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Universiteit Gent (2002)

Erosion of dry and wet sediment induced by wind and wind-driven rain : a wind-tunnel study

Cornelis Wim

Titre : Erosion of dry and wet sediment induced by wind and wind-driven rain : a wind-tunnel study

Auteur : Cornelis Wim

Université de soutenance : Universiteit Gent

Grade : PhD Physical Land Resources 2002

Résumé
The dissertation attempts to contribute to a better understanding of some key parameters that influence the onset of wind erosion, in particular the effect of moisture content and wind-driven rain on deflation and transport of sediment. Further, attention was paid the optimal design of windscreens. All conclusions made in this dissertation are based on wind-tunnel experiments, conducted at het International Centre for Eremology (ICE), Ghent University. In a first part, the aerodynamic characteristics of the ICE wind tunnel were investigated and zones where the air current is homogeneous were denoted. Further, a simple low-cost sediment catcher was designed to measure total mass transport rates in wind-tunnel experiments. In a second part, a simple moment balance model to predict the threshold shear velocity to induce deflation of dry loose particles was presented. The model assumes the aerodynamic effects to be independent of the Reynolds number and the cohesion force between two particles to be primarily dependent of van der Waals forces. The model was calibrated and validated using wind-tunnel data from literature and from our own experiments. In a third part, the effects of near-surface moisture on the deflation process was studied. An evaluation of widely-used deflation models for wet sediment showed a great divergence between the model predictions. The deflation model developed in part two was therefore extented for wet sediment. Existing theory on liquid bridge bonding (capillaire forces) and absorbed layer bonding (adhesive forces) was reanalysed applying state-of-the-art knowledge on soil water retention phenomena. The model was calibrated and validated using wind-tunnel data from our own experiments and from simulations with Chepil’s model. In a fourth part, the effect of wind-driven rain on detachment and transport of sediment was studied. Simple equations were presented for the vertical and horizontal mass flux, and for the trajectory length and height of particles in rain-splash saltation. A model was also presented to predict the total mass transport rate as a function of the normal component of the kinetic ernergy or momentum of the raindrops and the shear velocity of the wind. It was shown that considerable amounts of sediment can be transported by wind, even during periods of heavy rain. The final part went into more practical aspects of wind erosion, in particular into optimal windbreak design for wind-erosion control. It was shown that an evenly distibuted porosity of the windbreak results in the longest protected area with an optimal overal porosity varying between 0.20 and 0.32 m2m-2. The longest shelter zone was also obtained from single-row windbreaks compared to multi-row breaks.

Source : Pedon 14 - Physical Land Resources - Universiteit Gent

Page publiée le 29 octobre 2015, mise à jour le 25 janvier 2017