Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.
There is a swash and a backwash. When the backwash is greater than the swash, beaches erode. That is what the above graphic refers to deconstructive waves.
When waves break that is where the bottom is stirred and a lot of sand and other things such as rocks, shells or coins are brought into suspension. The first step for moving objects is suspension.
When the surface sand is moved, buried objects, such as coins, are uncovered so they can be moved, but they must be uncovered first. It is important to remember that as long as objects are buried under a protective layer, they won't be moved until the protective layer is moved.
The depth of the effect of a wave is one half the wave length. See the illustration below.
In deep water, the effect of the wave on the bottom is virtually nonexistent. The depth of the wave's effect is one half the wavelength. When you see the wave breaking, that means that the depth of the water is less than half the wavelength.
The water has to be fairly shallow for a wave to move the sand or anything else on the bottom.
The following illustration shows how the wave changes shape and eventually breaks as the water becomes more shallow.
Waves are only going to move sand in fairly shallow water. And since it requires more force to move other objects, such as coins, those will be moved only in rough shallow water. Where the waves break is the point of most turbulence and force exerted on sand and objects on the bottom.
In the above illustration, you can see that the first three waves (on the left) do not disturb the bottom. Therefore any coins on the bottom will not be disturbed by the passing waves above. Even though the waves are not disturbing the bottom, at the same time there can be currents that do, though they will seldom be sufficient to move things such as coins. Remember, relative to sand, it takes a lot more force to move rocks and coins than it takes to move the sand.
Often you'll see the waves break far from shore when there is a reef or a sandbar or something. That will remove much of the wave energy before the wave gets to the beach.
Rip currents are another situation. Then you have a backwash channeled in an area that takes sand out and leaves things like rocks and coins. Some really great coin holes can be found where there was a rip current, but you should stay out of rip currents unless you are an very good swimmer and know you can handle the situation. It is not worth drowning for a little treasure. The first time I got caught in a rip current I didn't understand what was happening and it was pretty scary.
So what about coin movement. Once again, there has to be enough force to suspend and move them, and then they'll be deposited where the force decreases enough. They won't be moved in deep waters unless there are strong currents. The greatest natural force that will move them is where the waves are crashing and the swash or backwash is strong.
Often when coins appear on a beach, it might seem like they just got washed up onto the beach, when what actually happened is that the sand washed away and uncovered the coins.
It takes quite a bit to wash coins up onto the beach and it doesn't happen real often. Before they are washed up, they first have to be exposed by any protective layers of sand being removed and then they must be suspended and moved by strong waves that haven't lost their energy before hitting very close to or on the beach.
I did some little informal experiments myself and was surprised by how much turbulence it takes to lift coins off the bottom. Perhaps you've done some fanning and noticed how the sand moves away but other objects like coins remain unmoved.
More on this in the future.
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Sorry I lost track of some of the sources for the illustrations. Mostly they came from pinterest.
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| Surf Chart for the Fort Pierce South Jetty Area from SurfGuru.com. |
The Treasure Coast high tides are very high today but the surf is small.
Happy November,
TreasureGuide@comcast.net