Written by the TreasureGuide for the exclusive use of treasurebeachesreport.BlogSpot.com.
|Same Cup of Water and Sand |
Just in: I have some great new pictures of treasure ship finds sent by Captain Jonah! Probably tomorrow.
I accidentally did an ingenious experiment. I should probably call it an observation rather than an experiment. I'll get back to that in just a minute.
I always enjoy receiving emails from this blog's readers. They are an educated and intelligent bunch.
Yesterday I talked about how objects of interest sink in sand. I received an email from Bill F., who has a degree in physical oceanography and did his undergrad thesis on beach erosion at Ponce Inlet.
Here is some of what Bill said.
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.
Above is a picture of the same cup that I showed yesterday. The ring and coin in this cup never sank so much as a micro inch while it sat undisturbed. That clearly demonstrates one thing Bill said - without wave/water energy, objects don't settle on their own.
I decided to see what it actually takes for objects to sink in the sand. 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 pretty much stayed in place. When I stopped, a little sand covered the objects, but very little. With gentle only the sand above the green line moved. This is important, and I'll get back to it.
When the objects got covered there were a couple of ways I could tell how deeply they got covered. You'll notice that the surface of the sand in the cup (below the yellow line) is not exactly horizontal.
A perfectly circular even motion moved the water so that the sand remained level across the cup, but by moving in a more jerky manner resulted in the sand moving from one side to the other of the cup to the other.
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 fast I moved the cup and therefore how fast the water moved.
I had no idea how important it was going to be for me to be able to see through the container when I started this experiment.
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 the picture of the cup moved when I moved the cup gently. The top of that layer moved the most, decreasing as the moving layer got deeper until there was no suspension and movement of sand at all.
The fastest moving sand was clearly suspended, and the grains appeared farther apart than the grains that were lower in the moving layer, which moved more slowly and less overall.
How deep do you think the objects sank in the sand when the cup was moved gently? Now this is an important principle. The objects sank to the surface of the layer of stable sand, and then stayed there.
I recalled how many times in the past few years that I dug an object and found it on the surface of a lower layer of sand. If you go back through this blog I'm sure you can find that. Often the lower layer was a different color or texture. Often it was a layer composed of course shells.
That might not be surprising finding, but it sure was interesting to see. 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, the objects sank no deeper than to the surface of the layer of stable sand.
I know this is not a highly controlled experiment and what happens on a beach will be somewhat different, but in I think this principle will hold. Just like in the previous report, the objects did not sink into the undisturbed sand no matter how long they sat there in the cup. In this demonstration the objects did not sink into undisturbed sand even though they did sink through the suspended moving sand.
Here are a few observations that held up all through these demonstrations with the moving cup.
(1) Only a relatively thin layer of sand was disturbed by the moving water.
(2) When the cup and water was moved more forcefully, a larger layer of sand was moved.
(3) The speed of the sand decreased in the moving layer of sand from top to bottom until there was no movement at the bottom of the 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.
In this demonstration I'm only addressing sinking targets, which occurs primarily when the trigger point for sand is reached but not the trigger point for targets.
This simple experiment has many limitations, but it reveals some important principles that I've observed in operation on the beach. I reasoned that objects in deeper layers of undisturbed sand would remain in place, but with this demonstration I observed it and am much more certain of it.
Another thing I observed with this little experiment is how it is only suspended sand that moves. I could see this on the beach just yesterday. The waves were crashing on a sand bar. In front of the sand bar was what I might call a flat topped creeping sand bar. You could see a clear edge of a layer of sand that was moving from the crash zone towards shore. The crash would suspend sand, and then that suspended sand would be washed ahead in the surge.
Where the sand was suspended, you couldn't see through the water. That makes sense. But closer to shore, where there was a dip beyond the creeping sand bar, and the water there was clear. That is where I found a couple of relatively new coins that had not yet been covered by the incoming tide and creeping sand bar.
As the tide comes in, the area where the waves crash and most sand is suspended closer to shore. Obviously that means that a greater area of sand is affected. The crash zone changes and the area of surge comes in and then goes out again.
If you've spent much time down in the Fort Lauderdale and Miami area, you know that they have much more calm water than we do. Many days the shallow water sits still almost like you are in a bath tub, especially before the sun starts to heat things up. The shallow water on the Treasure Coast seldom has that kind of flat water even when the surf is down to one foot. That small difference in roughness appears to make a big difference in the average sink rate of targets. They'll stay in detector range down there a lot longer than on the Treasure Coast. Couple that with the absolute number of people and what they wear and there is an even bigger difference between South Florida and the Treasure Coast. Bottom conditions are very different.
Well, I'll quit there for today. This is getting long. The principles demonstrated are important and help explain how things settle and therefore how and where things will be found. I know that I did not address all of the relevant factors and situations.
I'll do some more experiments someday to help answer some of the remaining questions.
I'm sure some of you will find this post tedious although several of you used the +1 button to indicate that you particularly liked the previous post. For my own purposes and understanding, I'm glad I did the demonstration.
There are no storms to watch in the Atlantic right now.
On the Treasure Coast we still have a one foot surf and will continue to have a calm surf for at least a few more days.