Sunday, June 28, 2015

6/28/15 Report - Trigger Points, Drop Points, Water Velocity and How Things Move And Get Sorted On A Beach.

Written by the TreasureGuide for the exclusive use of

There is a lot of information available on waves and coastal erosion.  What I do is start with that information and add my own observations and experimentation.  The academic literature is very good but it doesn't cover everything that a detectorist might want to know.

I could give you simple hints and suggestions such as to check dips below cuts, and that might would be good advice, but it won't help you nearly as much as knowing the principles of how beaches work. Situations differ, but if you know the important principles you'll be able to analyze any beach you look at and have a good idea if the beach is improving, and it will also tell you where to detect.  There is a lot to it, and it isn't simple, but if you continue adding to your knowledge base, you'll get it.  I keep learning more and more and understanding more and more.  Keep in mind the principles that you pick up and try to observe and test them in the field.

I'll try to put it in a way that I think is most understandable and applicable to a detectorist.  I'll add a little at a time, repeating some things and slowly adding a little new material to it.

As you would suspect, it takes more water velocity to move larger particles and larger items.  The relationship in "laminar" currents is pretty much a straight line when you are talking about items having the same densities and the same shape.

While particles will be transported by water moving at a given velocity, it takes more velocity to dislodge settled particles (and other objects) than it takes to keep them moving once they are suspended.

The velocity required to dislodge particles and get them moving is what I have often referred to as the "trigger point."  That isn't the scientific term.  It is just the term I use.  If we were to include that on a graph like the one above, there would be a line to the right of the red line, and it would not be a straight line because it takes more force to dislodge certain types of particles.

Clay is a good example.  It consists of very fine particles that transport very easily in water when suspended, but due to what I'll simply call the "stickiness" of clay, it takes a good bit more force to dislodge the particles and get them moving.

Different particles, in addition to having different trigger points, also have different "drop" points. When the water slows, there is a point at which the particle will drop out or settle.  The water has to be very calm before fine clay particles drop out, for example, while sand drops out while the water is moving a little more rapidly and pebbles will stop moving when the water is moving faster than that.

In this graph, the straight red line between the other two red lines shows the increasing velocity required to move larger particles and objects when a laminar current is assumed and other factors are not taken into account.  That line is very much like the graph above.

The curved red line to the right of the straight line shows that pebbles are moved with velocities of just less than 100 cm/s.  The same line curves to the left as particle size decreases because it requires less water velocity to move smaller particles such as sand.  It then curves back to the right again because it requires faster water to get silt and clay moving.

The curved red line to the right of the straight line on the graph shows it takes more water velocity to move clay than pebbles even though pebbles are much larger than particles of clay.

The most important thing to get from that is that objects such as sand, coins and rings and things have different trigger points and require different amounts of water force to get them moving.

There are times when you might have enough force to move sand but not coins.  Therefore, you might have erosion but not enough force to wash coins up onto the beach.  That would not be at all unusual.

The curved red line to the left of the straight line shows the decreased amount of force at which particles will drop out or settle.  Pebbles, for example, will continue to be moved until the flow slows to somewhere around 25 cm/s flow and will settle well before the water flow decreases to 10 cm/s.

Silt and clay, on the other hand, will remain suspended and continue to be transported as long as you have just a very little flow.

That is a good place to stop today.  Get that down and then I'll show you how it applies on a real beach to determine when there is erosion and when coins and things move and how they are deposited differently.  I'll add some additional factors.


Here is a good web site giving terminology and other good basic information about waves.

Knowing how a beach works is one of the most important things you can know for greater success with beach and shallow water metal detecting.


When you submit a photo of an object for ID, include some indication of size.  That can be something simple like a coin for comparison.  Also include a picture of both sides of the object.  You might think there is nothing to see on the other side, but some one who has not seen the item in person might benefit from seeing the other side even if there isn't much to see.  Maybe there is nothing to be seen there, but that is important too.  Sometimes there will be the tinniest of clues, a very small stub where something was attached, the slightest signs of usage, even corrosion, which by itself can tell something about the metal the item is made of.  Green corrosion is sometimes a sign that the object is cuprous, for example.  Even the shape is important.  An object might look flat on the back, but it might be just a touch convex or concave.  All of those are important clues.  I know it takes time.  I'm just saying, for the best chance for an ID, both size and pictures of both sides can make the difference.

An object that isn't easy to identify isn't easy to identify to start with, so any detail might help.


I've received some thoughts on the round mystery object I posted a couple of days ago.  I'll post that before long.


Has this been the longest period of smooth surf or what?  The Treasure Coast hasn't seen good beach detecting conditions for a very long time.

It has to happen.  I'm ready.

Happy hunting,