Sea Level Change
Sea Level Change variation in the average level of the surface of the sea. It is adopted as a datum plane, ie sea level datum for measurement of elevations and depths. global warming and subsequent climate change, and sea level rise are subjects of worldwide scientific and economic importance today. Bangladesh is most vulnerable to the effects of climate change and sea level rise due to the low elevation of the land areas, the low gradient of the rivers, and exposure to the sea. So the significant effects of sea level changes on flooding, drainage, agriculture, tides, waves, and vital wetlands cannot be ignored.
History of sea level research The occurrence of variations in sea level has been recognised since long; the cumulative evidence in early Indian literature (Upanisad), Tamil and Sanskrit, bear witness to it. Changes of land level to sea level and vice-versa were documented as early as the eighth and ninth centuries in China. In 1785, Frizi introduced the 'Theory of Earth Rotation' to explain the fall in sea level in Fennoscandia and its rise in the Mediterranean. More field evidence became available in the mid-nineteenth century to aid the explanation of the land/sea level relationship. The upward growth of coral atolls in the Pacific was suggested by Darwin (1842) as due to the sinking of the ocean floor. The theory of Glacio-eustasy introduced by MacLaren (1842) was illustrated by Daly (1920) and until the twentieth century this concept was widely accepted.
Only a few attempts have so far been made to reconstruct the Holocene sea level history of the bengal basin. S P Chatterjee (1961) noted the existence of seven marine terraces along Indian coasts as being records of sea level changes. According to him a submerged reef, the Adam's Bridge in the bay of bengal, which is hardly 4m below the sea level, is evidence of a post glacial sea level rise which caused the submergence of this connecting link between India and Sri Lanka. Rajamanickam and Loveson (1990) recognised a large number of coastal features as evidence of post glacial sea level variations along Bay of Bengal. Umitsu (1987) proposed a curve of continuous relative sea level rise for the Bengal lowlands. This curve has some convincing evidence of radiocarbon dates relating to relative sea level movement. In the same year Banerjee and Sen also drew relative sea level curve for the Bengal basin. Without postulating any curve, Monsur and Kamal in the early 1990s have accepted the Fairbridge (1961) eustatic sea level curve as applicable for Bangladesh and has reported the high sea level strand along the Bangladesh coast at about 6000 years BP (before present). Other workers like Peter Ravenscroft (1995) and Islam (1996) also introduced Holocene relative sea level curves for Bangladesh.
Changes in sea level Global mean sea level has been changing by several processes on different time and space scales. Glacio-isostatic rebound, oceanographic, atmospheric, and tectonic effects are important for such changes. Eustatic sea level variation is associated with the volume change of seawater. Relative sea level rise can be different due to local uplift and subsidence.
Sea level rise due to sedimentation, although significant near river deltas, is negligible on a global scale. Sea level changes on a short time scale (several years) are associated with El Nino/ Southern oscillations. Direct rainfall and river runoff water may be significant factors in short-term changes (seasonal) in sea level along some coasts. Rainfall and runoff water contribute significantly to producing the 100 cm sea level variation seen in the Bay of Bengal. The seasonal sea level changes in the Bay is remarkable and one of the highest in the world. Due to its funnel shaped geometry the maximum seasonal variation can be seen along the northeast coast of the Bay. Along Chittagong coast the annual variation is 1.18m.
The sea level movements along the coastal belts of Bangladesh are mostly dominated by regional and local components. In the Bay of Bengal during the last 9,000 years, five periods of marine transgressions, each followed by regression, have been recorded. A maximum relative sea level rise rate of 3.65mm/yr-1 has been estimated between 6315 and 5915 years BP; the average rate for the Bengal basin, during the Holocene, was 1.07 mm/yr-1
Effects of sea level rise The rise of global sea level by the end of AD 2050 would mean that there is a high risk of coastal inundation by sea water. In the case of Bangladesh the projected 1.44m rise of sea level would inundate 16% of the populated land, displace 13% of the population and lose 10% of the GDP. The effect of sea level rise is more critical, since the Bengal delta is subsiding.
Flooding and erosion/sedimentation Bangladesh experiences moderate to severe flooding every year. Frequent storm surges also cause severe coastal flooding. The flood situation is further aggravated by the high tide in the Bay of Bengal. It has been seen with a 1.4m rise in sea level water level rises to about 6m near the meghna estuary. Even with a 0.2m rise in sea level, water level rises between 4.5 and 5m near the estuary. Since most of the coastal area is below 1.5m above mean sea level (MSL) and the area near the confluence of the ganges and Meghna is below 3m above MSL, both depth and area of inundation will increase extensively. However, the water level in the Ganges and Upper Meghna also increases significantly due to backwater effect as a result of changes in the hydrodynamics of flow. Hence the severity and extent of flooding will increase even in the upstream portion of the river. On the other hand, a rise in sea level will also move the shoreline landward and this will result in loss of farmland, leading to the shifting of agriculture, reduced crop yields, and loss of cultivable areas. Increased flooding will cause problems with existing irrigation and drainage system too.
The erosion-sedimentation pattern will change with a sea level rise and associated backwater effect. There will be reduction in the discharge capacity of the rivers because of backwater effect. As a result, sedimentation may shift upstream. This would diminish the river gradient, resulting in a decrease in transport capacity and thus moving the river bed sedimentation further upstream resulting in the increase in flood frequency.
Tides, waves, and coastal erosion The present flood situation is also aggravated by the high tides in the Bay of Bengal. Tides in the Bay originate in the indian ocean. The tidal range along the Bangladesh coast varies between 3m and 6m. The high tidal wave action thus contributes to shoreline erosion in Bangladesh. The rise in sea level will move the shoreline farther landward and will reduce the bottom friction of the tidal flow and hence the tidal range will increase. Hence increasing tidal range and tidal wave action due to sea level rise is likely to augment coastal erosion in Bangladesh and to change the coastal morphodynamics.
Recently, in some parts of Chittagong coastal belt, erosion has increased at an alarming rate. A vast area including the export processing zone, the Naval establishment, and a large industrial estate could be in danger if the present rate of erosion continues. Port facilities will have to be adjusted to a higher sea level.
Agriculture Rise in sea level, increase in river discharge, and other potential changes in climate will significantly affect agriculture. In some areas even deep-water rice will be affected because of flooding too early in the rainy season for crop growth. The land area suitable for aus paddy and jute would be reduced, and farmers would grow lower-yielding varieties of aman, which are tolerant of deeper flooding. Even in the dry season, because of the slow rate of drainage, the land will be too wet for sowing dryland crops. Hence there will be changes in agricultural pattern and practice due to loss of cultivable areas and reduction in yield. This will create a need to modify existing cropping practices to cope with changing climate and rise in sea level.
Salt water intrusion Sea level rise will affect seepage and salt water intrusion, especially in the coastal areas of Bangladesh. There will be an increase in seepage in areas where the phreatic groundwater level is kept low in an artificial way (eg, polder areas as beel dakatia). The north-south gradient of groundwater in Bangladesh will also be affected by a sea level rise.
Sanitation, waste disposal and health hazards Because of increased flooding in low-lying coastal areas there will be immediate disruption of sanitation systems. The spread of infectious diseases could markedly increase if the flooding of coastal areas increases. The flooding of dumps containing hazardous industrial wastes could also lead to environmental problems.
Wetlands Two critical problems for wetlands that would result from sea level rise are submergence and salinity increase. Submergence occurs when vertical accretion of the wetland surface cannot keep pace with the rate of water level rise. Hence sediment input to wetlands is very important. If wetlands do not accrete vertically at a rate equal to the rate of sea level rise, wetlands will be stressed and ultimately disappear.
A deeper penetration of tide water due to increase in tidal range would cause the formation of tidal levees and extend the mangrove zone. However, in a country like Bangladesh, where inland areas are occupied by agriculture or settlements and protected by dikes, the mangrove zone would become narrower by erosion and might even disappear. [Sifatul Quader Chowdhury]
Bibliography MS Islam, Sea level changes in Bangladesh, Asiatic Society of Bangladesh, Dhaka, 2001; Selina Begum and George Fleming, 'Climate Change and Sea Level Rise in Bangladesh, part 2: Effects' Marine Geodesy, 20:1, 1997.