Friday, November 9, 2012

11/9/12 Report - Erosion and Refilling As It Affects Detecting

Written by the TreasureGuide for the exclusive use of

Today I'm going to do something a litte different.  I'm going to present a fairly detailed explanation of a common situation that you might encounter on a newly eroded beach.

Eroded Dune With New Sand at Base.
As you know, beaches erode and then fill again and then erode again.  It is a continuous cycle.  Thre process can reverse every day, erosion occuring at one high tide and then refilling at the next.  Coin holes therefore can appear and disappear that quickly.
Today I thought I'd take the time to diagram how that can happen and how it affects where you will find coins.

First, here is a photo showing one example.  It shows a cliff that has been cut into the dunes, and also at the foot of the dunes new sand that has accumulated after the erosion.

When the dunes are old, having not been disturbed for some period of time, targets can be buried at different levels in the dunes. If the sand has not been disturbed older targets are generally buried deeper. When the dunes erode, coins and other objects will be washed out of the dunes and will lay at least momentarily on the sand in front of the dunes near the newly eroded surface.

If three layers of sand holding coins from three different time periods are eroded, objects from all three time periods will fall out of the dunes as the face of the dune erodes. Older and newer coins from the different layers will all fall onto the newly exposed surface mixed together.

Generally each wave hitting the dune will affect only a few inches of the face of the dune. One wave will crack the first few inches of the face of the dune, another wave will cause a few inches of the the loosened sand to fall from the face of the dune to the foot of the dune, and then another wave will wash away the sand, leaving behind objects such as coins.

If the cliff is higher and more compact and the waves hit at an angle, the early waves will carve out sand near the base of the dune causing sand above that to fall to the base of the dune where it will generally then be washed away, again possibly leaving behind items like coins.

Most loose sand will eventually be washed away from the face of the dune , but so also will other objects such as coins if the wave action is strong enough and continuous enough. Objects at the foot of the dune can remain at the foot of the dune or be washed a short way from the foot of the dune or washed completely down the slope and into the ocean, depending up the strength of the waves, how the waves are hitting, and how long the waves continue to hit the face of the dunes.

Often you can get an idea of how long the dune was in place by looking at things such as roots or objects exposed at various levels of the dune face.  That will help you determine how likely it is that old objects were in the dune.

 Being relatively heavy and being flat, thus presenting a low profile to the waves, coins will hug the newly exposed surface and tend to lag behind as sand is pulled into the surf.

If the water is hitting with enough force and hitting the right way, though, coins can be swept all of the way down the slope and into the water. A number of factors can affect that process.  I've also seen coins fall out of the dune and roll down the slope.

In the case I‘m using as my example (Photo above.), there were coins directly at the base of the cliff laying very near the surface.

The last coins out of the dunes will generally be the closest to the face of the cliff. Those eroded earlier will tend to be farther away from the face of the cliff. They eroded from a part of the dunes that was closer to the water to begin with and also were exposed to the waves longer.

This diagram shows a cross-section of an eroded dune like that shown in the photo. Coins were found near the surface near the base of the cliff (A) and also a short distance from the face of the cliff (B & C).

It looks like this diagram got rotated 90 degrees.  Sorry.

In this case after the erosion occurred, the beach began to build again, placing a new layer of sand (shaded section) on top of the coins that had been exposed earlier. The depth of the newly accumulated layer of sand in this case varied from practically nothing to nearly a foot.

The coins found nearer the base of the cliff (A & B) were near the surface and gave loud signals, while those farther away from the cliff ( C ) were buried deeper and gave faint signals.

The new layer of sand can occur on the next incoming tide after the dune eroded, or not until after several tide changes. It can also continue for several tide changes, becoming deeper each time. The new sand can nearly reach the dunes, as shown in this example, or fill part way up the face of the dune, or fill all the way to the top of the dune again.

During Sandy I saw some places where the sand did not fill all of the way to the face of the dune and other places where the erosion completely disappeared again in a short period of time.

Very often people think that the objects are moving as they appear and disappear when the fact is that the objects are in the same location but are being uncovered and then buried more deeply again, coming into detector range and then being removed from detector range.    Of course, there are times when the objects do actually move. 

If you arrive when objects are uncovered, you’ll find a lot, but if you arrive when the objects are deeply buried, not so much.

In a case like this I would typically work the base of the cut first and remove the surface targets giving off louder signals. 

I've noticed many times that I miss a lot of the faint signals when there are a lot of loud signals, but after the loud targets have been removed, then I more slowly and carefully detect the same area and nearby areas while listening very carefully for the faint signals.

Sometimes that is how coin holes appear to move. One part of the hole is covered while another part is being uncovered.  In that case, the coins are not moving but the sand is.

I’m not talking about cases in which objects are coming in with the new layer of sand now. That does happen at times too. 

Well, I hope that explantion helps you to track down more coins.  You never know what you might find mixed in with a coin hole.

Happy hunting,