Assessment of hydro-morphological response of selected reach of the Jamuna river due to structural intervention using delft 3D model

Abstract

In this study, an attempt has been made to analyze the hydro-morphological responses of the Jamuna River due to structural intervention in a selected reach by preparing a morphological model. The study reach covers from 30 km upstream of Bangabandhu Bridge (BB) to 20 km downstream of this bridge. A two-dimensional mathematical model has been developed using Delft3D and studied various options with structural intervention of groyne(s) along a selected reach in the right bank of the Jamuna River at the upstream of Sirajganj Hard Point (SHP). The model has been calibrated and validated against the data for the year 2012 and 2013 respectively. Then simulations have been done with seven different options consist of groynes with various number and spacing. The length of the groynes have been selected as about one-fourth of the channel width. The spacing and length ratio (S/L) of the groynes have been set as 1.0, 1.5 and 2.0 respectively for the option simulations. In order to quantify the response of the river hydrodynamics and morphology for various options at different locations, model results for the month of July have been assessed. All the model output have been presented and discussed. For the purpose of discussion and interpretation of model output, four points as the upstream of the structure (u/s), downstream of the structure (d/s), Sirajganj Hard Point (SHP) and Bangabandhu Bridge Site (BBS) have been taken into consideration. Model results show that at the upstream of the selected intervention, water level increases about 10.84% and for the downstream is 6.72%. This rise of water level may be for the afflux effect due to the structural intervention(s). The velocity and bed shear stress show decreasing tendency around the structures which indicates the attainment of siltation zone. The effects are considerable at the upstream and downstream of the groyne(s) and for the other two points (e.g. Sirajganj Hard Point and Bangabandhu Bridge Site) the effect is very little due to the long distance. From the velocity vectors and bank line velocities as well as bed shear stress, it can be decided that for this specific reach of the river, the maximum allowable spacing between the groynes is 2.0 times of its length. Otherwise the bank will be in vulnerable condition due to erosion. The bed level variation surrounding the groynes is considerable. The effect of groyne extends up to 1 km inside the river and 2 km laterally from either nose of the groyne. The maximum erosion is -5.25 m and deposition is +3.70 m along the length of the groyne. In case of lateral section (along the river) the values are -6.0 m and +4.0 m respectively. For the structures, the near bank velocity and bed shear stress become very small with the increasing number of intervention. Thus it can be said that to prevent the bank erosion, this types of structures with increasing numbers and different spacing provides satisfactory results. Overall, the model results show that it has the capacity to assess the hydro-morphological river responses for various options particularly at the vicinity of the interventions.