Written by the TreasureGuide for the exclusive use of treasurebeachesreport.BlogSpot.com.
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Cup of Water and Sand with Coin and Ring on Top. Yellow line indicates the surface of the sand. |
I once did a post on common metal detecting misconceptions. One of those is the idea that coins sink in sand.
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| Quarter and Ring on Top of Sand. |
So to provide a demonstration, I got a cup, filled it with sand and water, and put a coin and gold ring on top. No surprise, after days of waiting, there was no sinking. The coin and ring stayed right on top. Coins don't sink in sand and neither did the ring. Who cares? That isn't like a beach.
The distinction is important. In what ways is the sand in the cup unlike that on the beach? And how do coins and dropped items end up buried deeply in the sand?
Without a force that disturbs the sand, the coins will stay right on top, unlike a coin falling through still water or through air. So, as I reported in an old post, I continued the informal experiement (See The Treasure Beaches Report Direct From Florida's Treasure Coast.: 9/4/14 Report - How Coins and Rings Sink In Beach Sand. Targets Only Sink As Far As The Disturbed Layer.)
I put my hand over the cup and moved the cup in a semi-circular motion so that the water moved. When the water moved the very top layer of sand lifted, and the suspended sand moved with the water while the objects stayed in place. When I stopped moving the cup, a little sand covered the objects. With ga little, only the sand that was above the green line on the cup moved. This is important, and I'll get back to it.
I could see how deeply the objects got buried either by sticking my finger in and finding the objects or I tilting the cup and moving it so that the sand then uncovered one side and one object. I could then see where the previously covered object had stopped sinking.
Here is an important point. The amount of sand that moved when I moved the cup was directly related to how vigorously I moved the cup and therefore how fast the water moved. That is no surprise either.
Years ago I posted a chart that shows trigger points and the amount of force required to move different particles such as sand, pebbles, clary, etc.
The graph from that post is shown below. See The Treasure Beaches Report Direct From Florida's Treasure Coast.: 6/28/15 Report - Trigger Points, Drop Points, Water Velocity and How Things Move And Get Sorted On A Beach.. I discussed the chart and implications in that previous post.
When moving the cup gently, a small amount of sand moved - only a thin layer - approximately the amount of sand above the green line, which I drew on cup. The top of that layer moved the most.
The fastest moving sand was clearly suspended, and the grains were farther apart than the grains that were lower and moving more slowly.
I recalled how many times that I dug an object and found it on the surface of a deeper layer. The lower layer will sometimes be of a different color or texture. It will be under the layer of sand that was moved and the objects rested on the layer just below that remained stable.
That might not be surprising finding either, but it was interesting to be able to watch the process. I could keep the water moving at a relatively slow rate and no more sand would be suspended or moved. No matter how long I kept the water moving at the same speed, and the objects sank no deeper than to the surface of the layer of stable sand.
The objects did not sink into the undisturbed sand no matter how long they sat there in the cup. The objects did not sink into sand unless the sand was moved. That is unlike coins sinking in still water - or air. In those cases, the force of gravity does the job.
Here are a few observations that held up all in these demonstrations.
(1) Only a relatively thin layer of sand was disturbed by the moving water.
(2) When the cup and water was moved more forcefully, more sand was moved, and deeper sand was moved.
(3) The movement of sand decreased with increasing depth down to a stable layer.
At first I moved the cup gently and only the layer above he green line moved, but when I moved the cup more forcefully, then the sand moved as deep as the light blue line, and that is where the objects ended up, and when I moved it even more forcefully, the sand moved as deeply as the dark blue line, and that is about where the objects ended up.
On the beach, different storms will disturb deeper layers of sand allowing targets to sink deeper. They will then often be covered again by incoming sand.
It is a matter of trigger points, or the amount of force required to move the sand and objects. The coin and ring have higher trigger points than the sand, so the sand was moved while the objects settled to the stable sand. For more about that trigger points to to another post by using the link above.
I've seen the same kinds of things on the beach, however on the beach you'll also see a lot more force, such as when waves cash and suspend sand that is then easily swept away.
I did this experiment quite a few years ago and the old post is pretty old, but increased my understanding in the meantime and tried to make this post a little clearer and more precise. I'm also sure that there are many people who have joined the blog in recent years that never read the original post and some that read it but forgot about it.
If you understand these principles, you can see them while you are at the beach metal detecting. That understanding will help you figure out what is going on so you can find where the coins are being deposited.
In this post I didn't get into the differences between the coin and the ring. There are some important differences. There are some important differences.
After I originally posted about the experiment I discussed again today, I received the following from Bill F., who has a degree in physical oceanography and did his undergrad thesis on beach erosion at Ponce Inlet.
Below are some of the points provided by Bill.
1) The finer the sand particle, the flatter the beach
2) The faster the current, the bigger the particles which are moved
3) As you show, without wave/water energy, objects don't settle on their own.
4) The heavier/denser an object, the less likely it is to move horizontally.
5) Shape makes a difference.
Thanks Bill! Those are some good points to remember.
You can see some of those illustrated in the chart on trigger points that I posted above.
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| Surf Chart by SurgGuru.com. |
As you can see nothing much until Thursday when we're supposed to have at least one day of bigger surf.
Good hunting,
Treasureguide@comcast.net