How wind direction and building spacing influences airflow patterns and sediment transport patterns around a row of beach buildings: A numerical study

Buildings at the beach change the near-bed airflow patterns in the surrounding area. This induces alterations in wind-induced bed shear stress and wind-induced sediment transport which, in turn, affect the bed topography in the vicinity of buildings. Three-dimensional computational fluid dynamics simulations using OpenFOAM have been performed to understand how and to what extent the buildings at the beach influence the sediment transport from the beach to the dunes. Herein, we explicitly account for the positioning of the buildings with respect to each other and the dominant wind direction. Also discussed are the airflow mechanisms that are responsible for sediment transport, and how they alter due to systematic changes in the gap spacing between buildings and the wind incidence angle. Simulations were performed, in which we model flow and initial sediment transport around a repeating row of ten parallel full-scale beach buildings when the gap spacings and wind incidence angles were systematically varied. The horizontal near-bed streamline patterns showed that there is a critical gap spacing, below which the neighboring buildings significantly affect each other. Furthermore, the airflow in the near-wake region behind the row of buildings is quite complex. The shape and the extent to which the sand drifts develop behind the gaps between buildings are largely influenced by the wind direction, relative to the buildings. We also computed the average sediment transport flux along different lines downstream of the buildings. Our findings showed that, depending on the buildings’ positioning at the beach, they could have negative effects on dune growth by obstructing the sediment particles from moving downstream, or they could have positive effects on dune growth by steering the airflow and supplying more sediment downstream

Simulation of aeolian sediment transport with inter-particle moisture using Discrete Particle Modelling

Moisture plays a critical role in the dynamics of aeolian sediment transport over coastal sandy beaches. It was found from field observations that the transport over wet beach surfaces is fundamentally different from that over dry surfaces (Swann, 2021). Despite many empirical findings about the moisture effect on the threshold and transport flux, the small-scale mechanics that inter-particle moisture affects the development of transport from initiation towards equilibrium remains poorly understood (Cornelis & Gabriels, 2003)

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

Exploring moisture-constrained aeolian sediment transport through a discrete particle modelling framework

Moisture is a crucial environmental factor that shapes the dynamics of aeolian sediment transport along coastal beaches. Despite the existence of empirical formulations, little is known about the mechanism through which moisture influences this dynamic process. To address this knowledge gap, we present a numerical modelling framework implemented in the open-source software package MercuryDPM [1].This framework combines a discrete particle model, a one-dimensional airflow model and a liquid migration model. The two-way coupling between the discrete particle model and the airflow model can accurately represent the momentum exchange between these phases, yeilding reasonable sediment transport rates [2]. The inter-particle moisture distribution is modelled by a liquid migration law, which governs the presence of liquid films covering the particle surfaces and liquid bridges spanning the particle contacts [3]. The liquid bridge model introduces a static capillary force as well as a dynamic lubrication force, which is necessary to model the dynamic effects of moisture. This comprehensive model effectively captures particle behaviour under moist conditions and demonstrates the dependence of bed erodibility on particle impact and wind entrainment for varying moisture levels.Our approach provides valuable insights on the moisture effect in aeolian sediment transport. It advances our understanding of this complex phenomenon, and gives insights on the development of geomorphological patterns at coastal sandy areas. With its flexilibity and versatility, it can be extended to study many more specific processes related to sediment transport