Can Bangladesh trap silt?

 
By Mostafa Kamal Majumdar
Last Updated: Saturday 04 July 2015

Recently in Dhaka, Dutch ambassador Bea Ten Tusscher suggested that Bangladesh, decidedly to be worst-hit by global-warming-induced sea level rise, could outpace this disaster by trapping the silt three great rivers carry through it to the sea. It is a fascinating idea from the ambassador of a nation actively associated with the development of Bangladesh's water sector since the 1950s.

Nobody's said this before
Lush green Bangladesh sits on 5-15 km thick deltaic alluvial deposits brought down from the mountains by the Ganga, the Brahmaputra and the Meghna, over millennia. According to experts, they carry an estimated one billion tonnes of silt that is deposited in the sea. Land elevation in 75 per cent of the delta ranges from one metre to less than three metres above the mean sea level. In a normal rainy season, floods inundate one third of the country; big floods submerge two-thirds. Either way, crops are damaged, injuring the economy. What will sea level rise, then, unleash?

As it is, with about 150 million people on 1,47,570 sq km of land, the population density of 904 persons per sq km is the highest in the world. Consequently, in addition to hordes of Bangladeshis seeking livelihoods outside the country, there is a clear trend of in-country migration from the less developed and economically deprived zones to cities and towns.

Anticipated here at 32 cm by 2050 and 88 cm by 2100, plus another 10-20 cm local rise due to land subsidence, sea level rise will multiply this injury. About 16 per cent of the current land area might go under water, damaging croplands, causing habitat loss and forcing people to migrate.

Outpace it, the Dutch ambassador suggests; adverse impacts would also not exist.

Is it realistic?
Emaduddin Ahmed, executive director of the Institute of Water Modelling (iwm), thinks raising the level of land threatened with inundation due to sea level rise is theoretically not impossible. The silt is huge. Yet, not all of the land that will go under is located near the rivers. Thus, transferring silt would be very costly, also in terms of ecosystemic re-engineering.

As precedence, the iwm chief draws attention to the reclamation of around 10,000 ha of land in the coastal area of greater Noakhali in the southern part of Bangladesh, since the 1960s till the 1980s, by constructing cross dams. These reclaimed lands are now fertile; farmers grow rice and other crops. Yet, the reclamation caused problems; in the adjacent mainland, drainage canals that took water south, into the sea, got clogged. An alternative arrangement had to be made to drain rain- and river-water towards the west, into the Meghna

Also, there has been little net land accretion in the Bay of Bengal, irrespective of the huge silt load. Ahmed says there is a shallow expanse of the continental shelf in the Bay of Bengal. Sometimes during low tide, tips of small islets are noticed; then they disappear. Coastal erosion also devours land from time to time.

Another way of raising land levels along the coast could be to trap sediment brought in from the sea by the tide. But would people accept their land being degraded by saline sediment?

Is it possible?
Monowar Hossain, an expert on the sediment load of the major rivers of Bangladesh, says the idea merits experimentation. Some of the sediment the rivers carry naturally balance out the land subsidence the delta experiences. About raising the land level of the inundation-prone areas, he believes that apart from dredging to transfer silt from riverbeds, diversion of silt might also be possible. But this calls for a careful study and experiment.

Water expert Ainun Nishat, now country representative, Bangladesh of the International Union for the Conservation of Nature (iucn), is more sceptical. He underlines an adaptive strategy, the need to strengthen coastal erosion measures--coastal dykes, embankments, and more cyclone shelters (about 2,000 are already constructed). Cyclone forecasting should improve, he believes, and warning and capacity enhancement for disaster management.

Comments eminent geographer and water expert Maniruzzaman Miah, "It is beyond my comprehension as to how much silt may be needed to build what extent of land over how long a period and that too with the swatch of no-land, the depth of which has never been known." The Dutch, however, know how to reclaim land from the sea.

This writer thinks Bangladesh may request the Dutch government to prepare a feasibility study. "Why not?" says Miah, a former vice-chancellor of Dhaka University.

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