Abstract
Dunes form critical agents of bedload transport in all of the world’s big rivers, and constitute appreciable sources of bed roughness and flow resistance. Dunes also generate stratification that is the most common depositional feature of ancient riverine sediments. However, current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments. For these dunes, the downstream lee-side is often assumed to be simple in shape and sloping at the angle of repose. Here we show, using a unique compilation of high-resolution bathymetry from a range of large rivers, that dunes are instead characterized predominantly by low-angle lee-side slopes (<10°), complex lee-side shapes with the steepest portion near the base of the lee-side slope and a height that is often only 10% of the local flow depth. This radically different shape of river dunes demands that such geometries are incorporated into predictions of flow resistance, water levels and flood risk and calls for rethinking of dune scaling relationships when reconstructing palaeoflow depths and a fundamental reappraisal of the character, and origin, of low-angle cross-stratification within interpretations of ancient alluvial sediments. Dunes in the world’s big rivers are dominated by lee-side slopes with angles of less than 10°, according to a bedform analysis of high-resolution bathymetric datasets.
Highlights
Dunes are a ubiquitous morphological element in all rivers that possess bed material grain sizes ranging from sands to gravels[1], and through their topographic steering of flow are a major source of turbulence within rivers[2]
In addition to multi-beam echosounder (MBES) data, dune shapes from data acquired by a single echosounder system in the Huang He (Yellow) River measured using the bedform analysis method for bathymetric information (BAMBI) method and dune data measured by hand from single beam echo sounder surveys of the Jamuna (Brahmaputra) River[18] are presented (Table S1 and Supplementary Figures 2 and 3) as additional data for comparison to dune leeside angles quantified from MBES data
Low-angle dunes with complex leeside shapes have been documented previously in the Brahmaputra[18], Fraser[20] and Amazon[34,35] rivers, and their flow dynamics has received limited study using physical and numerical models[21,25,26,27,28], the present results provide the most comprehensive analysis yet accomplished detailing the morphology of dunes in some of the world’s biggest rivers
Summary
Dunes are a ubiquitous morphological element in all rivers that possess bed material grain sizes ranging from sands to gravels[1], and through their topographic steering of flow are a major source of turbulence within rivers[2]. Recent work[32] has re-evaluated such dune scaling relationships by compiling data from both laboratory and field datasets (∼50) and concluded that there is a change in dune morphology from an asymmetric to a more symmetric shape and from high to lower angle leesides in shallow (
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