Monday, March 10, 2014
3/10/14 Report - Experiments On The Movement Of Sand And Other Objects On A Beach
Written by the TreasureGuide for the exclusive use of treasurebeachesreport.blogspot.com.
To the left is a picture of the beginning of an experiment.
Three lead sinkers were placed on a sand bar in shallow water where the water would get deeper as the tide came in.
At the time of placement the objects were in very calm water. What I'm talking about here today happened before the water got rough.
Which object do you think would be the first to disappear from view while the waves were still gentle?
The big heavy one? The little round one? Or the disc shaped one.
This might or might not be a surprise to you.
It was the disc shaped object that disappeared first. It disappeared quickly and well before either of the other two. The heaviest appeared to remain the most visible in those first minutes before the water got rough.
Some sand was removed from around the two egg shaped sinkers making a little depression around them, but the disc shaped one disappeared very quickly.
People often talk of things sinking. And they talk about heavy objects sinking more. That isn't always the case.
I should make one clarification here. When I just said sinking, it was as much a matter of being covered as sinking. There is a difference.
There is what I'll call a trigger point. It takes a certain amount of water forced to move different objects. Sand will be moved before the sinkers.
Very often when people think that things are sinking on a beach, they are actually being covered.
The trigger point is critical. It is one of the factors that helps to create coin holes and lines.
There is also a trigger point or threshold of force that determines when an item will be dropped or settle out.
Considering the change in waves and changing tides etc. different items will be moved and dropped at different times.
In the first few moments of the experiment above the sand was being moved, not the objects.
When the sand moves objects can sink as they settle down as the sand is removed from under them or when they are pulled down a slope.
I was flipping through TV channels the other day and happened to hit a program on USOs (Unidentified Submerged Objects, or something like that). They were trying to find a plane that wrecked in the water off of the California coast and couldn't find it where they thought it would be.
In order to try to figure out what happened to it they did a little laboratory experiment. I don't think it was a great experiment for what they were trying to accomplish but the experiment did reveal one thing.
I tried to find the video clip online but couldn't so I made the following diagram. If you can find the clip I hope you'll send me a link.
They had a big tank of water and a sloped surface in it covered with sand. They put what looked like a sinker on the sand. The sinker was partly in the sand but mostly exposed.
They then toke a hose and made a current aimed at the object and up the slope.
Guess what happened to the object.
The current pushed the sand away from the front and sides of the object as the water rushed around it. The object then STEPPED down the slope. I say "stepped" because it moved in small quick steps, pausing in between steps.
The current moved the sand from the front of the object but not the back of the object. Gravity on the object and also the sand behind the object then moved the object down the slope.
The current trigger or threshold level was sufficient to move the sand in the current up the slope but gravity moved the object down the slope.
That was a good illustration.
If you start putting all of these things together you will understand how gold and cobs and things move on a beach. It isn't simple though. There are many factors.
On the Treasure Coast we have beautiful weather again today. A lot of sand though!
The wind is out of the West and the surf is only around three feet. It will decrease even more for a couple of days.