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. ... Read more

Himalayan rivers in Nepal flow through the mountains with high gradient to emerge in lowlands as large rivers carrying enormous amount of discharge and sediments. They release significant quantity of sediment forming alluvial fan as a result of sudden decrease in gradient when they enter the lowland and gain braided form. This braided form has made the river morphologically more dynamic in nature. Division of channels into numerous anabranches leads to formation of temporary or permanent islands in between them. These islands in long run are either eroded gradually by the river channel or develop into vegetated islands. The development of vegetation may be long term with growth of trees or they may develop into grasslands that may be seasonal which is usually inundated during floods. The river channels and islands along with the surrounding floodplain with vegetation act as perfect recipe for the development of complex wetland ecosystem.

Koshi River in Nepal is among such rivers emerging from the mountains to flat plains of Terai thereby flowing into multiple channels within a large width of about 5 km, which is then controlled by Koshi Barrage at 41 km from the gorge. This dynamic river system feeds the Koshi Tappu Wildlife Reserve, a Ramsar site in the reach. The change in river course and vegetation of this large area which otherwise would be challenging to study can be done rather easily by the use of satellite imageries and cloud computing. Google Earth Engine (GEE) has been used in this study for analysing the morphological changes of the river as well as vegetation changes within the study area using the multiple satellite images taken at different times. NDWI has been calculated and used to identify the occurrence of water in the river channels, thus the morphological changes. While NDVI is used for intensity of vegetation. The temporal and spatial analysis of the morphodynamics and corresponding changes in vegetation is performed from 1987 to 2020 within the selected area.

The preliminary assessment of the results shows that the vegetation dynamics of the area has been affected by the continuous erosion and deposition caused by the morphological changes apparently due to the barrage. Over time, river has been channelizing and branching several times causing the existing islands to erode along with their vegetation as well as forming new islands with vegetation cover. This shifting of the river and resulting vegetation dynamics appear to have affected the habitat of the wild water buffaloes (Arna) as well as, other endangered species native to the area. Additional analysis on the effect of river morphology and vegetation dynamics to the flood pattern and other ecological components will be carried out to support the initial findings and draw generalized conclusions. ... Read more

Groynes have been replaced by longitudinal training walls in an 11-km long pilot project to optimize training of the river Waal in the Netherlands. These train­ ing walls improve navigability, reduce flood levels, create a sheltered second channel with more favourable ecological conditions, and decrease the erosive action on the river bed that is responsible for large-scale bed degradation. River managers wish to assess whether longitudinal training walls could have similar advantages along other parts of the Dutch Rhine branches (without excessive increase of maintenance costs). The required maintenance dredging depends on the amount of sediment entering the sheltered channel over an entrance sill situated at the upstream edge of the longitudinal training wall. Currently operational morphodynamic models cannot reliably compute this sedi­ ment flux. We present laboratory experiments to study the passage of bed sediment at different discharge distributions between the main and sheltered channel, and different degrees of submergence. ... Read more

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. ... Read more

The effects of sediment heterogeneity and sediment mobility on the morphology of braided rivers are still poorly studied, especially when the partial sediment mobility occurs. Nevertheless, increasing the bed sediment heterogeneity by coarse sediment supply is becoming a common practice in river restoration projects and habitat improvement all over the world. This research provides a step forward in the identification of the effects of sediment sorting on the evolution of sediment bars and braiding geometry of gravel-bed rivers. A two-dimensional morphodynamic model was used to simulate the long-term developments of a hypothetical braided system with discharge regime and morphodynamic parameters derived from the Waimakariri River, New Zealand. Several scenarios, differing in bed sediment heterogeneity and sediment mobility, were considered. The results agree with the tendencies already identified in linear analyses and experimental studies, showing that a larger sediment heterogeneity increases the braiding indes and reduces the bars length and height. The analyses allowed identifying the applicability limits of uniform sediment and variable discharge modelling approaches. ... Read more