Size-dependent asymmetry of barchans indicates dune growth controlled by basal area or bulk volume

We introduce a novel analytical model of the growth of barchan dunes in terms of their two flanks, from which we derive expressions for the size-dependence of the bilateral asymmetry of these bedforms in three cases where different mechanisms dominate the growth process. Analysis of the morphology of barchans on Mars and Earth suggest that there may exist two distinct attractor states for the asymmetry distribution. By comparing our analytical results with the observations, we show that the growth of barchan dunes appears to be dominated by processes, which are proportional to the basal area or volume of the bedforms, rather than being linear to their width as is typically assumed. We propose hypotheses explaining area-dominated growth as a result of variable wind regimes and volume-dominated growth from collisions. These predictions appear to be in line with the available data for the terrestrial swarms and barchan-like submarine bedforms and offer the potential of predicting patterns in inaccessible wind regimes from data on the morphology of the dunes. We introduce a model that predicts how the size dependence of bilateral asymmetry of barchan dunes depends upon the processes which govern the dunes' growth. Comparing to barchan swarms on both Earth and Mars, we reveal that the observed asymmetries are better explained by processes in which the dunes grow at a rate proportional to their basal area or bulk volume rather than by processes, which are linear in the width of the dune as is classically assumed. image