Bangladesh, one of the world's poorest nations is also the country most vulnerable to sea-level rise. The population is already severely affected by storm surges. Catastrophic events in the past have caused damage up to 100 km inland. It is hard to imagine to what extent these catastrophes would be with accelerated sea-level rise.

Digital terrain modelling techniques have been used to display the Bangladesh scenarios. A three dimensional view of the country has been overlaid with the current coastline and major rivers and potential future sea levels at 1,5 meters.

Since this scenario was calculated in 1989, the expected rate of sea level rise has been modified. At present expected rates, this stage will occur in about 150 years from now.

The data given here are course estimates, using the same parameters as Delft Hydraulics. (UNEP/Delft 1989)

Today, 46 million people live in flood-endangered areas. A sea-level rise of half a meter will put 92 million people at risk, while a rise of one meter increases the number to 118 million (this is without taking future population growth into consideration). Loss of land area can be significant for coastal and island states and flat lowlands such as the Netherlands, Denmark, the Maldives, and Bangladesh.

Although Denmark and the Netherlands are both rich, European countries, there are some differences in vulnerability. Compared to Denmark, the Netherlands has a shorter coastline to secure plus it already has a well-developed dike network and extensive experience in building dikes. In this sense, Denmark is more vulnerable than the Netherlands, but because both the Netherlands and Denmark are rich countries, they both have good possibilities to build dikes and take other actions to limit the economic damages and losses of life brought about by sea-level rise.

In contrast, agriculture is Bangladesh’s main industry, and it is one of the world’s poorest and most densely populated countries. It often experiences flooding that takes the lives of many people. A rise in sea-level could put large populated areas under water and put even larger areas at risk of flood. Because the country’s economy and the people’s access to food depend on agriculture, Bangladesh is very vulnerable to flooding and flood-related catastrophes. Moreover, a poor country like Bangladesh has little adaptive capacity because it cannot afford to build dikes and in other ways prepare for a rise in sea level. Bangladesh can thus be seriously affected and suffer great losses of both materials and human lives.

Summary of the consequences of sea level rise

For every centimetre the sea rises, about one metre of land is lost to the sea.  The consequences of this are huge.

• Most of the human population lives close to the sea.  In Bangladesh, for example, about 17 million people live less than one metre above sea level.

• Extensive flooding is a threat to human health.  Many people will die and moving huge numbers of people away from flooded regions, particularly in the developing world, will increase the risk of diseases spreading.  There is also likely to be a reduction in the quantity and quality of freshwater, further affecting human health.

• Important biological communities are likely to be lost because some species will not be able to adapt quickly enough to changes in salinity.

• Coastal regions are important for ports, fisheries, agriculture and tourism.  Flood defences prevent natural variations in the coastline and protect these economically important activities.  However, they can also lead to increases in water levels in coastal areas by isolating the sea from its natural coastal flood plain.  This can lead to catastrophic flooding if the sea defences fail.

An increase in the mean global temperature would result in a possible rise in mean sea level across the globe. Such a rise is attributed to (i) the expansion of the ocean's water volume due to water temperatures increase and (ii) the atmospheric warming which causes melting of mountain glaciers and other cryospheric systems, resulting into an additional amount of water in the oceans.

In the context of Bangladesh, sea level rise would result in, among other impacts,

• Inundation of low-lying coastal areas - Retreat of shoreline
• Intrusion of saline water further inland and into ground water - Increase of wave height

The combined effects of an increased mean sea level rise and increased wave heights would disturb the stability of the coastal infrastructures and also alter the circulation patterns inside the coastal polders and the estuaries.

The most direct impact of the rise in sea level is the inundation of areas that have been located at elevations just above mean sea level and below the highest tide. Low lying coastal areas, coastal wetlands (unprotected newly accreted char lands) and mangrove forest areas of Bangladesh belong to this category.

The study by Khan et. al. (1999) has shown that the mean tidal level at Hiron Point in the west coast of Bangladesh has been rising at the rate 4 mm/per year during the last two decades (1977-1998). The rise of mean tidal levels at the Meghna estuary (Char Chenga) and at Cox's Bazar are 6.0 and 7.8 mm per year respectively. This rise may be due to thermal expansion of the sea, geological subsidence and other reasons like intensification of monsoon winds and higher runoff from the landside. This shows that the relative sea level rise in Bangladesh coast is highly prominent, however, since the data length is short, it might not reflect the exact nature of the long-term change of mean sea level. It may be mentioned here that the average sea level rise for Indian coast (Continuous with Bangladesh coast) has been reported as 2.5 mm/year since the 1950s.

It is projected that by 2030 SLR would be about 30 cm and by 2050 it would be about 50 cm. Even a very cautious projection of 10 cm sea level rise, which would most likely happen within 2020, would inundate about 2500 km2 (about 2% of the total area of Bangladesh) in Khulna, Patuakhali and Barisal regions. SLR in Bangladesh would inundate 4% (about 6300 km2) of the country by 2050 (WB, 2000).

According to the IPCC (2001, Technical Summary of the Working Group-II Report) with no adaptation a 45-cm rise in mean sea level will lead to a potential loss of 15,668 km2 area (10.9%) of the country exposing about 5.5 million people (5%) of the country. assuming no adaptation. A 100 cm rise could cause loss of 29,846 km2 (20.7%) of land exposing 14.8 million (13.5%) people of Bangladesh under the same a assumption.

Geologically, Bangladesh is located in an active sedimentary basin known as the Bengal Basin. The Eastern Himalayan Rivers (for example, the Ganges, the Brahmaputra and the Meghna) carry a large amount of sediments, a substantial part of which is deposited each year on the riverbeds and on the floodplains.

It is believed that a part of the sediment load is also deposited on the shallow continental shelf of the basin. In addition, the basin is undergoing subsidence that may be attributed to the following two major factors: one is related to the isocratic adjustment of the crust (sediment load and rise of Himalayas); the other is related to de-watering and compaction of shale and mud of Proto-Bengal Fan. However, there is not enough evidence in favour of the latter factor. Tectonic subsidence usually occurs over a large extent of area, in a uniform manner and at a very slow rate. These areas are generally bound by active faults or hinge zones. Although the entire Bengal Basin is subsiding slowly, more rapid subsidence has been taking place in the Bengal Foredeep, particularly in the Sylhet Trough, Faridpur Trough and Hatiya Trough.

Existing literature provides a wide range of estimates of the rate of subsidence. It is therefore difficult to estimate the overall extent of relative sea level rise in the coast of Bangladesh. If it is assumed that the rate of subsidence in the Hatiya Trough (in the coastal areas) is equal to the rate of sedimentation, one may infer that the rate of change of mean sea level, as forwarded by IPCC, would be observed in the coastal areas of Bangladesh.

It would be noted that the rates of change of sea level, as presented in Table-4.1, are only indicative. Accurate predictions could not be made due to inherent weaknesses of the models. There is a wide range of variation concerning the extent of such changes in the literature (Yohe and Schlesinger, 1998). It is recommended that more detailed studies should be carried out in future on the projections of future climate change scenarios in Bangladesh

Any rise in the sea level will propagate upstream into the river system. In Bangladesh, this backwater effect will be more pronounced because of the morphologically dynamic rivers, which will adapt their bed levels in a relatively short time period (Huq et. al., 1996). This whole process will lead to decreased river gradients, increased flood risks and increased drainage congestion.

Since most of coastal plains are within 3 to 5 meters from the mean sea level, it was previously thought that a significant part of the coastal areas (as high as 18 percent of the country) would be completely inundated by rising seawaters (Huq et al., 1995; Houghton et, al., 1996). Such a speculation was made based on two major approximations; (a) the coastal plains are not protected and (b) the seawater front will follow the contour line. In reality, however, it is found that most of the plains in the coastal region is protected.

About 6000 km embankments have been constructed along the coastlines, banks of rivers and tidal estuaries to form polders. At present there are 108 polders, sub-polders in the greater Khulna, Barisal, Patuakhali, Noakhali and Chitta gong districts in the coastal zone of Bangladesh. Several thousand drainage sluices are provided to remove accumulated rainfall run-off from the polders to the sea or adjacent rivers by gravity flow during low tide. Automatic flap gates are provided with the sluices to prevent saline water intrusion inside the polder during high tides. The existing coastal polders are shown in Figure-4.4.

Existing embankments provide protection against flooding from high tides but are not , designed to prevent inundation by severe surges (GOB, 1992). In addition to the j protection against regular inundations and saltwater intrusion, the embankments can reduce the tidal forces. This has an adverse effect on the drainage conditions (siltation due to reduced tidal volumes) and the ecosystems (water logging and stagnant waters). These negative effects have already been visible in parts of the coastal area such as Khulna, Barisal, Patuakhali and Noakhali regions.

Drainage congestion may become an even more serious threat than higher flood risks. Due to the siltation and the poor maintenance of the drainage channel network in many parts of the coastal zone, drainage congestion is already a grave problem, and the problem is expected to increase considerably.