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 affect8 a _,izablepopulation in Bangladesh. To mitigate the river bank erosion llWl)fj ha.~initiated a 20 km capital dredging on the River Jamuna as pilot basis in order!o guide the flowto reduce the risk of failure ofSirajganj Hard Point and right guide bundh of the Bangabandhu Bridge. This study would be u~efult() assess th~ role of capital dredging in comprehensive riv~r management plan as well as the morphologi~al stability of an artificial dredged channel along the braided Jamuna river of Bangladesh. The sustainability of the dredged channel "as assessed by analyzing the timeseries satellite images, cross-section comparison, sediment concentration and depth average velocity. Geo-refereneed satellite image of ASTER and IRS LJSS from 2007 to 2012 ha., been superimposed to a,,,,,,s!he sand bar movement. During 2007 !o 2012, lateral movement of Sandbar-l (upstream of Sirajganj Hardpoinl) ranges from 800m to 1870m wher~as for Sanbar-2 (dowm,tream of Sirajganj Ilardpoint) ranges from 560m to 1060m, At the 8am~ tim~, .the longitudinal movement of Sandbar-] ranges from 150m to 600m whereas for Sandbar-2 l'"dIlgestrom 300m to 1400m. It is observed that, the channel is devdoping very fast along the western bank through deviation of flow l"warus the river banle It's is also obsened !hatlhe rate of bar translation is high and it may cause tbe siHalion of the dredged channel. Compariwn of cross-section indicales that there is no sigaificant positive impad of dredging has observed ncar Simjganj Harupoint area as well 8S tbe downstream of Hanghabhandhu Bridge. It i, also observed that the rate of siltation is higher, where the dredging alignment passes through the exisling char. It was happening, because Ihe hydraulic condition around the dredged area as well as ups!rcam river morphology remain favorable for the siltation on the existIng ehllr. Analysis ofobscrved data indicates that, more than 1.75 m/s flow velocity persist, during the peuks which covering the entire Sirajganj Hardpoint area and it persists from mid of May to September. AnaJysi, of sediment concentration and depth average velocity indicates thaI <;edimentconcentration at particular depth and velocity plays an important role in the em,ion or deposition process along the study reach. It is found lhat at a particular point under or over sediment concentration with respect to the sediment transport capacity cause ~ilhcr erosion or deposilion. The major fUiding of this study i~ that the dredging of a braided river would not be a sustainahle solution without changing upstream ri,~r morphology as well as hydraulic conditions. It is also observed that, if the dredging alignment pa'8e, through the exisling ~hannel not over the char area, the dredgoo channel would be more sustainable.