This paper presents a hydraulic and morphological analysis of the Lower Jamuna in Bangladesh with a focus on two key bifurcations that are important for stabilization of the Lower Jamuna reach. We used ground measurements, historical data, multispectral satellite images from various sources as well as numerical models. We carried out hydraulic analyses of the changes and their peculiarities, such as flow distributions at the bifurcation and hysteresis of the stage–discharge relationships. We supplemented our analysis by using numerical models to simulate discharge distribution at the bifurcations under various flow and riverbed conditions. We developed an advanced and automated satellite image processing application for the Lower Jamuna, referred to as Morphology Monitor (MoMo), using the Google Earth Engine. MoMo was found to be an effective tool for a rapid assessment and analysis of the changes in deep-channel and sandbar areas. It is also useful for monitoring and assessing riverbank and char erosion and accretion, which is important not only for morphological but also ecological impact assessment. The application can be adapted as an operational tool as well. Furthermore, we assessed the evolution of deep channels at the bifurcations based on regularly and extensively measured bathymetry data. The analysis was carried out in complement with morphological modeling, particularly for short-term prediction. In this paper we present the major findings of the analysis and discuss their implications for adaptive river management.@en

In this paper, we present flow and erosion problems in selected reaches of two large and dynamic river systems in South Asia, namely the Koshi River in Nepal (and India) and the Lower Brahmaputra (Jamuna) in Bangladesh. We attempted to analyse large- and meso-scale (short- and medium-term) morphological changes with a focus on the dynamics of deep-channels, revealing their importance for the river and riverbank erosion management. This focus on deep-channels is a key change of perspective as most morphological studies and analyses of large rivers are usually focused on sandbar and braiding dynamics. We used ground data, satellite imagery, and explorative morphological modelling to quantify and analyse the flow and morphological processes. We demonstrate how multispectral satellite imagery can be processed using Google Earth Engine to assess the spatiotemporal dynamics of morphological processes and changes. We also analysed bathymetric surveys to assess short-term changes of meso-scale morphology that are not fully captured by the satellite data analysis. The morphological modelling provided first results on reproducing essential processes, such as growth and migration of meso-scale features, particularly deep-channels, under varying flow conditions. Some features of these reaches of two rivers differ, but particularly the importance of deep-channel dynamics was revealed for both. We infer that the seasonal and annual discharge variabilities are key factors for the dynamic behaviour of bank, char (island), sandbars and deep-channels, particularly regarding short- and mid-term changes. We also infer that morphologically extreme situations do not always occur during high flows, but rather through the concentration of the flow along the deep-channels during medium and lower flows.@en