Ground-penetrating radar and its use in sedimentology: principles, problems and progress

[1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology.

/pdf/the-fluid-dynamics-of-river-dunes-a-review-and-some-future-2nrjtixo1r.pdf

The fluid dynamics of river dunes: A review and some future research directions

[1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology.

The fluid dynamics of river dunes : A review and some future research directions : Marine sandware and river dune dynamics

This review and commentary sets out the need for authoritative and concise information on the expected error distributions and magnitudes in observational data We discuss the necessary components of a benchmark of dominant data uncertainties and the recent developments in hydrology which increase the need for such guidance We initiate the creation of a catalogue of accessible information on characteristics of data uncertainty for the key hydrological variables of rainfall, river discharge and water quality (suspended solids, phosphorus and nitrogen) This includes demonstration of how uncertainties can be quantified, summarizing current knowledge and the standard quantitative results available In particular, synthesis of results from multiple studies allows conclusions to be drawn on factors which control the magnitude of data uncertainty and hence improves provision of prior guidance on those uncertainties Rainfall uncertainties were found to be driven by spatial scale, whereas river discharge uncertainty was dominated by flow condition and gauging method Water quality variables presented a more complex picture with many component errors For all variables, it was easy to find examples where relative error magnitudes exceeded 40% We consider how data uncertainties impact on the interpretation of catchment dynamics, model regionalization and model evaluation In closing the review, we make recommendations for future research priorities in quantifying data uncertainty and highlight the need for an improved ‘culture of engagement’ with observational uncertainties Copyright © 2012 John Wiley & Sons, Ltd

Benchmarking observational uncertainties for hydrology: rainfall, river discharge and water quality

Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga–Brahmaputra, Bangladesh)

The initiation and evolution of a kilometre-scale, sand braid-bar was monitored during a 28-month survey period from 1993 to 1996 in one of the world's largest braided rivers, the Jamuna River, Bangladesh. Repeated bathymetric surveys through two monsoon flood seasons, combined with bar-top surveys during exposure of the bar at low flow, provide the most detailed chronology of braid-bar growth yet compiled for a large sand-bed river. During rising and peak flow of the 1994 monsoon flood, a 1.5-km-long, 0.5-km-wide, 12-m-high, symmetrical mid-channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion. Initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the Jamuna channels at all flow stages. Bar-top aggradation continued through downstream migration of an `accretionary dune front', a 3-m-high, angle-of-repose slipface that was composed of amalgamated, 0.5- to 1-m-high dunes. At waning and low flow, the mid-channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core. A low-velocity zone in the sheltered wake region of the bar-tail led to the accumulation of substantial volumes of silts and clays. During the rising and peak flows of the next monsoon flood, the mid-channel bar extended its bar-tail by up to 1.5 km, as one of the anabranches became dominant, and flow was deflected across the bar-tail. Accretion at the bar-tail generated a lobate, transverse bar-front with a 10-m-high, angle-of-repose avalanche face. Emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid-channel bars. The conversion of a mid-channel bar to an alternate bar is contrary to many previous descriptions of the braiding process.

Morphological evolution and dynamics of a large, sand braid-bar, Jamuna River, Bangladesh

The three-dimensional subsurface alluvial architecture of a large (approximately 3 km long, 1 km wide, 12 m high), mid-channel sand braid bar in the Jamuna River, Bangladesh is described. Evolution of the bar and its depositional characteristics are assessed from a unique combination of ground-penetrating radar surveys, vibracoring, and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29-month period. This methodology permits identification of the formative processes of different packages of braid-bar sedimentation and provides a facies model for deposition within the entire bar.

Mid-channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes. These dunes created a bar-core that grew by (i) propagation of a downstream-accreting slipface, (ii) vertical accretion through stacking of dunes on both bar stoss and top, and (iii) lateral accretion on the bar margins during recession of the flood hydrograph. Braid-bar sedimentation is dominated by four radar facies: (1) large-scale, predominantly planar, dipping reflections interpreted as cross-stratification, up to 8 m in height and greater than 100 m in width, that is produced by the cross-channel migration of bar margins, (2) medium-scale, trough-shaped and planar discontinuous reflections interpreted as cross-stratification up to 4 m in height and 300 m wide, that is deposited from large, sinuous-crested sand dunes, (3) discontinuous reflections, up to 2 m high and 30 m wide, interpreted as small-scale trough cross-stratification, that is the product of smaller sinuous-crested dunes, and (4) high-amplitude, undulating reflections interpreted as mud drapes, deposited in regions of slow flow, often in the bar-tail region at low stage. Dune size decreases vertically within the bar, in response to the progressively shallower flows on the emerging bar top. Later evolution of the bar, as one anabranch channel became dominant, created a 1.5 km extension to the bar tail with an 8 m high, angle-of-repose, bar-margin slipface, formed by flow transverse to the long axis of the bar. Seven styles of deposition can be defined that constitute the alluvial architecture: bar-margin slipface, vertical accretion in channel, bar-top vertical accretion, upstream accretion, lateral accretion, downstream accretion, and low-stage mud drapes.

A model of braid-bar sedimentation is presented that shares many similarities with previous studies of smaller sand-bed braid bars with the dominance of dune-scale cross-stratification, the presence of large-scale, bar-margin cross-stratification, and the occurrence of lateral, vertical, upstream, and downstream accretion. However, the contribution of the bar-margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly. This study suggests a scale invariance in several aspects of mid-channel bar sedimentation in sand-bed rivers and proposes a model of braid-bar sedimentation that may be applied widely within studies of braided alluvial architecture.

Three-Dimensional Sedimentary Architecture of a Large, Mid-Channel Sand Braid Bar, Jamuna River, Bangladesh

The Brahmaputra-Jamuna River, Bangladesh, is one of the world's great rivers, ranking in the top three in terms of both sediment and water discharge. The high water and sediment discharges are generated by the monsoon-dominated floods and the tectonic setting, which provides abundant sediment from Himalayan uplift into the subsiding Bay of Bengal. Structural, climatic and autocyclic fluvial processes have produced a large multi-channel river in Bangladesh with individual channels up to 5 km wide that can scour down to 50 m. The Brahmaputra-Jamuna has experienced large-scale avulsion at the end of the 18th century, recent widening and westerly migration and exhibits rapid bank erosion in response to large floods, the most notable being during the recent floods in 1987, 1988, 1998 and 2004. This chapter presents a synthesis of: i) the broad setting of the Brahmaputra-Jamuna River in Bangladesh, ii) channel morphology and its recent historical change, iii) the major bedforms and their dynamics, iv) the nature and importance of channel bifurcations, offtakes and confluences, v) floodplain sedimentation, vi) the sedimentology of the Jamuna River, and vii) examples of applied geomorphology and engineering within some of the largest and most dynamic channels. Data and discussion are drawn both from published papers and recent multi-national studies on the Brahmaputra-Jamuna that have allowed the most detailed study ever conducted on this mighty river. This chapter concludes with an outline for future areas of research needed to understand better and manage one of the world's most majestic and strategically important large rivers. 1. Background Bangladesh is dominated by three great rivers - the Brahmaputra-Jamuna, Ganges and Meghna - that combine to feed sediment into one of the world's largest deltas in the Bay of Bengal (Fig. 1). Bangladesh has been shaped by, and is dependent upon, its rivers, which provide fertilie soils and a diverse flora and aquaculture but also bring significant flood hazard and risk to infrastructure for a large and growing population. The people of Bangladesh have adapted their lifestyle for centuries to live with river flooding - frequently moving their temporary bankside homes, planting on newly emergent river bars, and sometimes raising their homesteads above water level in flood periods (Paul, 1997). However, a growing population, coupled with the expansion of infrastructure and economic development, has resulted in an increase in the intensity of flood damage (FPCO, 1995; Paul, 1997; CPD, 2004). The lives of many millions of Bangladeshi citizens is thus reliant on these rivers, with up to 600,000 people living on the riverine islands and bars alone (Sarker et al., 2003). Bangladesh's rural economy relies upon annual 'normal' floods to bring moisture and fresh sediments to the floodplain soils (Paul, 1997): for instance, two of the three rice varieties (aus and aman) cannot survive without floodwater and the fish caught both on the floodplain during flood season and from the many floodplain ponds ('beels') provide the main source of protein for many rural populations (Chowdhury, 1994; Paul, 1997; de Graff, 2003; Shankar et al., 2004). However, the effect of 'abnormal' floods can be devastating and result in appreciable damage to crops and houses, severe bank erosion with consequent loss of homesteads, schools and land, and loss of human lives, livestock and fisheries (BDER, 2004; Shankar et al., 2004). For example, in the 1998 flood, over 70% of the land area of Bangladesh was inundated, affecting 31 million people and 1 million homesteads (Chowdhury, 2000). The 1998 flood, which had an unusually long duration from July to September, claimed 918 human lives and was responsible for damaging 16000 and 6000 km of roads and embankments respectively, and affecting 6000 km2 of standing crops (Chowdhury, 2000). In the recent 2004 floods over 25% of the population of Bangladesh, or 36 million people, was affected by the floods; 800 lives were lost; 952,000 houses were destroyed and 1.4 million badly damaged; 24,000 educational institutions were affected, including the destruction of 1200 primary schools; 2 million government and private tubewells were affected, and over 3 million latrines were damaged or washed away, this increasing the risks of diarrhoea, cholera and other waterborne diseases; 1.1 M ha. of rice crop was submerged and lost before it could be harvested, with 7% of the yearly aus (early season) rice crop lost; 270,000 ha. of grazing land was affected, 5600 livestock perished together with 254,000 poultry and 63 MT of lost fish production (BDER, 2004; CPD, 2004). In the districts that are dominated by the Jamuna River, the 2004 flood damage to infrastructure (homes, roads, culverts), tubewells and latrines, with ensuing unemployment of many of the population, were some of the areas of critical impact. Hence, due to the nature of these devastating floods, and their recent occurrence in 1987, 1988, 1998 and 2004, the possible influences on catastrophic flooding, such as the role of Himalayan deforestation (Kattelman, 1990; Mirza et al., 2001), the type of intervention and infrastructural response to flooding (e.g. the Brahmaputra Right Embankment (BRE) from the Teesta to Hurasagar confluences, see Fig. 1) and the nature, scope and need for the Flood Action Plan (e.g. Haggart et al., 1994; Paul, 1997) have thus received much attention and debate over recent years (see for instance, Boyce, 1990; Hossain, 1993, Haggart et al., 1994; Reavill and Rahman, 1995; Paul, 1997; Islan, 2001). Additionally, political debates concerning water usage and construction of dams have become a major issue for Bangladesh (e.g. Patel, 1996; Wood, 1999; Mirza, 2004), since over 90% of the catchments of these three great rivers lies outside the boundaries of the country. The Jamuna and Ganges rivers combine to form the Padma River, which carries the third greatest water discharge of all the world's rivers but is often ranked the highest in terms of sediment discharge (Schumm and Winkley, 1994). The Jamuna is the local name given to the river for its entire length in Bangladesh to the Ganges junction (Fig. 1). The Brahmaputra-Jamuna has one principal tributary input, the Teesta River in the north-west, and two major offtakes on the left bank that are the Old Brahmaputra (see below) and the Dhaleswari (Fig. 1). The Brahmaputra/Jamuna River contributes ~51% of the water discharge and 38% of the sediment yield to the Padma (Schumm and Winkley, 1994), with the sediment yield being estimated at 590 MT yr-1 and the sand fraction contributing 34% of this total (Sarker, 1996). The Jamuna can have a braidplain width up to 15 km in flood and scour depths of up to 40 m have been recorded (Klaassen and Vermeer, 1988). Thus, by any definition, the Brahmaputra-Jamuna is one of the world's truly great rivers, and has a direct and daily influence on the prosperity of its population and the country's economic growth and political stability. Since the seminal work of Coleman (1969), much research has been conducted on these rivers, especially in the last fifteen years as part of the Bangladesh Flood Action Plan (e.g. Thorne and Thiagarajah, 1994; Haggart et al., 1994; FAP24, 1996a-h; Paul, 1997), together with work from organisations such as the Center for Environmental and Geographic Information Services (CEGIS) and Water Resources Planning Organization (WARPO) in Bangladesh, and this has allowed a dramatic increase in our knowledge of the behaviour of the Brahmaputra-Jamuna River. This work, aided by major advances in monitoring techniques, such as frequent, all-year satellite imagery and whole-flow depth monitoring within the main channels at even the highest discharges, has resulted in the river being characterised in more detail than ever before. Recent attempts to predict morphological change have also met with some success (EGIS, 2002; CEGIS, 2003; Mosselman, in press), and together with numerical modelling (Jagers, 2003) offer some hope to both understand and predict river channel movement, and establishing management plans for such change. Additionally, development of large-scale infrastructure within Bangladesh, such as construction of the Bangabandhu Multipurpose Bridge across the Jamuna, and numerous bank-protection works (Mosselman, in press), has demanded an increased quantification of the alluvial channel processes and prediction of future channel change. This chapter seeks to provide a synthesis on aspects of the geomorphology and sedimentology of the Brahmaputra-Jamuna River within Bangladesh, between the northern Bangladesh border and its junction with the Ganges some 240 km to the south (Fig. 1), and examines issues of applied geomorphology in response to the flooding and migration of this huge and largely untamed river. Details of the Brahmaputra River upstream of the Bangladesh border are given by Singh et al. (chapter X , this volume), and a recent synthesis on the water resources of the Brahmaputra basin in India is provided by Singh et al. (2004), including a chapter on fluvial geomorphology by Bora (2004).

The Brahmaputra-Jamuna River, Bangladesh

Detailed vertical profiles of time-averaged flow velocities and sediment concentration were taken during threeperiods of mid-channel bar development in the Jamuna River, Bangladesh. Bar growth was initiated downstreamfrom a major flow convergence and generated a bar 4 km long and 1 km wide in a channel up to 15 m deep. Flowvelocities and the concentrations of sand-grade suspended sediment were quantified using an acoustic Dopplercurrent profiler (ADCP). Bed morphology was measured using echo-sounding and all positions were locatedusing a differential global positioning system (DGPS).These data reveal no evidence for channel-scale, coherent helical flow cells in either distributary around thebraid bar. Instead, the structure of flow is dominated by a simpler flow divergence over the bar head, flow convergenceat the bar tail and flow that is usually parallel to the thalwegs in each distributary. During the later stages ofbar growth, flow is directed over the bar top from one distributary towards the other as the bar begins to adopt amore asymmetrical morphology. In addition, large sand dunes migrate up the bar stoss side, producing an accretionarydune front at the bar head. These dunes are strongly linked to high suspended bed-sediment concentrationsas flow shallows on to the bar top. A shadow of low suspended bed-sediment concentration is located in the bar leeduring the early stages of bar growth, this also being a region of small sand dunes.The lack of coherent secondary flows, around large kilometre-scale bars, may be explained through the largewidth-to-depth ratio of these channels, the low curvature of the anabranches, the complexity of flow over the bartop as it interacts with flow in the anabranches and the significant influence of large-scale dune-bedform roughness.These factors suggest that current models for the processes of mid-channel bar creation, growth and preservation,derived from studies of smaller rivers, require substantial revision before application to kilometre-scale sand-braidbars.

J Roden

Papers

Morphological evolution and dynamics of a large, sand braid-bar, Jamuna River, Bangladesh

Three-Dimensional Sedimentary Architecture of a Large, Mid-Channel Sand Braid Bar, Jamuna River, Bangladesh

The Brahmaputra-Jamuna River, Bangladesh

Flow Structure and Transport of Sand‐Grade Suspended Sediment around an Evolving Braid Bar, Jamuna River, Bangladesh