Grains of truth show from riddle of the sands

SCIENTISTS believe they have unravelled the riddle of the sands. They have constructed a mathematical explanation for the ripples on a sandy beach.

The ripples are usually spaced at intervals of about 10cm and formed by the action of wind on individual grains of sand and not, as commonly thought, by the sea.

Why the ripples should form at all has mystified scientists for at least 50 years, but two Japanese researchers, Hiraku Nishimori and Noriyuki Ouchi of Ibaraki University, now believe that they are close to the answer.

A sand grain can move by rolling under the force of the wind or jumping after colliding with another grain, a process called saltation. This second movement is important because a jumping sand grain can dislodge another when it lands, causing more movement.

The scientists believe saltation is largely responsible for the formation of ripples and have attempted to explain all in a mathematical model described by John Maddox, editor of Nature, as 'a hybrid between a lattice calculation and one appropriate to a system with continuous variables, which means it may even be read when the beach season is over'.

Wind dynamics play an understandably important role in ripple formation on a beach. However, it is not horizontal windspeed that is important but the up-and-down movements of wind shear - responsible for many aircraft accidents during take-off and landing - that the scientists deem to be important.

The shape of the ripples will also influence their evolution. A grain of sand that is lifted into the air from the top of a ripple will tend to behave differently to one that is blown up from a lower point on the ripple's windward face.