Discrete element modelling of grain-scale aeolian sediment transport on moist beach surface

In coastal areas, aeolian sediment transport could show significant spatio-temporal variability as a result of varying beach surface properties. The observed morphological patterns also vary with surface conditions. Surface moisture is one of the most important factors limiting the sediment transport process [1]. Moisture between the sand grains can influence both the mechanism of aerodynamic entrainment and the momentum transfer upon the collision between a saltating particle and the bed. Next to those, the saltation features are likely to be different from those in dry cases, hence different subsequent bed form patterns [2].
To understand the intrinsic variability of large-scale sediment transport on moist beach and the features of morphological processes, it is necessary to quantify the sediment transport properties on the grain scale first. From the information on the grain-scale dynamic behaviour, the up-scaling from discrete state of transport to a continuum description of bed forms could be realized through a novel transport formula. With this aim, this study investigates the effect of surface moisture on the grain-scale transport mechanism by CFD-DEM coupling. The open-source package MercuryDPM is used for DEM simulation [3]. This includes a 1D RANS model for air flow field calculation and a liquid bridge model that simulates the liquid between the particles. From this study, it is found that particles behave differently in the lift-off process by wind and collision process because of the cohesion induced by liquid bridge. The moisture could change the critical wind condition for transport initiation, as well as the cessation threshold for saturated transport to be sustained. The dependencies of transport rate on the wind strength and moisture level are studied as well.