Wednesday, March 15, 2017

3/15/17 Report - Advanced Beach Dynamics : Fluidisation and Liquefaction

Written by the TreasureGuide for the exclusive use of

It is amazing what you can learn by just walking around and observing.

Yesterday Joe R.wrote about sand fleas as a biological factor contributing to objects sinking in the sand on a beach. Here is some of what wikipedia says about sand fleas (Emerita).
Emerita is adept at burrowing, and is capable of burying itself completely in 1.5 seconds.[5] Unlike mud shrimp, Emerita burrows tail-first into the sand, using the pereiopods to scrape the sand from underneath its body.[11] During this action, the carapace is pressed into the sand as anchorage for the digging limbs.[11] The digging requires the sand to be fluidised by wave action, and Emerita must bury itself in the correct orientation before the wave has passed to be safe from predators.[11]
As the tide changes, Emerita changes its position on the beach;[5] most individuals stay in the zone of breaking waves.[6] This may be detected by the physical characteristics of the sand. As the tide falls, the sand is allowed to settle; when Emerita detects this, it uses the temporary liquefaction from a breaking wave to emerge from its burrow, and is carried down the beach by the wave action.[6] Longshore drift may also drag Emerita laterally along a beach.[6]

Notice the word "fluidised."  Another similar word is "liquefaction."  I've never used those words, but have described those processes to some extent.  If you ever watched the guys that build docks on the river, they use a pump to pump water into the sand and push the poles into area created where the sand has been fluidised.  You can see the same process if you watch people using water to run pipes under the road or driveways.

Here is a little of what I wrote a few days ago about how objects sink.

An item can be uncovered and covered several times before the sand below it is moved enough for it to sink down.  Another way an item can sink is when a wave crashes over the item.  That can push water into the sand, agitating the sand and pushing the grains farther apart while putting downward pressure on the item. 

That is a description of how I would describe what I've observed.  I recently found a study that talks about liquefaction.  "The criterion of liquefaction occurrence is estimated as the critical overpressure required to overcome the effective weight of the soil."

LIquefaction and fluidization are very important processes that can help explain how sand and other things move on a beach, but I was not previously familiar with those terms and have never used those specific words before. I've thought a good bit about how crashing water forces grains apart and puts them into suspension.

A few years ago I posted an amazing picture of a wave that appeared to show sand being sucked up into the wave.  It is easier to understand how that happens when you think in terms of the sand be fluidised.  When sand is fluidised and moves as a liquid, it is easier to imagine how the water and sand mixture becomes part of the wave.  Back then I described that as the sand being sucked up into the wave.  (My discussions of trigger points are also relevant.)

Below is a paragraph from the conclusion section of a scientific study on liquefaction.  The experiment was conducted in a kind of wave tank with a sloped loose soil base leading to a wall.  Waves were generated and the effects measured in a variety of ways.  .

CONCLUSIONS  A physical model for studying liquefaction occurrence in the wave loading against a vertical wall was described. For large enough wave conditions, and for a loose and partially-saturated bed, an excess pore pressure is recorded within the soil and a liquefaction threshold is reached. A large zone of the bed then clearly behaves as a fluid. Sand grains displacements were quantified. Phases of soil compaction and dilatancy were identified. As runs are repeated, the bed becomes more compact and better saturated, and the liquefaction phenomenon no longer occurs.

Soil dilatancy is another interesting phenomena that is relevant and could be discussed at length, but I won't try to do that now.

I'll have to study the article a bit more.  There are specifics in the paper that I'm sure I haven't yet captured.  The jargon they use is a bit strange and needs to be put into common terms.

Now think of a cliff that has been created by erosion on a beach.  A similar thing will happen to the sand in front of the cliff that happened in their wave tank in front of the wall.  If waves crash in front of the cliff and the sand "fluidises," sand will flow away with the water at a rapid rate.

There are a lot more details that I could get into, but I won't do that just yet.  I'm sure I'll add to that at some later time.

Here is the link to the study I was talking about.

I also just found a very interesting video demonstrating soil dilatancy. It shows how a weight will sink into agitated and fluidised sand while a buried ping pong ball rises through the sand to the surface.

I think you'll find the demonstration interesting.

I talked about all of this to some extent before but did not have the studies to back it up, or the terms to make it clear.  The addition of these two new terms makes it easier to discuss and visualize the processes that I had thought about and referenced before.  Quicksand is another  illustration of sand fluidising.  Thinking of the suspended sand particles as  a fluid provides another conceptual tool that changes how we conceptualize its movement.  The studies also increase confidence in the conclusions.

I might not be a hundred percent correct in how I discussed the study today or how I used the terms in every case, but I found both the study and terms helpful in presenting my thinking on the subject.  I'm sure I'll increase my understanding of both in the near future and perhaps be able to explain some of the things we see on the beach a little more clearly.

That is all of that for now. It would take a lot of pages to put this all together for you.  I haven't felt like doing it yet.  It will be a big project.


The wind has shifted.  We're not going to have much surf this week, but we are going to have some good tides.  We'll also have some good north winds.

Happy hunting,