The Proposal of Strengthening Embankment in Sundarban : Myth and Reality
Kalyan Rudra
Since Aila or the tropical cyclone of wind speed 120km/hr ravaged Sundarban on the 25th May, 2009, the Government of West Bengal, as reported in newspapers, holds the opinion that strengthening the embankments with concrete would save the people from ensuing disasters. This hasty opinion emerged in complete isolation from an understanding of the holistic eco-hydrology of the Sundarban and had no honest commitment to either science or society. Vast areas are still under water and cultivation of paddy seems impossible as seeds did not germinate due to salinity of soil above the threshold limit. Most people have not been able to rebuild their damaged huts and are virtually living under the sky. They are have nots in true sense. The victims are starving as supply of relief materials was miserably inadequate and now, has almost ceased. The deep despair overshadows lives in Sundarban. The large scale exodus has started.
A tropical cyclone in the Bay of Bengal is a natural hydro-meteorological event which causes disaster because of the socio-technical failure to cope with this natural phenomenon. Disaster management in West Bengal is a reactive programme; it starts to operate after the event strikes. There is needs to be a paradigm shift from erstwhile relief-centric and post-event response to a more pro-active approach encompassing prevention, mitigation and preparedness-driven disaster management - declared by the National Disaster Management Authority in its flood management policy of 2008. The Government of West Bengal indeed maintains a Disaster Management Department but it reacts late as usual after the event when the farthest terrains are difficult to approach and the afflicted community is in deep danger.
Sundarban is an area of intricate network channels and intervening islands. Since the British rule, 54 islands out of 102 were deforested over four phases: these are now home to more than four million people. The remaining islands are still virgin, covered with dense mangrove forests. The Sundarban was declared as a Biosphere Reserve which includes the Tiger Reserve, the National Park and Wild Life Sanctuaries. Geographically, six blocks of the North 24 Parganas and thirteen blocks of South 24 Parganas bounded by Hugli river in the west, the Raimangal-Hariabhanga rivers in the east, the Bay of Bengal in the south and the Dampier and Hodges Line in the north is identified as Indian Sundarban. The following table furnishes the geography of Sundarban at a glance.
- Area within West Bengal : 9630 km2
- No. of Islands : 54
- Height above mean sea level : 3m
- Tidal Height : + 5m
- Mangrove Forest Area : 4267 km2
- Reclaimed Area : 5363 km2
- Area under water : 2069 km2
- Population : 42 lakh (2001)
- Length of Embankments : 3500 km
The damages caused by Aila were unprecedented. The 20th century witnessed 44 tropical cyclones striking the coast of West Bengal but Aila surpassed all previous records in terms of damage and economic loss.
- Villages affected: 4249
- Population: 25, 62,442
- Missing: 8000
- Death: Official- 70 Unofficial-300
- Embankment breached: 400km.
- Cattle lost: 2,12,851
- Agricultural land: 1,25,872 ha
- Estimated loss in agriculture: Rs.337crore.
- Houses fully damaged: 1,94,390
- Houses Partially damaged: 1,94,701
- Total loss: Rs.1495.63 crore.
In view of predictions made in the fourth assessment report of Inter Governmental Panel on Climate Change that there will be more extreme hydro-meteorological events affecting live and livelihood of the people living in Indian Subcontinent, we need a pro-active disaster management plan. The Sundarban is extremely vulnerable in terms of its location, topography, climate and hydrology. The people living there constantly fight with many odds and evils that have emerged out of wrongly- directed management plans being followed since the colonial period.
Delta Building Mechanism:
The proper management plan for Sundarban needs to have, to begin with, an understanding of processes involved in delta building, especially the sediment dispersal mechanism during tidal fluctuation. The delta has a long depositional history covering more than 70 million years. It is the area where fluvial and marine land building processes have been working simultaneously with cyclical advancement and retreat of the Sea during past geological ages. The sea face gradually retreats southwards and sedimentation continues to build up new land on the continental shelf. The delta is divided in to three parts: the moribund, mature and active delta. A large part of the sediment reaches the sea through two marginal estuaries- Hugli in the west and Padma-Meghna in the east. But the tide pushes back a portion of sediment load through creeks and rivers. The silt-laden water spills over the flood plain during high tide and recedes during low tide and thus land along the flood plain of these rivers gets elevated. So Sundarban area is identified as active delta. The part of the delta is identified as matured when it is elevated above highest high water level. Then the area gets sediment deposit only during flood. When the rivers area decayed, the area is described as moribund delta.
The Sundarban is a part of the tidally active delta where the land building process is incomplete till date. Numerous interlacing channels, intervening islands with the elevation of less than three metres above mean sea level and dense mangrove forests have given this littoral tract a distinctive geographical identity. The Ganga- Brahmaputra system carries suspended sediment load amounting to more than one billion tonnes per year. This is most conservative estimate as other experts estimated the load as 1.67 billion tones.The subaqueous delta, as seen in the satellite image, extends far south from the coastline into the Bay of Bengal. Despite receiving such sediment load, there has been no appreciable growth of new land along the coast during last three centuries. There are various factors which are together responsible for this condition of the delta. It is reported by the experts that delta building is impeded due to siphoning of sediment away from the coast through the Swatch of No Ground or the submarine canyon of the Bay of Bengal. Secondly, destructive waves continuously attack the southern littoral tract and erode land. Thirdly, slow subsidence due to auto-compaction of newly deposited sediment surpasses the effect of accretion.
Encroachment on the Pre-mature land
After the battle of Plassy in 1757, Mir Jafar conferred to the East India Company the area covering 882 square miles lying south of Fort William to the east bank of Hugli river. Shortly after that, the forest cover over extensive areas were cleared and the land was divided into smaller zones, which were numbered as Lots and given to farming within the legal frame of the Zamindari land management. The premature land reclamation in Sundarban was completed in four phases from 1770 to 1971:
- First Phase (1770 -- 1780)
- Second Phase (1780 -- 1873)
- Third Phase (1873 -- 1939)
- Fourth Phase (1945 -- 1971)
The clearing of forests did not facilitate beginning of agriculture on the flood plain which was subject to submergence under saline water during high tides. The British built embankments along the bank of creeks to prevent the ingress of saline water. Thus the spill-over of silt-laden water on floodplains was restricted, but the practice impeded the dynamics of sedimentation. As a result silt was trapped within the embankments and river beds were filled up to make the channels increasingly shallow. While river beds were elevated, the flood plain remained at the same height. Rainwater falling on the floodplains remained stagnant for a longer period. The tidal inflow was transformed into a tidal bore which had the power to breach the embankments.
Now water levels in many embanked creeks remain at least two metres above the adjoining flood plains during high tides. The destructive wave during Aila not only breached a 400 km stretch of embankment but also overtopped at many places. The water remained stagnant and marooned more than two million people for several days. The word ‘disaster’ seems a rather weak expression to describe the distress of the afflicted community. The hapless people have been living under subhuman conditions. The supply of drinking water has become the most serious challenge. It was water everywhere but there was not a single drop to drink. Most of the thatched houses have either collapsed or have been damaged. The farmland has gone under saline water making it non-productive.
Embankment as a protective measure
The creeks of Sundarbans are classed as meso-macro tidal, having an amplitude ranging from 2 to 5.5 metre. It is important to note that height of most of the areas in the active delta or Sundarban is < 3m above mean sea level. Most of these creeks do not have any fresh water supply from upstream areas. The term ‘fresh water’ includes both rain and snow melt water. The water of the Ganga-Brahamputra basin covering more than 1.50 million km2 is discharged into Bay of Bengal through two marginal estuary of the delta- the Hugli in the west and the Padma-Meghna in the east. Out of the eleven intermediate estuaries of undivided Sundarban, Sapatamukhi, Zamira, Matla, Bangaduni and Gosaba drain the Indian Sundarban and have no supply of upstream fresh water. These creeks are beheaded and replenished by tidal flow.
The efficacy of embankments as flood control measures have already proved inadequate in many parts of this subcontinent. The embankments in Sundarban caused increasing sedimentation on the river bed which gradually went high above the floodplain. The water level goes further above the crest of the embankment during cyclonic upsurges. The impinging waves scour the base and cause breaches in the embankment. The wave achieved a height of three metres during the Aila and overtopped the barrier. The reversal of gradient causes stagnation of water on floodplains for a protracted period till evaporation or infiltration. So the proposed concrete embankment is not a solution to present drainage congestion in Sundarban, nor will it ensure any security to the people. Since deposited sediment layers of Sundarban are not yet fully consolidated and the area is slowly subsiding, the overlying load of concrete embankment may cause collapse of bank. Moreover, deep foundation of embankments would impair the dynamic relationship between ground water table and river as both the influent and effluent seepages would be retarded. The concrete embankment on the bank may be affected hydrostatic pressure causing uplift and dislodgement of such structure. The scouring of the bank along the concave side of meander and heavy weight of overlying structure would possibly lead to collapse of bank. The application of polymer geo-tube packed with sand as a bank protection measure may be more effective but it is neither cost effective nor eco-friendly option.
Comparison with the Netherlands
The delta management in the Netherland is often compared with the case of Sundarban. But delta of the Netherlands is hydro-geomorphologically different from Sundaban. The notable differences are:
- The estuaries of the Netherlands are aligned in an east-west direction but those of the Ganga are aligned north-south and allow easy invasion of tide far inland. The off-shore currents in Netherlands flow parallel to the coast while Coromondal and Martaban currents of the Indian Ocean flow northwards and strike the southern face of Ganga-Brahmaputra delta.
- The sediment load carried annually by the Ganga- Brahmaputra system into the Bay of Bengal is about 1000 million tonnes and this is more than four times the load carried by all European rivers together.
- The maximum possible tidal amplitude in estuaries of the Netherlands is < 3.5 m and those are classed as meso-tidal while many estuaries of the Sundarban are said to be macro-tidal having maximum tidal amplitude close to 5.5m.
Comparison with US: Hurricane Katrina
In the month of August in 2005, hurricane Katrina devastated thousands of lives in southern Florida, Louisiana, Mississipi, and Alabama in the United States. One of the worst affected cities was New Orleans, home to nearly 1.4 million people where storm surges were more than five metres above normal tide levels. Despite a large-scale pre-disaster evacuation, thousands of people had decided to stay behind, mostly the poor and the sick; these people and were trapped by rising waters. The death toll was around 1500 (Kelman, I, 2007, Peck, 2008).
The city of New Orleans is geographically situated below the nearby water levels of the sea, river and lakes. Levees protecting the city failed miserably, resulting in flooding that was several metres deep (ibid). This is much comparable to the situation in Sunderban after Aila. Talking of Katrina, Economist dated September 10, 2005 opined: “ The disaster has exposed some shocking truths about the place: the bitterness of its sharp racial divide, the abandonment of the dispossessed, the weakness of the critical infrastructure” (quoted in Peck, 2007).
Sustainable Options
- The plugging of breaches is an immediate necessity to prevent further inflow of water into floodplain. The height of embankment should be determined based on highest recorded storm surge which may be more than five metres above high tide level. Since the area is environmentally fragile and ecologically sensitive no non-biodegradable material should be used as far as possible; the local soil and bamboo can be better options. But the presently soil are being borrowed from river front creating hollows at the base of embankment. This makes the embankment vulnerable.
- All the embanked creeks of Sundarban should be allowed wider spill area to reduce the hydrostatic pressure on embankments. The suspended solid in the creeks is estimated to be 15mg /litre. It should be learnt from experience that unless we allow spill over of suspended solids carried by the river, the decay of rivers are inevitable.
- There should be a long-term sustainable planning for the Sundarban. The greatest challenge is to combat the reversed slope between river and floodplain caused by uninterrupted sedimentation in the river bed.
There are two options:
- The dredging of channel to a depth below the adjoining floodplain. This is neither an economically viable nor a technically feasible solution. The depth of water in many creeks remains less than the draught required for plying of the modern dredger. Moreover, in view of tide velocity asymmetry, dredging is not a foolproof solution. The water travels fast towards north during high tide for about four hours and retreats at a slower speed during subsequent eight hours leaving behind substantial sediment load which was carried up. So spill over of the sediment on the floodplains can be best possible option which will again elevate the land to the desired height. Note that the non-reclaimed part of Sundraban (i.e the forest area) now stands above reclaimed part as the land building processes were not impaired there.
- We need to construct retired embankments about 500m away from the older ones and allow regulated spill of silt-laden water in the tract lying between the old and new embankments. The settlements trapped between embankments should be evacuated and temporarily rehabilitated elsewhere until the land is elevated to the desired level by the process of accretion. Plantation of mangroves along the banks of rivers can ensure protection from bank failure.
The management of Sundarban does not need any foreign collaboration. There is no reason to depend on the expensive technology of using geotube. We can combat the challenge with indigenous materials (mud, bamboo etc) and local labour force. It is a long-term or perhaps unending project which may offer rural employment guarantee. The project can be included in the National Rural Employment Guarantee Scheme. (NREGS).
The agrarian economy of Sundarban has been facing a serious challenge due to ingress of saline water. The natural process of flushing out of salinity from the top soil may take two or three rainy seasons. This has been causing large scale exodus of labour force from Sundarban. The exhaustive research is urgently needed before introduction of salinity resistant crops. The area which remains perennially waterlogged may be used for aquaculture.
However, before adopting any management plan, we must have a clear understanding of the hydro-geomorphic processes operating in Sundarban and also bear in mind the long-term development implications of any such plans on the socio-economic conditions of the millions of lives involved.
References
- Kelman, I (2007) Hurricane Katrina Disaster Diplomacy, Disasters, 31 (3), pp 288-309.
- Peck, J (2006) Neoliberal Hurricane: Who framed New Orleans? in Panitch, L and Leys, C (ed) The Socialist Register 2007, London: Merlin, pp 102-129.
— Prof. Rudra is a member of Ganga Monitoring Committee
collapsed huts at Kumirmari and Chhoto Mollakhali
collapsed huts at Kumarmari and Chhoto Mollakhali
the disaster left her alone
ruins of a home
breached embarkment
