On The Beach

I seem to have an obsession with sand, because it appears that I write about it quite regularly. Like here (dangerous sandcastles on beaches) and here (killer crystalline silica dust on beaches). Just a couple of days ago I was wondering how the sandy river deltas formed (my explanation: the rivers slow as they reach the sea, and widen as they slow).

This morning, still slowly pursuing the most recent line of inquiry, I started wondering about beaches again.

There are a lot of things that puzzle me about beaches (which may be why I keep going back to think about them again and again). Why is it that all beaches everywhere in the world seem to be composed of sand, and not mud or silt or gravel? Where does the sand come from? How does it get to the beaches? Why does it stay on the beaches, and not just slip off?

And why are beaches usually very smooth and flat and firm underfoot near the water edge, but softer and hillier further inland (and also a bit hilly underwater offshore)? Why are there sometimes sand dunes inland from beaches?

Sand is actually composed of quite a lot of things. Most of it is made up of quartz. But there are also broken sea shells mixed in. And sometimes coral fragments.  The deltas I was considering a couple of days ago seem to be the sources of a lot of sand, probably because there are no sea shells or coral fragments in them, and they’re purely composed of quartz crystals. Quartz is highly resistant to chemical weathering, and that’s why it sticks around a long time. The quartz, composed of silicon and oxygen, SiO4, comes from granite. We make glass from silica, SiO2.

In fact not all beaches are made of sand. In Devon, beaches like the one at Sidmouth, are composed entirely of large rounded pebbles. And further east at Lyme Regis, where the beaches are made of sand near the water’s edge, there are banks of pebbles along the top of the beach. You have to go further east to places like Charmouth to find beaches almost wholly composed of sand. Which suggests that beaches are graded from top to bottom with the largest pebbles at the top, smaller pebbles (sand) in the middle, and the smallest pebbles (silt) at the bottom. And this probably happens because sand and silt-bearing water flows easily down the beach between large pebbles, and the fine silt-bearing water flows easily further down the beach between grains of sand. The beach acts as a sieve.

But why is the smooth sand just above the water level so firm underfoot? If you’re building sandcastles, dry sand is no good: it falls into a conical heap. And wet sand spreads into a similar heap. For some reason or other, you need damp sand to build sandcastles. Damp sand seems to possess much greater mechanical strength than wet sand or dry sand. Why is that?

I suspect the reason is that as water runs away between grains of sand, some drops (ringlets) remain at the points of contact between grains, and surface tension in the ringlet surface acts to pull the grains of sand towards each other, holding them together. These ringlets of water remain in place, and only gradually evaporate. And they are what serves to give damp sand greater strength than wet or dry sand: the grains of sand are glued together by water.

And there is a lot of air space inside dry sand. The density of quartz is about 2,650 kg/m³, and the density of dry quartz sand ranges from about 1,300 to 1,600 kg/m³. So dry sand is composed of 40 – 50% air. And when sand is filled with water it will drain quite rapidly to leave drops of water adhering to sand grains. Silts composed of smaller grains will have many more, narrower channels between grains, and will drain much more slowly. Pebble beaches will drain quickest of all.

So when a wave rushes up a beach in a thin sheet of water (photo at top), carrying lots of grains of sand in it,  these grains of sand are deposited on the beach when the water slows to a halt, and the grains of sand fall more or less vertically onto the beach underneath. But they are also more firmly deposited by the action of the water flowing down between the sand grains beneath the beach surface. Each wave wets the sand, and it immediately starts drying until the next wave arrives a minute or so later. The beach is a sieve which collects sand. And the damp, drying sheet of sand, bound together by droplets of water, must be in slight tension, which will act to stretch it flat. The same slight tension is what holds sandcastles together. And it also makes damp sand firm underfoot.

And maybe this is why most beaches are composed of sand. Silts that are composed of very fine grains remain waterlogged for long periods of time, and have no tensile strength. Layers of pebbles that are composed of large stones which dry rapidly also lack tensile strength. It may be that only sand drains quickly enough, yet also adheres together strongly enough, can serve to form beaches. And that’s why most beaches are made of sand.

The shape of sand grains may also affect the mechanical behaviour of beaches.

Angular sand grains may bind together much like toothed cogs in an engine, while smooth ones will slip easily over each other. Beaches made up of angular sands might be able to slope more steeply than ones made of rounded sand.

The slope of a beach may also affect its behaviour. Steeply sloping beaches will drain more rapidly than shallower ones, and so dry quicker. They will also be more difficult for waves to sweep up, and so will be wetted less often. Beaches may change their slopes depending on how frequently waves waves break on them, and how much water arrives with each wave. During storms, beaches may get steeper in order to reduce the rate at which water is deposited on them, and speed the drainage of water from them. If a beach can’t drain itself of water quickly enough, it is likely to become completely waterlogged, and the separated grains be swept away.

That maybe begins to answer one or two questions about beaches. I have plenty more.


About Frank Davis

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20 Responses to On The Beach

  1. garyk30 says:

    Many lakes and rivers have beaches that are composed of dirt/mud.

    The ‘Delta’ of the Mississippi River is mostly mud.

    But then, dirt is just very fine sand.

    The Earth is a strange place.

    • beobrigitte says:

      The shape of the sandcorns play an important role in the building trade. The sand from beaches mostly consists of round sandcorns (due to tidal movement) and therefore is useless when it comes to using it for concrete. It will be brittle.
      The demand for concrete increases with increasing demand for new buildings.
      Now there is a black market for sand.

      • nisakiman says:

        Yes, I read recently that the demand for sharp sand (for concrete) was creating a black market which was destroying the environment – in Africa somewhere I think, although I can’t remember exactly where. People digging up river banks and altering the way the waters behave, resulting in the drying up of vital wells. Nigeria, perhaps? There were some figures (which I also can’t remember) for the amount of sand which is used daily in the building industry, and the quantities were staggering.

        • nisakiman says:

          Couldn’t find the article I was referring to, but this is along the same lines:

          INDIA’S “sand mafia” is doing a roaring trade. The Times of India estimates that the illicit market for sand is worth around 150bn rupees ($2.3bn) a year; at one site in Tamil Nadu alone, 50,000 lorryloads are mined every day and smuggled to nearby states. Gangs around the country frequently turn to violence as they vie to continue cashing in on a building boom.


          Sand may appear plentiful, but is in fact becoming scarce. Not all types are useful: desert sand is too fine for most commercial purposes. Reserves also need to be located near construction sites; as transport costs are high compared with the price, it is usually uneconomical to transport sand a long distance. That, though, does not stop countries with limited domestic resources (and deep pockets). Singapore and Qatar are big importers; the Burj Khalifa skyscraper in Dubai was built using Australian imports.


          It seems insane, really, that countries like the UAE are importing sand from Australia. It’s reminiscent of the joke about selling refrigerators to the Eskimos.

      • beobrigitte says:

        You’re right, Nisakiman! I watched something about it on youtube quite a while ago. Unfortunately I remembered only some of it – and got it the wrong way round…..

  2. waltc says:

    OT –on Idle Theory.
    There’s an interesting article in this weeks New Yorker magazine, headlined “How civilization started (was it a good idea?)” that you might want to look up a link for. (i’m reading the hard copy.) But here’s the passage that made me think of you:

    “It turns out that hunting and gathering is a good way to live. A study from 1966 found that it took a [tribal African Bushman] only about 17 hours a week on average to find an adequate supply of food; another 19 hours were spent on domestic activities and chores. The average caloric intake of the hunter gatherers was 2300 a day, close to the recommended amount. [In 1966] a comparable week in the US involved 40 hours of work and 36 of domestic labor. The [tribesmen] don’t accumulate surpluses; they get all the food they need and then stop.”

    • Frank Davis says:

      One difference between African Bushmen and Americans would probably be that the Americans would also be funding idle time activities – like going to bars and restaurants and cinemas, or buying books or magazines or clothes – using the money they earned during their 40 hour week. How much of their work was to earn what they needed for basic necessities, and how much for amusing pastimes? If the latter were stripped out, their idleness might be comparable with Bushmen.

      Unfortunately, Tobacco Control and their puritan chums seem to want to actually strip out all the amusing pastimes, including smoking and drinking.

      • waltc says:

        Point well taken, but maybe they spend their idle time telling stories around the campfire, doing rain dances, carving bison on walls, painting each others faces, smoking wild tobacco plants, drinking fermented berry juice and having terrific sex. Just a thought. 🤔

  3. More than I ever knew about sand before! Thank you Frank! :)
    Amazing about the black market and environmental problem stuff: totally new to me.

    As for this though: “Why is it that all beaches everywhere in the world seem to be composed of sand, and not mud or silt or gravel?” I’ve been to at least one beach (Along Puerto Rico’s northwest coast during a bicycle activist circumnavigation of the island years ‘n years ago.) that was composed of smooth irregularly roundish black pebbles of a half inch to an inch or so. No idea how/why it originated. And I was reading recently about a beach in Australia that was so heavily composed of unweathered shards of sea shells that you had to wear boots or you’d slice your feet up.

    – MJM, “Be The Sand.”

  4. waltc says:

    I’ve been watching the first parts of the Ken Burns (10 part) documentary series “The Vietnam War” on Public Broadcasting. It’s a searing but I think necessary experience at least for Americans–almost a kind of penance. (It may also be online at pbs.org and someone said it’s on itunes.)

    I don’t mean to reduce the gravity of that fucked up waste to the comparatively tiny issue of smoking. But. There was a clip of the young John McCain as a desperate POW, in deep physical pain, smoking a cigarette, and I thought about the later McCain having been the author of the proposed federal anti-tobacco legislation that was so strict (banning smoking in almost every building in America and imposing high federal taxes) that it led to the tobacco companies settling (The Master Settlement Agreement) in order to preclude it. And I wondered how he forgot.

    Not to mention (in light of the IDF ban) how combattants on both sides were shown constantly smoking.

    • Joe L. says:

      I’m sure McCain received enough money from antismoking lobbyists to cause selective amnesia.

      I didn’t realize McCain was an ex-smoker. I wonder if we’ll soon hear that his brain tumor (diagnosed at age 81) was caused by a moderate smoking habit 50 years ago.

    • nisakiman says:

      Thanks for the heads-up on the Vietnam War series, Walt. I’ve just started downloading the full series – all 47.2 GB of it! At my download speed (I cap it at 1.2 Mbps, but the speed depends on the seeds) it’s going to take a while…

    • Frank Davis says:

      First episode: Deja Vu. I was a distant non-participant in the Vietnam War.

      This might be the Vietnamese version, unfortunately.

      • nisakiman says:

        I was travelling in Asia (’67 – ’71) during the time the Vietnam war was raging, and met and got to know several GIs who had gone AWOL and were hiding out in India, Pakistan and Afghanistan. They were to a man seriously fucked up by their experience there, and some of the stories they told me were horrific. They, and their adversaries, had been thoroughly dehumanised, and all normal moral parameters had been cast aside.

  5. RdM says:

    “Where does the sand come from? How does it get to the beaches?”

    You might find these two posts interesting reading:

  6. roobeedoo2 says:

    Bad habits could be down to a single neuron in the brain, apparently…

    “This cell is a relatively rare cell but one that is very heavily connected to the main neurons that relay the outgoing message for this brain region,” says Nicole Calakos, an associate professor of neurology and neurobiology at the Duke University Medical Center.

    “We find that this cell is a master controller of habitual behavior, and it appears to do this by re-orchestrating the message sent by the outgoing neurons,” she says.


  7. Pingback: Missive From ‘Merica: Write On! – Library of Libraries

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