Ever noticed that steeper beaches often have bigger grain sizes? The mechanisms are complicated, but one of the main factors is a thing called percolation.
It’s coming up to mid-summer in Europe and most of you are probably surfing beachbreaks. Clearly, some beaches are steeper than others. This affects the waves that break on them; for example, steeper beaches tend to have steeper, more powerful waves because the sudden depth change makes the waves dissipate their energy more quickly over a shorter distance. In contrast, flatter beaches have gentler waves because the energy is dissipated more slowly over a greater distance.
You may have also noticed that steeper beaches frequently (but not always) have larger grain sizes. Think of the fine-grained beaches of North Devon compared with the large-stoned beaches around Madeira. Which ones are steeper?
One of the first pieces of evidence that steeper beaches tend to have larger grains was way back in the early 1950s, when Willard Bascom did some experiments at Half-moon Bay, California. Half-moon Bay contains progressively different sized grains along its length. Bascom noted that the beach got steeper as the grain size got bigger. I wonder if he also noticed a big wave off Pillar Point at the northern end of the bay, and I wonder if he imagined how famous it would become half a century later.
Anyway, why are course-grained beaches steeper than fine-grained ones?
At the shoreline of any beach, there is an area where the water just swashes up and down. It’s called the swash zone, and consists of an uprush and a downrush. If you stare at it for a while, you’ll notice that, sometimes, the uprush seems to just keep coming up without ever flowing back again. You wonder where all that water is going. Then you realize that some of it is sinking into the sand, filtering, or percolating, its way through the grains.
Now, if some of the downrush is lost due to percolation, the sediment that the moving water drags along with it (the sediment transport) will be dominated by the uprush. In other words, more grains will move up the beach than down. As a result, the sediment piles up and the beach becomes steeper.
Of course, the beach can’t keep steepening forever. There must be some other force that stops it steepening once it reaches a certain point. That force is gravity. As the beach gets steeper, the effect of gravity on the flow increases, the uprush struggles to climb up a steeper slope and the backwash has an easier ride back downhill. As the beach steepens, the uprush becomes weaker and the downrush stronger. This effect is exactly opposite in polarity to that produced by percolation. Eventually, a point is reached where the increase in downrush due to gravity exactly compensates for the loss of downrush due to percolation. At this point, the beach stops steepening.
And here is the crucial part. A beach with bigger grains allows more water to sink into it than one with smaller grains, and so more of the downrush is lost through percolation. Therefore, a course-grained beach has to steepen more in order for gravity to compensate for percolation. In contrast, fine-grained beaches do not have much percolation, so they do not have to steepen very much before equilibrium between gravity and percolation is reached. The overall result is that coarse-grained beaches are steeper than fine-grained ones.
Cover shot: Helio Antonio.