The Jamuna-Brahmaputra River, Bangladesh

Bangladesh is dominated by three great rivers – the Jamuna–Brahmaputra, Ganga, and Meghna – that combine to feed sediment into one of the World's largest deltas in the Bay of Bengal. The Jamuna River has developed in a region of significant tectonic activity associated with Himalayan uplift and development of the Bengal foredeep. The bedload, although only ~10% of the total sediment load, is critical in generating a wide array of bedforms of different scale that drive channel change and migration. Within the Jamuna River, the ubiquitous occurrence of bifurcations and confluences is a key aspect of the river channel pattern and dynamics, and these features form important nodes in the braidbelt. The nature of floodplain sedimentation and inundation is vital in planning annual crop growth and may adopt great significance in the ongoing debate on the sources, causes, and accumulation of arsenic in the groundwaters of Bangladesh.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Rivers, Ports, Waterways and Dredging Engineerin

The planform mobility of river channel confluences: Insights from analysis of remotely sensed imagery

River channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristics of river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea-level change). This paper investigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80% of large confluences are mobile over this 40-year window; conversely this figure is < 40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

The sedimentology of river confluences

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record

Evolutionary, multi-scale analysis of river bank line retreat using continuous wavelet transforms: Jamuna River, Bangladesh

In this study continuous wavelet transforms are used to explore spatio-temporal patterns of multi-scale bank line retreat along a 204 km reach of the Jamuna River, Bangladesh. A sequence of eight bank line retreat series, derived from remotely-sensed imagery for the period 1987-1999, is transformed using the Morlet mother wavelet. Bank erosion is shown to operate at several characteristic spatial and temporal scales. Local erosion and bank line retreat are shown to occur in short, well defined reaches characterised by temporal persistence at the same location, and separated by relatively stable reaches. In contrast, evidence of downstream propagation of bank line retreat patterns is evident at larger spatial scales. The intensity of localised bank line retreat (i.e. at scales of 0 - 20 km) is strongly related to the magnitude of monsoonal peak discharge, but this relationship weakens as the spatial scale of erosion increases. The potential of continuous wavelet analysis to enhancing our understanding of morphological evolution in complex fluvial systems with multi-channel planforms is discussed

Assessing channel changes of the Ganges-Padma River system in Bangladesh using Landsat and hydrological data

© 2016 Elsevier B.V.The Ganga/Ganges1is an important river system in South Asia which supports the life and livelihoods of millions of people both in India and Bangladesh. The system has a number of names throughout its length. Below its confluence with the Brahmaputra at Aricha it is known as the Padma, which in turn merges with the Upper Meghna at Chandpur below which the channel is known as the Lower Meghna. There is a growing concern about this large river system because its channels are subject to frequent migration, threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability, land use change, and agricultural intensification as the basin experiences rapid population growth. Concerns have been expressed that the construction of a barrage just upstream of the Indo–Bangladesh border has adversely affected the Ganges reach in Bangladesh. Partly in order to investigate this, the planform changes of the Ganges and the Padma within Bangladesh was analysed over the period 1973 to 2011 using multitemporal Landsat images and long–term flow data in eight epochs with an average duration of 4.5 years. The Padma reach is less affected by the barrage and provides a useful control study. Areas of erosion and deposition were determined from sequential changes in the bankline positions. Mean channel width, sinuosity and braiding index were analysed using a Geographic Information System (GIS). Flood frequency, duration and magnitudes were studied using long-term discharge records. Generally, channel planform evaluation indicated that both the Ganges and the Padma experienced contraction, expansion and readjustment in configuration over the last 38 years. Erosion and deposition statistics of the Ganges indicate that 57 km2 of land was lost along the right bank whereas around 59 km2 has been gained along the left bank during the assessment period, suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank. The width of the Ganges varied from a maximum of 5.36 to a minimum of 3.23 km during the observation period. Changes to sandbar area are, in general, much more radical than changes to the overall width and area of the channel. Measurement of areas of erosion and accretion showed that both banks of the Padma experienced considerable loss of land. The total net loss for left bank and right bank was 155 and 28 km2, respectively. The Padma is approximately twice the width of the Ganges and the changes to its channel area are not as temporally dynamic as the Ganges. The relationship between bank curvature and erosion/accretion of the river banks for both rivers was analysed and the results contradict established meander theory. Regression analysis between bank erosion rates, annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the Padma (r2 = 0.6283) and that the Ganges bank erosion rate is influenced by mean flood flow (r2 = 0.6738). The flood frequency shows generally good stability across the first eight of the nine epochs for the Ganges but for the Padma the frequency showed even greater stability. We were unable to support the widely held belief that the upstream barrage has a deleterious effect on the Ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices