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| Fort Pierce South Jetty Park. |
I like this camera angle.
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| Gold Ring and Quarter In Cup of Sand and Water. |
In the passt I often talked about beach cynamics, including how beaches move but also how objects, especially coins, move in sand. I haven't done that for quite a while and thought it would be a good time to address that topic once again.
I try to avoid repeating myself, but repetition is how we learn. And in many cases, we don't get everything the first time. Information rich text might require several readings before you get it all.
I don't expect people to remember what I said a year ago or ten years ago. And sometimes I can add something to make what I said clearer.
In any case, I decided to go back and look at a simple experiment that I think is very illuminating although I'm sure some people will think it is not.
You can try this yourself.
You will often hear how heavy objects sink in sand. I'm sure many people will think the distinction is not important, but I think it is. It is actually more about density than heaviness. So what is the difference? The illustration I've used is that a ton of Styrofoam is heavy but will not sink in much of anything '- not even water. A cruise ship is heavy but will not normally sink. There are other factors. Density is more important than heaviness. The shape of the object also has an effect.
If you take a cup or other container filled with sand and water and place a coin and or gold ring on the sand it will stay where you put it as long as you leave it undisturbed. It will just sit there.
How much do you think the quarter and ring sank in the a week?
Here is the answer. Not at all!
The cup is obviously different than a beach. On a beach there are forces that move the sand. A beach is a dynamic system.
If you start moving the cup so the water and sand moves, the sand will move more than the coin or ring and the sand will eventually cover the objects. It will take more force to move the denser objects but the sand will move more easily and move more. The effect is that at some levels of force, the coin and ring will remain relatively stationary while the moving sand covers them.
I should also mention that the shape of the objects is also important and I've done and reported on experiments that show how objects of different shapes are moved more or less.
A coin presents a relatively larger surface area to the sand below it which slows the rate of movement of sand below it. The coin's low profile also minimizes the horizontal forces of the water against the coin.
To me, saying that coins or other objects sink in the beach sand seems to suggest that gravity just pulls the objects down through the sand, which is not what happens. Other forces are required to move the sand. In most cases it is water, but it can also be wind or people walking or running, or sand fleas burrowing, I suppose.
You might say that the cup experiment is a poor experiment, and it is nothing like a beach where the water and sand move. Precisely! It is not like a beach. Everything is stable, and gravity alone will not pull coins or a rings like these down through the sand. Other forces are at work when an object settles down to lower layers of sand on the beach.
On a beach the water and sand moves, and the moving sand covers objects like these that do not move as much as sand. But that is just part of it.
The objects will move some too, but it is the sand that does MOST of the moving. It is more a matter of the objects moving less than the sand rather than the objects being pulled by gravity down through the sand, which is why I don't like to describe it as "objects sinking" in sand. It is something like describing the sun as "rising." It might seem like it is rising, and it can be a useful description for some purposes, but it is does not help understanding what is actually going on.
Lets focus on density for a bit. The site linked through the following link presents a list of common metals along with their density.
http://www.engineeringtoolbox.com/metal-alloys-densities-d_50.html
Here are some of the most relevant metals listed on that web site. (Density is given in Kg. per cubic meter.)
Aluminum 2712
Brass 8400 - 8700
Copper 8940
Gold 19320
Iron 7850
Lead 11340
Platinum 21400
Silver 10490
Steel 7850
Titanium 4500
Tungsten 19600
Zinc 7135
This correlates somewhat with how things will be distributed on a beach - but not perfectly
Aluminum is the least dense of the above group followed by titanium. You'll see those often during poor beach conditions.
Copper and zine are sort of in the middle, with zinc being slightly less dense than copper. Zinc cents will be found more towards the outer boundaries of coin holes, and copper cents just a bit closer to the center.
There are other factors with the zinc cents. The surface corrodes and gets rough and actually adheres to sand and other substances. You've probably seen those badly encrusted zinc cents.
Lead is slightly more dense than silver.
The most dense of those listed above are platinum followed by tungsten, followed by gold.
If you look at that list, you'll have some indication of how those objects will sink into sand. But as I showed above using the cup, they won't sink by themselves unless there are other forces at work. And the shape of the objects will affect the rate of movement
The water moving the sand is a very big part of it. I've explained before about trigger points. Different objects require different amounts of water force to move them. Grains of sand move relatively easily. They have lower trigger points. It takes less water force to move sand. Objects like the coin and ring require more force to get them moving. They have a higher trigger point.
When the water isn't moving much, the sand will be moved while the object remains relatively stationary. Sand will be washed over the object, out from around the object, and if the shape is right, even from under the object. This movement of sand is one of the biggest factors in determining how a object will sink into the sand. With stationary sand, there is little to no sinking.
The shape of the object is also important in determining how an object will be moved on a beach.
I once showed an experiment in which lead sinkers having different shapes but the same weight were moved by crashing water - a disk shaped sinker was moved less by the same water than a egg shaped sinker, which was moved more than one of those fish shaped sinkers. That is exactly what I predicted before the experiment.
The disc shaped sinker provided less surface for horizontally moving water to push against. It hugged the surface of the sand something like a coin would.
Flat thin sheets are moved more by water - much more so than most other shapes. As a result, even though copper is about three times as dense as aluminum, you will find thin copper sheets being deposited on a beach very much like aluminum.
Again, the shape of an object has a lot to do with how an object will be moved around and deposited on a beach.
For a more complete discussion of trigger points and the lead sinker experiment see my 3/10/14 post.
http://treasurebeachesreport.blogspot.com/2014/03/31014-report-experiments-on-movement-of.html
I also once showed an experiment that showed how an object "stepped" down a sand slope when the water current was rushing up the slope. What happened is that the current moved the sand from around the object and from in front of the object as the object sat on the slope, and when enough sand was removed from the lower side of the object, the object slipped down the slope into the void, where it came to rest until the current moved enough sand for it to slip down another step. Again, it was more about the sand being moved than the object sinking through the sand.
Of course loosely packed sand will move more easily and permit objects to "sink" lower more quickly than would packed sand. That affects how much the sand moves too. I can't get into all of the factors here, but wanted to make it perfectly clear that objects do not sink into sand simply by the force of gravity pulling the object down through the sand. It is more a matter of the objects relative to the movement of the sand.
If you understand these forces you will be better able to figure out what is going on at the beach and where various objects will most likely be found.
Some of this discussion was posted a few years ago. Here is the link to the original post.
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| Source: Surfguru.com. |
Looks like we might have something to look forward to.
Good hunting,
TreasureGuide@comcast.net
