Coastal Erosion
Coastal Erosion the tidal process that loosens and moves coastal land and vegetation from one place to another. The deltaic coastline can be recognised as consisting of two basic physiographic units: the inactive or abandoned Ganges tidal plain and the active Meghna deltaic plain. While the Ganges tidal plain is relatively old, the Meghna deltaic plain is geologically very young. The Meghna deltaic plain extends from the Chittagong coast in the east to Tentulia Channel in the west. This area is highly dynamic, changing its planform continuously. The erosion-accretion processes of the channels and islands of the Meghna estuarine delta are well documented.
Tidal characteristics along the Bangladesh coastline and adjacent areas appear to be affected by the refraction of the incoming tidal wave from the bay of bengal. The refraction is caused by the submarine canyon, the swatch of no ground, resulting in high tidal ranges on both sides of the canyon and low tidal ranges at the head of the canyon. More inland, the funneling effect is obvious in the Sandwip and Hatiya Channel tides. Most areas of the active delta are dominated by mesotidal activity (mean tidal range between 2 and 4m), while a small area near Sandwip is macrotidal (mean tidal range greater than 4m). Tidal currents exceeding 3m/sec occur in Sandwip and Hatiya channels during spring tides. In west Shahbazpur and Hatiya channels, ebb currents often exceed 3m/sec during the monsoon. In areas north of Sandwip island, tidal bores occur during spring tides, with spectacular meetings of the two bores coming through the Sandwip and Hatiya channels. Sedimentary characteristics of different channels indicate that the Sandwip and Hatiya channels have a silty environment and the Shahbazpur and Tentulia channels have a fine sandy environment.
Broad map comparisons indicate that the delta of the Ganges-Brahmaputra rivers has not grown significantly toward the sea over the last two centuries. This has happened despite the fact that huge amounts of sediments are delivered by the Ganges-Brahmaputra rivers. However, considerable developments occurred in the Meghna estuarine delta during the last two centuries. Among others, changes can be noticed in Sandwip and adjacent islands, in Hatiya island, in Bhola island, and in the coastline of the Noakhali mainland. Sandwip island is reduced in size during last 200 years. Sandwip Channel was nearly isolated from the main distributary network of the rivers in 1764-1793. The Hatiya Island is elongated and migrated considerably southward during this period. Bhola Island is also elongated north-south. The islands, in general, prograded by about 50 km southward. The meghna migrated southwest, leaving its original course. A dying branch of the Lower Meghna silted up, partly naturally and partly because of human intervention. This added new landmass to the Noakhali mainland.
Chittagong coastline remained relatively stable over the years. In the north, considerable accretion took place due to a closure dam on the feni river. The reason for its relative stability is perhaps the near-isolation of Sandwip Channel from the distributary network in recent times. At present, Sandwip Channel is tide-dominated, allowing net import of fine sediments. '''''
Noakhali coastline underwent considerable changes over the years. These changes are primarily the result of accretion in the old lower meghna riverbed. The accretion was promoted by its closure by two cross-dams. The total accreted land during the period from 1957 to 1985 was about 52,000 ha. This land area can be compared to the total annual sediment carried by the rivers. Although the overall Noakhali coastline had a south and southwest build-up, there are localised areas of erosion. For example, the Noakhali bankline at the Upper Shahbazpur Channel has places with considerable erosion. ''''''''''
Sandwip Island is surrounded by the tide-dominated East Hatiya Channel, the Sandwip Channel, and the channel linking Urirchar. Available data for the last 75 years (1913-1988) show that Sandwip was reduced to about 50% of its original size, with considerable erosion northwest and accretion southeast. Map comparisons show that erosion on the northwest coast of Sandwip accelerated after 1963. It was about 200m per year between 1913 and 1963 and about 350m per year between 1963 and 1984. Urirchar grew from 3 sq km in 1963 to 46 sq km by 1981.'''''
Hatiya Island is surrounded by the South and West Hatiya channels and East Shahbazpur Channel. East Shahbazpur is a flood channel and the southwest Hatiya is an ebb channel. Available data on Hatiya Island for the last 36 years (1952-1988) indicates that considerable erosion of the Island took place at the north, with simultaneous accretion at the south. Detailed map comparison indicates that the erosion rate on the north of Hatiya was negligible before 1963 but became one of the highest in the region at a rate of some 400m per year between 1963 and 1984.
Bhola Island is surrounded by Tentulia Channel and the north and east Shahbazpur channels. It is surrounded by ebb currents. Erosion rate in the highly eroded place of the northeast bankline was about 150m per year between 1940 and 1963. Comparison of maps between 1963 and 1989 shows the narrowing and the lengthening tendency of the island by simultaneous erosion and accretion. Erosion is primarily at the northeast bank and accretion at the south.
The highly dynamic nature of the Meghna estuarine delta is the result of interaction between the two strong opposing agents of delta-building activities - fluvial and marine processes. While the fluvial process brings huge seasonal fluxes of water and sediment, the primary marine process is a semidiurnal tide with strong fortnightly variation. The morphologic changes of Sandwip Island and adjacent areas are primarily the result of a strong tidal current, while the same changes of the Bhola Island are primarily the result of a river current. Therefore, the erosion at Bhola is highly seasonal. Hatiya Island and adjacent areas have the influences of both the tide and river flux, yet in some areas such as the morphologic changes of Urirchar the development is dependent on the tidal meeting dynamics, which are dominated by the amplitude and phase differences between the two tides coming through the Hatiya and Sandwip channels. [Sifatul Quader Chowdhury]
Bibliography Barua K Dilip, 'The Active Delta of the Ganges-Brahmaputra Rivers: Dynamics of its Present Formations';, Marine Geodesy, Special volume on the sea level problems of Bangladesh, 1997.
See also offshore islands. [Chowdhury, Sifatul Quader Professor of Geology, Dhaka University]
