Dynamics of bar in the braided river Jamuna

Abstract

Braiding is one of the major river patterns of alluvial rivers. To respond the pulsation of discharge and sediment load during the flood, the morphological features (bars and channels) of braided rivers experience major changes in area, shape and spatial distribution; that make the river network complex. Moreover, braided river network shows scale invariance property; the morphological and dynamical properties of a small part of a braided river can be applied to the larger part of it, from one braided river to another of different size, or from a laboratory model to a real braided river. Hence, this study focuses to understand the dynamics of the braided bar/island development process of braided river, Jamuna which is the downstream continuation of Brahmaputra river in Bangladesh. Three types of data have been analyzed in this study- time series hydraulic data, data derived from the dry season satellite imagery analysis and the numerical modelling. The dry season satellite imagery analysis was basically used to understand the bar development process. At the same time the change of bar characteristic with time were also assessed using this data. Several hydraulic data such as water level, discharge, sediment, river cross section were used to assess the time series change of the bar development process. Numerical model was used to understand the detail of the development process of the bar at unsteady flow condition. The bar development process in a dynamic river like Jamuna is very complex. This study indicated that, though during the last few decades river's discharge increased slightly and water level did not change significantly; river sediment load decreased drastically. As a response of the process, river reduces its water surface slope and increased its width. The bar area showed higher sensitivity to this process compared to the channel area. Large floods may not have a very significant role in increasing the bar area, but it contributes to the increase of sand area through lessening the vegetation coverage. Temporal analysis of bars showed that at present maximum bars (53%) of the river are very young. The vegetation can cover bar area upto a limit of 70%. The vertical growth of bar stabilizes from 8 to 10 years. This study also found that the average height of the bars above low water level to be around 5 m which is comparable to the findings of several previous studies. The numerical model study using Delft 3D indicated that several braid bar development phenomenon like adjacent channel shifting, formation of cross-bar channels and channel abandonment can be simulated through numerical model. This study also showed that several bar property like bar amplitude, aspect ratio are related to the active channel's (the channels that carry the 90% of the sediment in a cross-section) property of the river. With one unit change in active channel width-depth ratio both the bar amplitude and aspect ratio increased about 0.1 %. The results of the numerical model also indicated that the average rate of sedimentation over the bar top during the wet season is 3.5 m/year which is comparable with the observed data (3.11 m/year during the young age). Numerical model study also revealed that the growth of bars in lateral direction depend on the hydraulic property of the adjacent channel. The increment of channel length-width ratio also caused bar’s length/ width ratio increment. With one unit change in channel length-width ratio, bar’s length/width ratio also increases more than 5%.With the change in discharge the adjacent channels adjust their sinuosity causing the lateral growth of bar. The growth of crossbar channel depends on the water depth that the bar experienced during moonson. Adjacent channel abandonment process is mainly depend on the bifurcation angle with the main channel. The process of braided bar development is as an essential part of the morpho-dynamics of the river. The river like Jamuna is subjected to future changes like climate anomalies, human interventions etc. If such changes occur hydro-morphological condition of the river will be altered. Finally this study recommends that before any interventions in the river, it should be considered that the river may not behave as the same as it do now. Hence detail study is needed prior to any intervention.