Global Climate Change & Bangladesh....

Erosion Dynamics or Causes of Erosion

Whatever may be the net result of the accretion/erosion process, it is very difficult to identify separately the contribution to erosions due to different dynamic causes of erosion. The main causes of coastal erosion in Bangladesh are:

• Heavy discharge current
• High astronomical tides
• Strong monsoon water current
• High storm surges

A brief description on the possible impacts of SLR on erosion via media the above- mentioned four major causes of erosion in Bangladesh will be made here (Ali, 1989). The erosion theory will be discussed in the next section.

DISCHARGE CURRENT

A huge amount of water discharges through and from Bangladesh into the Bay of Bengal. The main thrust is on the Meghna estuary at the north-east corner of the Bay of Bengal. The strong discharge current causes considerable erosion in the coastal area. The SLR will push the coastline as well as the river mouth landward and is likely to modify the discharge current and hence the erosion. The 1.5 m contour line is very near the coastline in the Meghna estuary. The 4.5 m contour is also quite near. As a result the gradient flow may increase and also the erosion. However, there are a lot of uncertainties about this. The mechanism thus needs to be explored.

TIDE

Tide is another major cause of coastal erosion in Bangladesh. It is mostly semi-diurnal in character and shows an increase from the Indian coast in the west to the Meghna estuary where it is the highest (range is as high as 5 m or so on the average). Then the tide shows a gradual decrease in the south-east direction along the east coast of Bangladesh (Au, 1996). Tides in the Bay of Bengal originate in the Indian Ocean and get amplified at the Head Bay due mainly to nonlinear shallow water effect and the northward convergence of the Bay of Bengal. The high tidal water action thus contributes to the erosion problem in Bangladesh. The rise in sea level is likely to increase the tidal range in the Bangladesh coast. This has been demonstrated by Flather and Khandkar (1987) through numerical modelling. Furthermore, the inundation due to SLR may increase the nonlinear effect and hence increase the tidal range, thereby increasing the erosion effect.

MONSOON CURRENT

The south-west monsoon wind that flows over the Indian Ocean and the adjoining land areas generates strong water waves and water current in the area. The water waves and water current cause heavy erosions in Bangladesh coast, particularly in the Meghna estuary where water rises due to piling up of water by south-west wind (Ali, 1995). The wind also stirs the water in the shallow coastal area almost over the whole depth. The result is erosion. The areas likely to be inundated by SLR may be subjected to wind effect, thereby causing more erosion in the area.

STORM SURGES

Storm surges are generated by cyclones forming in the seas and oceans. Tropical cyclones forming in the Bay of Bengal and the associated storm surges combined together cause innumerable losses to the life and property in Bangladesh. The storm surges in Bangladesh are among the highest in the world. These are greatly responsible for changes in the coastal configuration, causing erosion and accretion. Global climate change and the sea level rise are likely to influence the cyclone activity and storm surge phenomenon. Possible impacts of climate change and sea level rise in the context of Bangladesh have been discussed, among others, by Johns and Ali (1987), Flather and Khandkar (1987), Pramanik and Ali (1989), Ali and Ahmad (1992) and Ali (1996). Ali (1996), using a storm surge model for the Bay of Bengal, has given different scenarios of cyclone and storm surge activity in Bangladesh with respect to April 1991 cyclone that took a death toll of about 138,000 in Bangladesh.

It may be said that if cyclone activities and storm surges change due to climate change, the erosion problem is likely to be more aggravated.

 

Theory of Erosion Due to Sea Level Rise

The sea level rise is one of the driving mechanisms of coastal erosion. While this has an indirect effect on coastal erosion through the dynamical processes, it itself has a direct contribution to erosion as well. With a rise in water level, the coastal morphological system will adjust itself to the high water level situations by creating a new coastal profile at a higher level at the coast of the presently existing coastal profile. This process, as described by Vellinga (1986), is as follows

a) By a rise in water level, the water line will shift landward.

b) As the coastal profile becomes steeper, erosion will occur until a new dynamic equilibrium is reached at a higher level.

c) The natural filling rate of lagoons and tidal basins will increase with an accelerated rise in sea level; the sediments required for the filling will come largely from the surrounding areas through erosion.

d) Rising sea level will cause a shoaling effect in rivers as a consequence of which (shoaling) sediment yields from rivers will reduce; these sediments will not be available to compensate for any erosion in the coastal area.

We shall discuss here the theory of erosion due to sea level rise, following mostly the method developed by Brunn (1962).

The erosion phenomenon due to sea level rise is depicted in Figure-2. The figure shows the present coastal profile with respect to the present mean sea level. Higher sea levels will result in coastal recession because the waterline will shift landward due to submergence. Not only this, erosion will occur, as pointed out before, until a dynamic equilibrium is reached and a new coastal profile is formed. Figure-2 shows the new coastal profile under the future sea level rise situation. The continuous line is the present profile and the dotted line is the new profile after rise in sea level and after quantitative balance between shore erosion and bottom deposits. The eroded and sedimented areas are shaded. Amount of sediments eroded will be equal to the amount of sediments deposited, assuming that eroded sediments are not lost (if lost, then by an insignificant amount) into the deep sea.

Erosion Situation in Bangladesh

Erosion is a worldwide phenomenon. About 70% of the world’s coastline has shown a net erosion over the past few decades, less than 10% has net degradation and the remaining 20% or so have remained relatively stable (Bird, 1985). For Bangladesh, studies are scattered depending on the particular purposes in most of the cases. A brief overview of some of the accretion - erosion studies done for Bangladesh coast is given below:

Miah (1975) made a qualitative discussion on the accretion and erosion problem in the Meghna estuary covering the period 1779-1975. Jabbar (1979) made a qualitative assessment of accretion/erosion by comparing the Survey of India Map of 1931 and a Landsat map of 1977. It was found that during the period a net accretion of 493 km2 took place in the mainland. This was however the result of the building of a cross dams for reclamation of land. Erosions were observed in a major islands, namely Bhola (accretion 85 km2 and erosion 376 kin2) Hatiya (accretion 64 km2 and erosion 172 kin2), Sandwip (accretion 35 km2 and erosion 227 kin2). During the period, many small chars such as Char Udaykal and Char Clark were completely eroded and some new chars like Char Dhal, Char Shabani, Nijhumdwip were fonned.

Pramanik et al., (1981) made a comparative study of seven different maps of the coastal region between 1779 and 1979. The results of the study are summarised in Table-2. It is observed that while the islands Hatiya, Sandwip, Shahbazpur, Manpura and others decreased in area, the mainland increased in size due to construction of cross dams. It is to be noted that around 1950 Hatiya island got larger and then it broke into two islands, the northern one joined the mainland.
 

TABLE 2 : Land area in the Meghna Estuary (in sq. kin)

Source: Prnmanik et at. 1981

A detailed computer analysis of Landsat data for the years 1972 and 1979 was made by Pramanik (1983) for the coastal region. This study, however, showed a net accretion of land by about 213 km2 during the period 1972-79. Erosion was observed to be taking place in the north-eastern part of Bhola, northern part of Hatiya and north-western part of Sandwip.

A SPARRSO (1987) study on coastal dynamics of Bangladesh for the period 1960-84 showed a net erosion. The amount of land accreted and eroded for major islands is given in Table-3, which shows a net loss of about 844 km2.
 

TABLE 3 : Change detection study for the period 1960-84 (in sq. kin)          

Source: SPARRSO Report, 1987

A study by Pramanik (1988) compared the Landsat imagery of 1972 and 1987 and found that major erosion occurred at south-eastern and southern part of Sandwip, northern part of Hatiya and north-eastern and north-western part of Bhola. A summary representation of this study is given in Table-3. The chars and islands show a net erosion. A total of 11 chars/islands totally disappeared. About 40% of the land accreted in the mainland appeared to have come from the reduction of river widths which seems to be precarious for flood discharge because this may slow down the discharge of flood water into the Bay of Bengal.

TABLE 4 : Areas of mainland and char/islands and number of chars/islands in 1973 and 1987 (in sq. km)    

Source : Pramanik 1988

Siddique (1988) has given some erosion rates for different islands. This rate had been about 150 rn/year between 1940 and 1982 for Hatiya. The northern tip of Hatiya which was almost stable during 1940-63 showed a severe erosion rate of about 400 rn/year during 1963-82. The Sandwip island had a rate of erosion of about 200 rn/year between 1913 and 1963 which increased to 350 rn/year during 1963-84.

MCSP (1992) made an accretion/erosion study for the whole coastal region of Bangladesh. The study made a review of maps of the last few hundred years and made a comparison of eroded and accreted land between the years 1971 and 1991. The study also made a prediction/projection of erosion and accretion in the coastal area of Bangladesh for the next 25 years.

In a later work undertaken by SPARRSQ (1993), accretion and erosion were studied for (i) the entire coast, (ii) the Megbna estuary and (ii) two small islands - Srizonee and Char Montaz. The study period considered was 1976-90 and the study was made using remotely sensed data. The results of study are shown in Table-5. It is seen that accretion and erosion in the entire region as well as in the Meghna estuary are comparable. But accretion is quite significant in Srizonee and Char Montaz and in the surrounding areas.

TABLE 5 : Comparative statement of erosion and accretion            

Source: SPARRSO, 1993

It is apparent from the above-mentioned studies on accretion and erosion that neither accretion nor erosion is of alarming rate. Both are compensating each other to a reasonable extent.

Source: Huq. S., karim, Z., Asaduzzaman, M., Mahtab, F., 1999, Vulnerability and Adaptation to Climate Change for Bangladesh, Kluwer Academic Publishers, The Netherlands.