Written by the TreasureGuide for the exclusive use of treasurebeachesreport.blogspot.com.
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| Source of diagram: http://www.seafriends.org.nz/oceano/beach.htm . |
I added the A and B labels to the original diagram to point out the advance and retreat of waves that hit the beach. Point A is the most advanced point of one wave, and point B shows the point to which the water of that wave recedes down the slope again.
I also added the small black arrows between A and B and the x. Those arrows show the advance and retreat of the most mobile sand during the surge and retreat of one wave. The x shows the start and end point of the mass of moved sand at the beginning and end of each wave.
Most of the time the beach is in a state of relative balance. It isn't in precise balance. There is always some gain or loss of sand along the beach, but usually it isn't a huge gain or loss and wouldn't be noticed by casual beach-goers unless they had a special interest and made careful measurements.
I'm talking today about what happens when the waves are hitting the beach at something near 90 degrees - not when the water is hitting from a good northeast or southeast direction, which is an entirely different situation.
If you watch successive waves that hit the beach straight on, you'll see that normally each wave transports sand (which my video that I wanted to upload clearly shows on a micro level). Each wave normally moves some sand up the beach a little (towards point A), and then as the wave recedes, a small amount of sand retreats back down the slope about the same amount (towards point B).
It is almost like there is a smaller wave of sand (black arrows) within the wave of water. The sand moves a smaller distance than the water, but also advances up the slope and then retreats with each wave. Point x, where the bulk of moved sand settles after each wave, does not change much with each wave that hits the beach, but does advance up the slope as successive waves move up the slope with the tide. And it retreats down the beach as the tide goes out.
The amount of sand that moves with each wave is small, of course, but the accumulated effect of many waves can be considerable.
When I was making my video clip the distance that the wave of sand moved was a fraction of the distance that the water moved with each wave. The wave of sand was something like 20% of the distance that the water moved, but varied considerably depending upon things such as the timing of waves and how they came together.
The distance that most of the moved sand will move (length of the small black arrows) with each wave depends on a variety of factors.
Remember, what I am talking about is when the waves are hitting the beach at a 90 degree angle. The movement of sand is small and the advance and retreat of sand pretty much balances out. Another way of saying that is that point x does not change much with each wave, but does change more with the tides.
In the video I took to illustrate that common situation, there was one notable shell that was in the area where the sand moved with each wave. It must have been a hundred or thousand times heavier than the grains of sand that were moving. How do you think the shell moved in relation to the sand under these circumstances? Do you think the shell moved more or less than the lighter grains of sand with each wave.
The answer is neither. Under those conditions the shell moved very much with the sand. It moved up the slope about the same amount as the wave of sand and then back down with each wave the same amount as the bulk of moved sand.
I've often said that weight is not the most important factor that determines how an object gets moved on a beach. This illustrated that point very well. The shell was much heavier than the grains of sand that were being moved, but it stayed right with the moving wave of sand wave after wave.
I won't go into that entire discussion again now. I don't want to complicate this too much at this point.
Here is a big factor to add to all of the above. When one of these 90 degree waves recedes, it will hit another wave coming in behind it.
What do you think happens to the carried sand and other materials when the incoming wave hits the receding wave?
The momentum of the water stops and the carried materials drop out. At that point you will get a little pile of material.
Also, despite its much greater weight, the shell stayed on top of the moving sand, wave after wave.
Back to the movement of sand. From what I've said so far, it would appear that you would neither lose or gain sand during a tidal cycle. It would balance out. And it often does. But there are other things that could change. For example, the surf might be rough during the incoming tide but then calm down during the outgoing tide. Or the period between waves might change. Or the direction of the wind and waves might change. All of those things can affect the flow of sand and other materials.
That is all for that topic today. It takes me quite a while to make that understandable. I hope I succeeded.
Here are two class rings from the same school but many years apart.If you do much water hunting, you'll find a fair share of class rings.
They can be fairly heavy. I've found a few over an ounce. These two are closer to .75 oz.
I think they are making a lot of lighter ones these days. And even a few are made of silver or stainless steel now.
But class rings are among the more common finds for water hunters.
On the Treasure Coast today the surf is around three feet. It will be about that this weekend too.
I'll keep trying to figure out how I can get the beach dynamics videos done.
Happy hunting,
TreasureGuide@comcast.net
