Morphological assessment of a selected reach of Jamuna river by using DELFT3D model

Abstract

Simulation of sediment transport rate at the river Jamuna and variation of bed level along the river is carried out by using a two dimensional morphological model. Noncohesive sediment transport module of Delft 3D Flow is used for the simulation. The upstream boundary of the model is taken at 30 Km upstream of Bangabandhu Bridge and downstream boundary is taken at 20 Km downstream of Bangabandhu Bridge. Simulation period is taken from April 2010 to December 2012. Simulation is carried out for hydrodynamic calibration and for sediment transport rates. The cross-sections are taken at the locations which are vulnerable, such as Subaghacha, Sirajganj, Jamuna Bridge and also in the upstream near Kazipur and downstream near Chauhali etc. In the Morphology tab, the morphological scale factor has been set to 8.25 which extend the 121 days hydrodynamics to about 1000 days of morphological change. Calibration and validation are carried out against field observations (water level) of 2010 and 2011 respectively. Comparisons between simulated and observed water level are taken at the Sirajgonj station. The results showed satisfactory agreement with observed values. For hydrodynamic and morphological computation, a time series discharge data is used at the upstream boundary and water level data as downstream boundary. Observed and simulated bed level elevation of 2010 has been compared and the comparison showed a very good agreement. After calibration of the model, the net amount of erosion and deposition along the river reach is computed. Finally, crosssectional variation of bottom level during the monsoon seasons from 2010 to 2012 has been observed. Results reveal that erosion takes place in the channel bed and the deposition mainly takes place to the adjacent char areas and increased its width and area. It is also evident that the channel has been shifted westwards of the reach due to shifting of the bank line of the river. Many tributary and distributaries have been appeared due to progressive erosion. In Sirajgonj, the sediment transport capacity seems to be the xvii highest due to higher velocity of flow. The zones of higher velocity has higher sediment transport capacities therefore occurs more erosion