On the effect of two-direction seasonal flows on barchans and the origin of occluded dunes

TL;DR

This study uses grain-scale simulations to explore how two-directional seasonal flows influence barchan dune shapes and dynamics, revealing conditions under which dunes adapt or become rounded, potentially explaining Martian occluded dunes.

Contribution

It introduces a novel numerical approach to analyze barchan morphodynamics under seasonal flows and links high-frequency seasonal effects to the formation of occluded dunes on Mars.

Findings

Low-frequency flows allow dunes to recover their shape.

High-frequency flows cause dunes to become more rounded.

Grain trajectories and forces are characterized for the first time.

Abstract

We inquire into the morphodynamics of barchans under seasonal flows. For that, we carried out grain-scale numerical computations of a subaqueous barchan exposed to two-directional flows, and we varied the angle and frequency of oscillations. We show that when the frequency is lower than the inverse of the characteristic time for barchan formation, the dune adapts to the new flow direction and recovers the barchan shape while losing less grains than under one-directional flow. For higher frequencies, the dune has not enough time for adaptation and becomes more round while losing more grains. For both cases, we show, for the first time, the typical dynamics of grains (trajectories and forces). In particular, the round barchans are similar to the so-called occluded dunes observed on Mars, where seasons have very high frequencies compared to the dune timescale, different from Earth. Our results represent a possible explanation for that shape.