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.

Sand wave morphology and dynamics on continental shelves vary substantially, and we hypothesize that these spatial variations depend on local bed properties and hydrodynamic characteristics. To date, process‐based modeling studies have not been able to simulate realistic equilibrium sand wave heights and empirical studies are mostly limited to case studies. In order to explain the spatial variation in the morphology of equilibrium sand waves on continental shelves with processes and local bed conditions, a large‐scale investigation is required. In this paper, we use high‐resolution multibeam echo soundings, hydrodynamic models, and databases and sedimentary data for the analysis of, respectively, sand wave shape characteristics and the comparison to hydrodynamic and sedimentary characteristics for the Netherlands Continental Shelf. The results are quantified lengths, heights, and asymmetry of all sand waves in the Dutch part of the North Sea. Furthermore, we show that the mode of sediment transport (bed load or suspended) is a dominant factor in explaining sand wavelength, height, and asymmetry. Full results of shape characteristics of all sand waves on the Netherlands Continental Shelf together with the tidal velocity, water depth, surface wave height, and median grain size are provided in a repository with this paper (http://doi.org/10.4121/uuid:0d7e016d‐2182‐46ea‐bc19‐cdfda5c20308). These results are highly valuable for applied offshore engineering projects and to modelers for validating their morphodynamic model results.

[1] The understanding of the morphodynamics of harmonic bed forms on the seabed is essential for modeling marine sediment transport and coastal morphologic development. Previous research has mainly focused on the type and distribution of bed forms, but areally extensive data and time series of seabed features are scarce. Multibeam and side-scan sonar data from four expeditions reveal the contrasts between a coastal site with asymmetric and flattened, three-dimensional (3-D) compound sand waves on a shoreface-connected ridge and an offshore site with asymmetric and sharp-crested, 2-D compound sand waves. Migration rates of the coastal sand waves are 6.5-20 m yr À1 , while migration rates of the offshore sand waves are À3.6 to 10 m yr À1 . This contrasting morphology and dynamic behavior of compound sand waves at the two North Sea sites is explained by differences in the relative importance of tidal currents and wave activity near the bed. These new field data provide parameters and boundary conditions for sand transport models, while the empirically derived behavior of sand waves may be used to validate sand transport and sand wave models.