I’ll be standing on the sand, facing the ocean just at the edge of the surf-line. When a wave comes up and over my feet, it just washes without really disturbing the sand under them.
When the wave washes back to sea, however, it pulls the sand out from under my heels, sometimes sucking it out from under my feet so hard that I almost fall over backwards.
What I think is happening is that the sand and water blends into a slurry on the way back out to sea and the weight of my body squooshes the mix out from under me.
Is this true? If it is, why doesn’t it do the same thing when the wave comes into shore the first time?
There are several things going on all at once; the water is soaking into the sand, lubricating the individual grains, agitating them somewhat and also buoying them up a little. Your feet cause turbulent flow in the receding water, washing out some of the sand from around and under the edges of your feet - this means your weight is now distributed over a smaller area of sand.
IAMNA… whatever kind of expert would deal in this, but “Yes, it’s partly true.”
When the wave comes in you are standing on dry(ish) sand. Compact, stable, able to support your weight. When a wave comes in it washes (mostly) in (mostly) on top of the sand, so you’re still on a stable plattform. But the water also sinks into the sand, so what you are standing on becomes more and more like very thick quick sand (sort of, I think), and when the wave flows back it pulls along some of this slurry, and the water which has sunk into the sand further up also flows back, through the sand, and increases the effect. Or so I believe.