Dunes in the world’s big rivers are characterized by low-angle lee-side slopes and a complex shape

Abstract

Dunes are present in all the worlds' big rivers and form critical agents of bedload transport, constitute appreciable sources of bed roughness and flow resistance, and generate stratification that is the most common depositional element of ancient alluvium. Yet our current models of dunes are conditioned by the geometry of bedforms observed in small rivers and laboratory experiments, and in which the downstream leeside angle is often assumed to be at the angle-of-repose. Here we show, using high-resolution bathymetry from a range of the worlds great rivers, that dunes are instead characterized predominantly by low-angle leeside slopes (<10 • ), complex leeside shapes where the steepest portion is near the base of the leeside slope, a mean wavelength:height ratio greater than 100, and a height that is often only 10% of the local flow depth. This radically different shape of dunes in the world's big rivers demands that we incorporate such geometries into predictions of flow resistance and water levels, rethink the scaling relationship of dunes when reconstructing alluvial palaeoflow depths, and calls for a fundamental reappraisal of the character, and origin, of low-angle cross-stratification within ancient alluvial sediments.

Supplemental Material: Dune-scale cross-strata across the fluvial-deltaic backwater regime: Preservation potential of an autogenic stratigraphic signature

“…S5A). Most of the dune height data points are scattered through a range of flow depths without any significant trend, as has been found in other large rivers in the world (Cisneros et al, 2020). To take the hydraulics of backwater flow and grain size of the channel bed into account, the suspension number, defined as the ratio between shear velocity (u*) and settling velocity (ws) (Karim, 1995), is calculated for a given reach-averaged median channel bed grain size and corresponding reach-averaged flow depth (Fig.…”

“…Simulations were conducted over spatial steps of 5 km. (Yalin, 1964), or H = 6.96h 0.95 (Bradley and Venditti, 2017)), but may vary by two orders of magnitude (Bradley and Venditti, 2017;Cisneros et al, 2020).…”

Supplemental Material: Dune-scale cross-strata across the fluvial-deltaic backwater regime: Preservation potential of an autogenic stratigraphic signature

“…S5A). Most of the dune height data points are scattered through a range of flow depths without any significant trend, as has been found in other large rivers in the world (Cisneros et al, 2020). To take the hydraulics of backwater flow and grain size of the channel bed into account, the suspension number, defined as the ratio between shear velocity (u*) and settling velocity (w s ) (Karim, 1995), is calculated for a given reach-averaged median channel bed grain size and corresponding reach-averaged flow depth (Fig.…”

Robust methods for the decomposition and interpretation of compound dunes applied to a complex hydromorphological setting

“…The general reliability of the outlined methodology was validated using a freely available MBES data set from Rio Paraná, Argentina, which was originally investigated by Parsons et al ., (2005). After detecting prevailing bedforms along a total of 150 longitudinal transects from a single (snapshot) bathymetry, dune dimensions and abundances were compared to results obtained from an analysis in accordance with the wavelet analysis tool developed by Gutierrez et al ., (2018) and those published in Cisneros et al ., (2020). Especially regarding the results of the former, the examination of this independent study area reveals good resemblance between the proposed and at least one established approach, except for the occurrence of outlying ‘high small dunes’ and ‘low large or very large dunes’, respectively, which are deemed critical by the authors.…”

“…Bathymetric data of the Rio Paraná, which were used for a validation of the introduced dune tracking approach, is provided as an exemplary file of the Bedforms Analysis Toolkit for Multiscale Modeling (BedformsATM) developed by (Gutierrez et al ., 2018). Complementary bedform results for this region were carefully extracted from a database hosted by the University of Illinois at Urbana–Champaign (Cisneros, 2020). Exemplary code for the presented dune tracking approach is available from the corresponding author on request.…”