Modeling the dynamics of aeolian meter-scale bedforms induced by bed heterogeneities

Significance

We present a model to explain the emergence of meter-scale bedforms that grow in coarse-grained interdune areas or on moist beaches. We show that the existing theory of dune dynamics must be extended to account for the spatial variation of wind transport capacity over bed heterogeneities, with enhanced transport over consolidated rather than erodible surfaces. The quantitative agreement between the model predictions and a unique set of high-precision field data acquired in the Namib Desert allows us to theoretically explore the different dynamics of such emerging bedforms, which can eventually disappear or lead to dune formation. This work provides ways to interpret the initiation and evolution of small bedforms, and facilitates the estimation of aeolian transport in diverse environments.

Abstract

Desert surfaces are typically nonuniform, with individual sand dunes generally surrounded by gravel or nonerodible beds. Similarly, beaches vary in composition and moisture that enhances cohesion between the grains. These bed heterogeneities affect the aeolian transport properties greatly and can then influence the emergence and dynamics of bedforms. Here, we propose a model that describes how, due to transport capacity being greater on consolidated than erodible beds, patches of sand can grow, migrate, and spread to form bedforms with meter-scale length. Our approach has a quantitative agreement with high-resolution spatiotemporal observations, where conventional theory would predict the disappearance of these small bedforms. A crucial component of the model is that the transport capacity does not instantly change from one bed configuration to another. Instead, transport capacity develops over a certain distance, which thereby determines the short-term evolution of the bedform. The model predicts various stages in the development of these meter-scale bedforms, and explains how the evolution of bed elevation profiles observed in the field depends on the duration of the wind event and the intensity of the incoming sand flux. Our study thus sheds light on the initiation and dynamics of early-stage bedforms by establishing links between surface properties, emerging sand patterns, and protodunes, commonly observed in coastal and desert landscapes.