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THIRUVANANTHAPURAM, March 7

ALTHOUGH sand is one of the world's most plentiful resources, constituting as much as 20 per cent of the earth's surface, clean sand is fast becoming scarce to source, according to an expert.

Presenting his paper at the workshop on sea sand mining conducted here by the Swadeshi Science Movement, Dr K. Ravindran, former Director of the Central Institute of Fisheries Technology, said the coastal shelves and basins are now filled by muddy deposits generated by rampant soil erosion and this is what has affected the availability of clean sand.

Explaining the wave action on sand, Dr Ravindran said that with each wave, the sand is moved. When a wave enters shallow water, it starts to rise. Crests become shorter and steeper, whereas the troughs in between do exactly the opposite. With each crest comes a forward rush of water over the bottom, followed by a more gradual return movement as the trough moves over. As a result, the sand moves towards the beach.

The depth of a wave is not measured by its size, but by its length. The height represents the quantum of energy it packs. Small, short waves add sand to the beach whereas tall and long waves shift sand in the deep water. As a wave spills and rushes onto the beach, it drags sand up the beach slope. The retreating waves washes it, but being less powerful, leaves some sand behind. Under the influence of currents (often induced by winds), the sand is easily moved in the wave zone along the beach. This is called littoral alongshore sand drift.

The shape of beaches, dunes and the sea bottom is maintained by the balance of the forces that create them (tides, waves, wind and the sun) and those that pull them down (gravity, rain). The wind and the weather have a decisive discipline, causing the beach profile to change from day to day, season to season and year to year. When sea sand is drawn out from the deep, a new balance would have to be found, and along with it, a new profile.

In this manner, mining of the seabed for sand can have a profound effect on the whole system and its entire profile. Where sand moves quickly (for instance, on the beach), changes are felt soon, but where sand moves slowly (deep water), the changes may not manifest themselves for decades.

The cleanest, and thus the most valuable sand is found in the foredunes, beach and near-shore. But this is also the most sensitive band since its slope is the steepest. Mining sand from this area will inevitably result in beach erosion. To prevent beach retreat, one should mine the areas with the least slope, viz. the rear dunes and the deep sand.

Sand removed from a depth (6 to 10 metres) will most likely be replaced by beach sand moving in to restore the balance, since the slope here is the steepest. Deep storm waves will eventually move sand from deeper (10 m-25 m) reaches towards the beach, but this may take decades. When it happens, the sand is moved massively and over a huge area. So far, though, scientific knowledge about this process is almost entirely lacking, Dr Ravindran said.

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