Deep-channel dynamics: A challenge for erosion management in large rivers

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.