves lap against the shore. Lap, lap, lap, and they lap endlessly. If you look closely, you will see the incoming wave move sand grains in toward shore, and as the wave goes out, the grains roll out also. Back and forth the waves go; back and forth the grains move. You hypothesize that the wave motion must have something to do with sorting.

However as you watch the waves moving in and out, you cannot see any sorting because all the sand grains seem to be about the same size. Perhaps they have already been sorted, and that nature continues sorting anyway - seeming to just make sure the job was done right. But what if you had some sand that was a mixture of sizes. Would the waves separate them? Would they become sorted into different sizes - big ones over there, medium sized ones here, and smaller ones over there?

This would really be nice if the different sizes had different colors - then a before and after pair of photographs might tell the whole story and get you a wonderful grade! Perhaps at a pet shop you can find different sizes of sand - and in different colors, too! Check them carefully to make sure that they are not all the same size but just dyed in different colors. Then you can either pester your parents to take you to the beach to test them out (no playing and having fun at the beach is allowed, right?).

If you cannot find the different colored sands of different sizes, then you can make your own. Put on safety goggles, and start whamming some gravel with a hammer. Scoop up all the pieces - even the powder. Try that in your sheet cake pan. If you want to do the "sorting" at the beach, make sure that the rock you have crushed is of a markedly different color than that of the beach sand.

THE EXPERIMENT: At first, you might get a sheet cake pan, and put some of the mix of the various sands in the pan at one end and then slowly slosh water back and forth in the pan. Do the sands become sorted? Take pictures!

Next you should try real beach. As you will see this is so easy and so much fun, you might want to try two or three different beaches.

But what should you do at the beach?

At the beach you might hammer in four stakes (the black circles in this diagram) to be used a references. Between them you would lay down lines of the mixed sands.

If the waves actually do sort the sands by sizes, you might see patterns such as shown here:

CONCLUSIONS: In the hypothesis part you have asked a lot of questions. Now you have results as you see the various colors in the waves before you, or on your photographs. So let's look at the questions again. Where do the fine dusts go? Do they go further up onto the beach or do they get washed out to sea? What about the larger rocks? And do you see any evidence of any "lateral" movement of the sand? Do the sands move sideways at all? You should be able to answer all these questions. And you might write arrows on a photograph of your beach showing which way the sands are moving. You picture might look something like the one next to the title, at the beginning of this page.

PUBLISH: You haven't finished your science project unless you write up your work and present it as a report or a poster or, if you are a professional scientist, in a scientific magazine called a journal. Your report on this beach sand subject ought to have lots of pictures! And next time you or any of your classmates visit a beach you will understand the sorting that has taken place.

This is just a starter experiment. Surely, as you were getting results, you were seeing other things that you could do. More questions were coming to mind, and you had your guesses - oops! - hypotheses that you could test in experiments, and then draw even newer conclusions and finally publish a second report. That's how science marches on - step by step.


The picture next to this page's title is of Cinnamon Bay in the U.S. Virgin Islands National Park.


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