21 Aug MINE SEA SAND – Why not?
Prof. (Dr.) Thrivikramji. K.P. (Retd.)
Kariavattom Campus 695 581
Sea sand – a part of the mixture of clay and sand that gathered in the sea bed during the last several 10’s of million years – finally graduated as a promising substitute for vanishing river sand or the fine aggregate, along with cement in the construction industry. The proposal for dredging of seabed is wrought with several objections, viz., a). Loss of the fishing ground (hence livelihood of thousands of fish workers and their families); b). “Death” of the mud bank and consequent “demise” of Chakara; c). Deepening of the seabed to the disadvantage of opportunities for fisheries; d). Aggravation of beach erosion by waves, and e). Aggravation of water scarcity in the already water scarce coastal belt due to water washing of salty sea sand to remove salt for sale.
Firstly, some unknown facts about sand. For the layman sand is the granular, nearly clean accumulation of relatively hard natural material covering the riverbed, the sandy beaches, small and large deserts and a chief component enhancing soil quality and hence profitability of the farmer. If sand is not available from any of these sources, then one will go blast the rock –the ultimate source – crush and grind it to manufacture sand.
For the specialist sand is mineral quartz, SiO2, and has no doubt about the existence of an extensive sheet of sandy-clayey sediment and associated heavy minerals, hidden below the vast expanse of seawater of the
, adjacent to the mainland in the continental shelf with in a depth of 100 m. As far as Kerala goes, the area of the continental shelf is almost as large as 4 to 5 sixth of the 38, 855 Km2 land area.
Most people do not worry about the ultimate source of sand and for them, source is where it is found. It is untrue. Sand is an original constituent (~1/3 of the bulk) of the common crystalline rocks, The latter is now also marketed as sawed and polished slabs, as raw stone for foundation. Lions share of the 2/3 of bulk is feldspar – an aluminum silicate of potassium or sodium and calcium. The accessories (the ferromagnesian silicates) and heavy minerals (ilmenite, sillimanite, zircon, garnet and monazite) together contribute to 1 or 5 % of the total.
A process known as chemical weathering, Wc, unique to the monsoon climate of clear wet dry spells, is responsible for transforming the hard crystalline rock in to a state (soil formation) enabling running water to erode and transport the constituent rock minerals (soil erosion) to new sites of deposition. Wc also chemically converts feldspars and ferromagnesian minerals to a group of minerals called clays. Quartz and heavy accessories are an exception. This Wc, an extremely slow process, and a 1.0 m. thick slab of crystalline rock takes at least a million years to weather to soil or laterite. The water erosion did remove all the minerals including the clay. While the quartz and heavy (black sand) traveled down the river by rolling and jumping on the riverbed during flood, the micron sized clay physically floated down river in times of flood at the very same velocity of floodwater to reach the final resting place in the sea bed.
Land, Sea and Sand
In fact, a portion (6500 Km2. of the coastal land) of today’s land area (38855.0 Km2) of Kerala, is only a later addition, in the last 12,000 years ago, i.e., by end of the last glaciation. Coastal land added by deposition of SW monsoon wave driven seabed sand.
During the Pleistocene (1.8 ma ago), due to at least 4 different phases of continental ice sheet and glacier ice advances, the sea level did stand well below 100.0 m. exposing vast expanses of the continental shelf around the continents. Kerala was no exception. As a result, a shelf area nearly equivalent of present extent of Kerala stood above sea level, enjoying a sub-aerial environment and forcing the Kerala Rivers to have a longer run up, say 30 to 40 Km. more to the then shoreline. Thus, today a large alluvial deposit of sandy-clayey nature sits in the sea bed.
But then, on set of monsoon climate in Kerala ~50-55 M.a. ago, initiated Wc, Consequently during the same time span at least 50-55 m of crystalline rocks would have been transformed to laterite/soil, and that too along the length and breadth of the 30000 Km2 (pre-Pleistocene area) of the state. And handsome portion of the detritus (heavy mineral laden sandy-clayey sediment) must be now sitting in the Laccadive sea bed. A portion of the latter is has already been deposited (since the Pleistocene) as the low coastal land. This process of deposition continues even today, driven by the monsoon waves to renew the blacksand deposit in the Chavara – Neendakara – Thirkunnapuzha sector.
Myths of Sea Sand Mining Uncovered
The response of fisheries potential to sea sand mining is beyond my specialty and hence not addressed in the following. Let us examine the concerns one by one. The chakara, a relatively thick suspension of bottom mud (like a submerged break water) triggered by the onslaught of the SW monsoon waves on the seabed sediment appears between Thirkunnapuzha and
Kozhikode. The nutrients stirred up from bottom, attracts large schools of fish and fish-workers like a magnet. The fundamental requirements for chakara are the wave climate and substrate of mud. The dredging process automatically enhances the mud content in the sea bed and hence only adds to strength and vigor of chakara.
The dredging removes large volumes of sand and hence will lower sea bed. Look at the ship channels in the
harbor that require dredging of huge volumes of sediment to maintain a draft of 13.8 m. This dredging is due to reasonably fast redistribution of bottom sediment by the currents. In the sea, currents are much vigorous and probably of longer duration, tending to repair the depth changes at the heel of dredging. Further, the dredging related disturbance at no point in time exceeds a tract of 15-20 times the diameter of the length of the dredger. More over the monsoon wave climate should be a quickest sediment redistributor and a healer of the submarine scars.
The Chief Hydrographers maps do not show any depth changes either. The sea bed has a very rich in sediment reservoir. The plume of murky water of about 20 to 30 times the diameter of the barge is in fact very short lived, especially in a region with the monsoon wave climate. Secondly, the dredger will have to remain in-operational during the same period for safety reasons.
Secondly, the clayey sandy sediment with ilmenite and other placer minerals is stirred up by the monsoon waves and is transported shoreward toward the mining lease tracts of the IRE and KMML. But the finer clay particles do not settle in the beach and is kept in suspension and is transported seaward, southward or northward toward Thirkunnapuzha and beyond to nurture the source sediment of Chakara. The proof of northerly transport is absence of chakara in any place to the south of Thirkunnapuzha.
The acceleration of beach erosion is a natural concern of the public. In Kerala, no single beach is known to have consistently undergone erosion. It is always a mixed bag. When there is erosion it is featured in the press and media, but not when there is accretion and gain. In fact about 6000 Km2 area in the coastal land was created by wave deposition only and that too in the last 12,000 yr. Therefore, erosion and deposition are two sides of the same coin and dredging as such does not influence both as long as seabed has an optimal reserve of sediment.
Finally about the waste of water in washing the sand coming off the dredger before it is sold. Take a look at the bridge constructed by the L&T Ltd., connecting
Kochi with Island or the wharfs in the harbour. These are structures on concrete columns bathed in brackish water for 24 hours of the day. There are salt water tolerant cements and hence concrete (e.g., Kochi ). That is the sand does not require to be washed to the purity of a downstream use in a pharmacy industry.
Therefore, we can always look at the sea not only for food but also for the mineral wealth including sea sand waiting to be mined or exploited. The choice of words is yours.