Modelling the airflow in foredune blowouts using Computational Fluid Dynamics

Abstract

Due to past coastal management strategies, sediment transport from the beach to the dunes has been severely limited, negatively affecting biodiversity in the backdunes. To amend this, through-like ’depressions’, called foredune notches, have been constructed into the foremost part of the dune ([Ruessink et al., 2018]). The notches facilitate wind-driven sediment transport from the beach to the backdunes. The wind inside the notch is however strongly steered and accelerated by topography. The degree of steering and acceleration, and therefore the effectiveness of the notch at facilitating sediment transport, is dependent on both the incident wind direction and notch geometry. The relationship between key geometric parameters, such as the notch width and steepness of the lateral walls, and the effectiveness of the notch at facilitating sediment transport is currently not precisely known. Here we show that the geometry of the notch can significantly impact its score on key sediment transport metrics using computational fluid dynamics. First, 2 real-life notches (notch 3 and 4) from the Zuid-Kennemmerland were considered. For these notches, sonic anemometer measurements of the wind field were available for a period of almost 1 year. These measurements were used to validate the CFD model. The CFD-derived wind fields were also used to determine which notch had the most effective geometry. It was found that notch 3, which was narrower and had steeper lateral walls, was the most effective at facilitating sediment transport. To assess the relationships between geometric parameters and the notch effectiveness in more detail, simulations were performed on idealized notch geometries. We found that the widest (75m) and deepest (20m) notches considered were the best at facilitating sediment transport. The effectiveness of the notch can also be significantly improved by giving its floor a slight slope, in the order of 5.5◦. Giving the notch a floor plan that gradually narrows along its long axis however decreases its effectiveness. The findings are largely in line with previous observational studies with regards to sediment transport in foredune notches ([Riksen et al., 2016], [Pye and Blott, 2016]). No evidence was found for flow separation over the lateral walls of real-life foredune notches. In runs for idealized geometries, flow separation was only observed when the lateral walls were sufficiently steep. These results are indicative that large-scale geometries are generally better for facilitating sediment transport, with the slope of the basin floor allowing for further enhancement of sediment transport.

[Ruessink et al., 2018] Ruessink, B., Arens, S., Kuipers, M., and Donker, J. (2018). Coastal dune dynamics in response to excavated foredune notches. Aeolian Research, 31:3–17. The Ninth International Conference on Aeolian Research- ICAR IX (Coastal Dune Processes and Aeolian Transport).

[Riksen et al., 2016] Riksen, M. J., Goossens, D., Huiskes, H. P., Krol, J., and Slim, P. A. (2016). Constructing notches in foredunes: Effect on sediment dynamics in the dune hinterland. Geomorphology, 253:340–352.

[Pye and Blott, 2016] Pye, K. and Blott, S. J. (2016). Dune rejuvenation trials: overview report. report to natural resources wales. Tech. Rep. KPAL Report 19099, Kenneth Pye Associates Ltd.