Sand fences are widely applied to prevent soil erosion by wind in areas affected by desertification. Sand fences also provide a way to reduce the emission rate of dust particles, which is triggered mainly by the impacts of wind-blown sand grains onto the soil and affects the Earth’s climate. Many different types of fence have been designed and their effects on the sediment transport dynamics studied since many years. However, the search for the optimal array of fences has remained largely an empirical task. In order to achieve maximal soil protection using the minimal amount of fence material, a quantitative understanding of the flow profile over the relief encompassing the area to be protected including all employed fences is required. Here we use Computational Fluid Dynamics to calculate the average turbulent airflow through an array of fences as a function of the porosity, spacing and height of the fences. Specifically, we investigate the factors controlling the fraction of soil area over which the basal average wind shear velocity drops below the threshold for sand transport when the fences are applied. We introduce a cost function, given by the amount of material necessary to construct the fences. We find that, for typical sand-moving wind velocities, the optimal fence height (which minimizes this cost function) is around 50 cm, while using fences of height around 1.25 m leads to maximal cost.

No abstract

Flows through porous media can carry suspended and dissolved materials. These sediments may deposit inside the pore-space and alter its geometry. In turn, the changing pore structure modifies the preferential flow paths, resulting in a strong coupling between structural modifications and transport characteristics. Here, we compare two different models that lead to channel obstruction as a result of subsequent deposition. The first model randomly obstructs pore-throats across the porous medium, while the second model always blocks the pore-throat with the highest flow rate. By subsequently closing pores, we find that the breakdown of the permeability follows a power-law scaling, whose exponent depends on the obstruction model. The pressure jumps that occur during the obstruction process follow a universal power-law distribution with the same scaling exponent as the avalanche size distribution of invasion percolation. The fact that this exponent is also independent of the model suggests that the clogging processes and invasion percolation may belong to the same universality class.