Tuesday, September 2, 2014

9/2/20 Report - How Objects Sink in Beach Sand, Density of Common Metals. Movement of Sand. Tropical Storm Dolly

Written by the TreasureGuide for the exclusive use of treasurebeachesreport.BlogSpot.com.

Gold Ring and Quarter In Cup of Sand and Water.
I always read  about how "heavy" objects sink in beach sand.  I understand why people say that, but it is not correct.

To the left you see a cup of genuine Treasure Coast beach sand and tap water.  I poured about half a cup of water into the cup and then about a half a cup of sand.  The sand filled the cup up over three fourths of the way with about a half inch of water on top of the surface of the sand.  I then put a quarter and thin gold ring on the sand.

How much do you think the quarter and ring sank in the last week?

Here is the answer.  Not at all!  

The way some people talk, or write, it sounds like gravity pulls objects like these right down through sand.  That didn't happen in this cup. 

You might say that this 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 this 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 much more often it is the sand that does MOST of the moving.

Before I go on I want to address another part of this.  People say that "heavy" objects sink, but it is not the weight of the object alone that determines if an object will find its way to lower levels of sand.  As I've explained in the past, a one-ton block of Styrofoam weighs one ton but will float in water and will not sink into the sand.  It is not the weight, but the density that has more to do with it.  But it isn't just the density of the object either that determines how an object will sink in the sand.  The shape of the object will have something to do with it too.

Lets focus on density for a bit.  The site linked through the following link presents a list of common metals along with their density.


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.

Lead is slightly more dense than silver.

The most dense of those listed above are platinum followed by tungsten, followed by gold.

(You might remember the tungsten ring that I found not long ago.  I caught it in the right light yesterday and saw on the inside of the ring that it was marked as tungsten.  On a detector the target ID came up as nickel.  I could tell when I picked it up that it was heavy for its mass, although without really thinking, I called it titanium at first before being corrected.)

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.

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, shape has a lot to do with how an object will sink in beach sand or be moved around and deposited.

For a more complete discussion of trigger points and the lead sinker experiment see my 3/10/14 post.

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.

This might not seem important, but to me it is very important to me because it helps me understand how different types of objects will move on a beach, and therefore, where they are most likely to be found.  Sometimes when an object seems to be sinking, it is actually being covered rather than sinking.   And, of course, there are times when the water force is enough that more dense objects are moved forcefully by the water.  Coins have been observed to flip up and over a sand berm, for example.

That's all of that for today.

On the Treasure Coast we're supposed to have a one to two foot surf for a week or so.

There is a new tropical storm Dolly, but she will be hitting Texas.

Happy hunting,