Q: While walking down a stony beach and skimming some stones into the sea, I noticed that the flattest pebbles (and the best ones to use for skimming) tended to accumulate at the top of the beach, furthest from the sea. Why is this?
The amusing answer that all of the flat stones are found in this location because those nearest to the sea have already been used up by people skimming earlier was provided by an astonishing 25 people. Perhaps there is a ring of truth in their suggestions. However, we suspect that the true reasons are given below.
A: The coastal sediment system is dynamic. Wave action constantly washes the sediment along the coast until it reaches a point where the wave energy is reduced by the local topography to the extent that transport of the sediment is no longer possible, and it is deposited.
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Prevailing weather conditions, the aspect of the beach and the coastline, wave fetch, offshore topography and so on cause same beaches, over time, to be subject to a higher wave energy regime than others. Consequently, sediments are sorted by their location or graded by their ease of transport, and beaches which are subjected to a higher wave energy regime are constructed largely from those sediments which are more difficult to move and vice versa. In a similar manner areas that are further up a beach will have a much higher energy regime than areas nearer the water, as only the more energetic waves are capable of reaching them.
Normally, the sediments which are mare difficult to move by wave action are those which are larger and heavier or those which are more spherical and have a smoother surface, as these have a relatively low surface to weight ratio. However, most stones above a threshold size, certainly including those large enough to be held in the hand for skimming, are not picked up entirely by most waves, and are instead rolled to a new location. The shape of flat stones resists this rolling action and therefore they are more resistant to movement by waves, despite their large surface area relative to their weight.
Furthermore, when taken as a group rather than individually, stones with flat surfaces display characteristics of increased resistance to wave energy. On any shingle beach it can be observed that they sometimes tend to stack like coins and wedge between each other. This grouping makes them much more difficult for a wave to move and explains their location in an area of the beach that has a higher energy regime. Of course, their deposition in groups of this nature would also make them more noticeable and more enticing as a supply of skimming stones.
A: Sediment that is transported by waves running up a beach tends to reflect the contrast in power between the swash and the backwash. Because the wave鈥檚 energy is dissipated as it runs up the beach, and water tends to be lost by percolation into the beach sediment, the backwash is weaker than the swash. The swash is often able to carry coarse and fine particles up the beach, but the backwash is only capable of carrying the fine particles, leaving the coarser particles stranded at the top of the beach.
This crude sorting process usually reflects relatively high magnitude, low frequency storm events. The waves associated with these events leave behind 鈥渓ag鈥� deposits of coarse sediment which tend to stay put until they can be shifted by the next event of a comparable or higher magnitude. The 鈥榓rmoured鈥� beds associated with mountain rivers commonly display a similar accumulation of coarse sediment.
However, it is not just a particle鈥檚 size which determines the ease of its transport, and hence the sorting of sediments, but also its shape. The weight of a particle, and the friction that it generates by its contact with the layer below it, determine its resistance to flowing water. And its cross-sectional area that is exposed to the flow determines the magnitude of the fluid force that is required to move it from its resting position.
Flat particles within a flow of water will tend to rotate to lie flat. This gives them a high friction contact with the surface on which they are resting and a relatively small cross section in relation to their weight exposed to the flow. Because of this, they tend to be less mobile than a sphere of eqtial weight. Therefore, when a mixture of particles of different shapes but uniform weight is present, flat particles, once they are in position at the top of a beach will tend to remain there for the greatest time.
This week鈥檚 questions
The moonies: On 23 December 1995, around eight o鈥檆lock on a clear frosty evening, we were looking out at a gibbous Moon and noticed that the full disc of the Moon could be seen faintly. Most of it looked like a coin, dark grey, with one brightly lit edge.
Usually only the illuminated part of a gibbous Moon is visible. What made the shadowed side visible on this night, but not on others with similar sky conditions and with the same amount of background light from the countryside?
Bubble trouble: I have noticed that gas bubbles rising in a column of water behave differently according to their diameters. Bubbles with a diameter of less than about 1.5 millimetres rise straight to the surface. Bubbles larger than about 4 millimetres also rise towards the surface in a straight line but deform randomly from their spherical shape, while bubbles of intermediate diameters appear to oscillate regularly and follow a helical track upwards. The variation in bubble size is, I assume, dependent on surface tension and viscosity, but how does their regular motion came about? Are there equations which predict this behaviour?
Cooking salt: Is it true that food tastes more salty if the salt is added at the final moment of cooking rather than if it is added before or during the early stages of cooking? And, if this is the case, why?
Deep breath: Is it true that every time we take a breath of air or swallow a mouthful of water, we consume some of the atoms breathed or swallowed by Leonardo da Vinci (as I read in a children鈥檚 science book in 1960)?
Correction: In the answers to 鈥淧ower power鈥� (The Last Word, 10 February) it was incorrectly stated that the carbon cycle in main sequence stars involved the conversion of hydrogen into carbon.
Some words were excluded inadvertently leading to an erroneous sentence. The second sentence of Mike Dworetsky鈥檚 letter should have read: 鈥淭his is especially true for the carbon cycle in main sequence stars (in which hydrogen is converted to helium) and for the triple-alpha reaction in evolved stars (in which helium is converted into carbon).鈥� Our apologies for the confusion that this may have caused.
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