Morphological stability of a dredged channel for river training a case study along the braided Jamuna river

Abstract

The erosion and shifting of river courses, loss of land, especially along the Jamuna river have long been recognized as a national problem that affects a sizable population in Bangladesh. To mitigate the river bank erosion BWDB has initiated a 20 km capital dredging on the River Jamuna as pilot basis in order to guide the flow to reduce the risk of failure of Sirajganj Hard Point and right guide bundh of the Bangabandhu Bridge. This study would be useful to assess the role of capital dredging in comprehensive river management plan as well as the morphological stability of an artificial dredged channel along the braided Jamuna river of Bangladesh. The sustainability of the dredged channel was assessed by analyzing the time-series satellite images, cross-section comparison, sediment concentration and depth average velocity. Geo-referenced satellite image of ASTER and IRS LISS from 2007 to 2012 has been superimposed to assess the sand bar movement. During 2007 to 2012, lateral movement of Sandbar-1 (upstream of Sirajganj Hardpoint) ranges from 800m to 1870m whereas for Sanbar-2 (downstream of Sirajganj Hardpoint) ranges from 560m to 1060m. At the same time, the longitudinal movement of Sandbar-1 ranges from 150m to 600m whereas for Sandbar-2 ranges from 300m to 1400m. It is observed that, the channel is developing very fast along the western bank through deviation of flow towards the river bank. It’s is also observed that the rate of bar translation is high and it may cause the siltation of the dredged channel. Comparison of cross-section indicates that there is no significant positive impact of dredging has observed near Sirajganj Hardpoint area as well as the downstream of Banghabhandhu Bridge. It is also observed that the rate of siltation is higher, where the dredging alignment passes through the existing char. It was happening, because the hydraulic condition around the dredged area as well as upstream river morphology remain favorable for the siltation on the existing char. Analysis of observed data indicates that, more than 1.75 m/s flow velocity persists during the peaks which covering the entire Sirajganj Hardpoint area and it persists from mid of May to September. Analysis of sediment concentration and depth average velocity indicates that sediment concentration at particular depth and velocity plays an important role in the erosion or deposition process along the study reach. It is found that at a particular point under or over sediment concentration with respect to the sediment transport capacity cause either erosion or deposition. The major finding of this study is that the dredging of a braided river would not be a sustainable solution without changing upstream river morphology as well as hydraulic conditions. It is also observed that, if the dredging alignment passes through the existing channel not over the char area, the dredged channel would be more sustainable.