CFD modelling and field measuring of offshore wind flows over coastal dune fields at Magilligan Point, Northern Ireland

Foredune evolution has been conceptualized in a number of beach-dune interaction models. A fundamental aspect of this interaction is the input of sand from the beach to the dune by wind processes, which is essential in calculations of coastal dune budgets. The procedure to predict sediment input by wind to the adjacent coastal dune often excludes the role of offshore winds, which precludes explanation of the existence of extensive aeolian dunes on leeside coasts (where the dominant wind direction is offshore). Recent research points to the existence of ‘topographic steering’ and flow reversal under offshore winds as important mechanisms for dune maintenance. Findings at Magilligan Strand, (Northern Ireland) show that post-storm recovery of wave-scarped foredunes relied almost exclusively on offshore winds. Flow reversals from these winds transported sand from the beach onto the scarped section and thus aided re-construction of a dune ramp. This study consists of a series of experiments that seek to examine the effects of offshore wind flow events on localised aeolian sediment transport in the beach-dune zone. We combine a high density array of 3-D sonic anemometers with computational fluid dynamics (CFD) to enable a more thorough examination of natural dune forms and their associated airflow fields. Our working hypothesis is that offshore winds contribute substantially to foredune behaviour on leeside coasts. The field experiments took place at Magilligan Strand, Northern Ireland, over a four week period in September-October 2009. The foredunes are of sufficient height (up to 11m) to induce significant secondary airflow effects during offshore SW winds. Twenty four ultrasonic anemometers (3D Gill HS-50 model), were configured into a series of vertical arrays to capture vertical and horizontal components (u,v,w) of the wind as it flowed over the foredune and across the beach surface. Data was sampled at 50 Hz and transferred to the serial communications directly back to the control station position where they were time-stamped and buffered before transfer to a desktop PC. A LIDAR survey of the entire Magilligan foreland was completed in June 2008 using a LADS MKII LIDAR. To supplement topographic LIDAR mapping, a detailed DGPS survey of this smaller area consisting of more than 48,000 points was also undertaken September to October 2009 using a Trimble 4800 RTK at a point sampling resolution of 1m x 1m. OpenFOAM CFD simulations of offshore wind flows were carried out using a steady state, k-epsilon turbulence model, with 2nd order momentum discretisation. Preliminary results show strong reverse flow eddies in the seaward side of the foredunes during offshore wind events. These secondary flow reversals have been above velocity threshold and are transport capable. CFD simulations permit mapping of shear velocity vectors at heights of 0.3 m above the ground, providing information of potential areas for sediment transport. The extent and severity of flow separation regions (airflow reversal and reattachment zone) shows spatial and temporal patterns that may be related with incoming wind speed and direction. The use of high resolution anemometry over a complex dune terrain allows for CFD model validation, which in turn can enable pre-fieldwork simulations that can be used for optimal subsequent deployment of field instruments. This maximizes equipment efficacy and location relevance within complex terrain such as foredune systems. Using CFD modelling across a high resolution LIDAR surface of the dunes and beach we have isolated key areas of wind direction and velocity patterns which are important in aeolian transport budgets under offshore wind events.