Aeolian and subaqueous bedforms in shear flows

Abstract

A sediment bed sheared by an unbounded flow is unconditionally unstable towards the growth of bedforms called ripples under water and dunes in the aeolian case. We review here the dynamical mechanisms controlling this linear instability, putting the emphasis on testing models against field and laboratory measurements. We then discuss the role of nonlinearities and the influence of finite size effects, namely the depth of the atmospheric boundary layer in the aeolian case and the water depth in the case of rivers.

Direct Numerical Simulation of Transverse Ripples: 2. Self‐Similarity, Bedform Coarsening, and Effect of Neighboring Structures

“…We observe the wavelength to height ratio of the ripples in the self‐similar phase to be approximately 25, consistent with previously reported data from DNS of Kidanemariam and Uhlmann () as well as laboratory experiments and field studies (Flemming, ). The dashed line in Figure corresponds to an aspect ratio of 20, which has been reported by Andreotti and Claudin (). They argue that when the ripple aspect ratio approximately reaches this value, the ripple amplitude saturates and the slope on the lee side of the ripple corresponds to the angle of repose.…”

Migration and Morphology of Asymmetric Barchans in the Central Hexi Corridor of Northwest China

“…Nevertheless, our observations clearly showed how the asymmetric sand supply and dune collisions are affecting the dune shapes in the Hexi Corridor (as summarized in the conclusions, 1-5, above), and are providing valuable data on sediment size distributions in a field where asymmetric barchan shapes emerge because of local sediment conditions and dune interactions. Recent modeling work has helped to push forward our understanding of sediment transport and dune migration dynamics [64][65][66][67], and the data provided in the present manuscript is valuable to inspire future applications of such theoretical modeling and numerical simulations. It would be interesting to investigate the dune processes in Hexi Corridor by means of numerical modeling, in particular, to verify the conclusions made here on the effect of the sediment source distributions, barchan collisions, and asymmetry in the sediment supply on the dune shape.…”

Pattern formation in the geosciences

“…Some topics have been well studied over several decades, such as sand dunes and sinuous rivers. The results that are presented here show how understanding of these ideas, once developed, can be fluidly applied in other contexts, such as sand bars [6] or submarine channels [10]. The inclusion of kinneyia [20] shows how new applications of linear stability theory can give insight into processes that happened long ago in our planet's past.…”

“…For example, Petroff et al [3] here demonstrate that springs eroding into a slope will generically split, or bifurcate, at an angle of 2π/5, while I present a crack-ordering mechanism that links columnar joints with polygonal terrain and mud-cracks [4]. There are many other situations where regular patterns are generated, from the largest scales of geomorphology, such as the curving subduction arcs of the Earth's crust [5] or the meanders of river networks, to ripples on the beach [6] and periodic chemical precipitation patterns [7]. The details of such patterns can provide quantitative information about the systems in which they form and the mechanisms underlying them.…”

“…If the bed has an uneven surface, grains can be preferentially eroded from regions of higher wind stress and deposited elsewhere. Andreotti & Claudin [6] review how this mechanism leads to the linear instability of dunes of well-defined wavelength. They show how such dunes scale under different fluids, such as liquid water or the atmospheres of Earth, Mars and Venus, and how these small-amplitude dunes grow and coarsen until they are limited by the thickness of the flowing fluid layer.…”