Abstract:
Bangladesh is located in the lower part of the delta having flat land formed mainly by the sediments deposited by the world’s three largest rivers (GBM), the Ganges, the Brahmaputra and the Meghna. Huge quantity of flow during monsoon coming from the upper catchments (located outside the country) makes its way towards the sea through these rivers. Due to the geographical location of Bangladesh on the globe, the river systems here are morphologically very much active which is evident from the continuous wide spread bank erosion, channel shifting and sedimentation processes. Bangladesh, on average, is losing more than 8,000 ha of land annually due to bank erosion. Although there is a tendency of decreasing rate of erosion due to implementation of large number of riverbank protective works during the last few decades by Bangladesh Water Development Board (BWDB). Failure of implemented bank protective works is also remarkable. Under these circumstances, this study has been conducted to determine the probable cause of the damage of riverbank protective works constructed on the right bank of Jamuna River at some selected locations of Sariakandi. BWDB implemented the protective works at Titporol and Debdanga along the Right Bank of the Jamuna River during November 2004 to April 2005. From field investigation on 28 June 2005 it was found that the revetment work at Debdanga performed well during early flood after construction. But some portion of upstream revetment at Titporol, however, damaged in June 2005 and mitigation measures by dumping of sand filled synthetic bags were carried out to stop further collapsing of river bank. In order to find the probable causes of failure of protective works at Titporol, investigation has been carried out through checking the adequacy of the design of the revetment, slope stability analysis and field investigation.
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Design of the revetment has been reviewed using the standard procedure mentioned in the Design Manual of BWDB and have been found satisfactory. From the analysis of field condition it is revealed that the damage of revetment works occurred due to low shear strength of soil and the presence of pore water pressure developed behind the geo-textile filter. It is apprehended that for the lack of free drainage, pore water pressure developed behind the geo-textile filter and this resulted failure of bank slope. At the damaged portion, subsoil water might have been drained from underground source or from the existing ponds behind the revetment works as was found at the protective site of Titporol. On the other hand, there found no such kind of underground source of water or existing ponds near the revetment- works site at Debdanga. Therefore, the protective-works at Debdanga performed well.
