Airflow and sediment transport dynamics in coastal dune blowouts

Abstract

Blowouts are erosional depressions that occur on pre-existing vegetated sand deposits. Their morphodynamic behaviour can reflect changes in anthropogenic activity, climatic conditions and animal behaviour; however patterns of deflation are poorly understood as near-surface airflow is complex. Previous research has indicated that flow is topographically manipulated as it moves through the landform, causing steering, reversal and jetting of the airflow. However, empirical data on these effects have been limited due to inadequacies in anemometer and sediment trap deployments. As a consequence knowledge of flow behaviour inside blowouts is limited to crude conceptual approaches.

This study used ultrasonic three-dimensional anemometry (50 Hz) to validate a three dimensional computational fluid dynamics (CFD) model. This enabled a detailed description and explanation of near surface wind flow within a bowl blowout from a variety of wind direction and wind speeds up to hurricane force. Three-dimensional anemometry complemented by a CFD simulation was used to quantify wind conditions, whilst high resolution (25 Hz) electronic load cell traps and saltation impact responders, measured sediment transport within a trough blowout.

The results demonstrate that considerable flow streamline compression, expansion, steering and reversal occur within trough and saucer blowouts. Airflow within blowouts alters with incident wind direction but does not change structurally with wind speed. Sediment transport flux and intermittency vary considerably within the landform, whilst the best correlation between wind flow and sediment transport varies between wind speed and turbulent kinetic energy (TKE). Where wind speed correlates best with TKE, the optimum averaging interval is much lower than those locations which correlate best with wind speed.

Date of Award	Oct 2012
Original language	English
Sponsors	Department for Employment and Learning, Northern Ireland
Supervisor	Derek Jackson (Supervisor) & Andrew Cooper (Supervisor)