Saeb Faraji Gargari 1*, Derek Karssenberg1 , Gerben Ruessink1
1 Department of Physical Geography, Faculty of Geosciences, Utrecht University, The Netherlands
* Corresponding author: s.farajigargari@uu.nl
Introduction
Coastal dunes play an important role in preventing marine flooding, especially during storms, making it essential to investigate their formation and migration for effective hazard management strategies. Understanding the wind flow over dunes is pivotal in comprehending sediment transport, influencing the dunes' development and evolution over time. The interaction between dune geometry and wind velocity creates a mutual coupling effect. While experimental and field studies have explored the impact of wind velocity and dune geometry, they often face certain constraints. Numerical methods offer a viable solution, utilizing computational fluid dynamics (CFD) to simulate air flow by solving the governing equations. This study aims to investigate variations in near-bed wind speed and direction across a foredune, exploring how these changes are influenced by the direction of the inlet wind and geometric characteristics of the foredune, including its height and the stoss (i.e., seaward) slope.
Objective and Methods
Air flow was simulated over a coastal dune by solving the governing three-dimensional Navier-Stokes equations. OpenFOAM, an open-source code utilizing the finite volume method (FVM) to solve partial differential equations (PDEs), is employed for the wind flow simulation [1]. The CFD model was first evaluated through a comparison of its results with measured data [2] obtained from a roughly 15-meter high foredune (1:2 stoss slope) at Egmond aan Zee, the Netherlands. Subsequently, the model was applied to simulate wind flow over synthetic foredune profiles, with variations in foredune height ranging from 6 to 25 meters and stoss slope between 0.25 and 0.5. The investigation of wind flow over synthetic foredune profiles allows us to explore the impact of different geometric properties of the dune profile on the wind speed-up factor, that is, the ratio between the wind speed at the dune crest and the dune toe.
Results
To validate the CFD model, Fig. 1a displays the CFD model’s wind velocity speed-up factor from the crest to the toe of the foredune for various inlet wind directions ranging from 0 (dune normal) to 90 (alongshore) degrees. The model results are compared with measured data, demonstrating the accuracy of the CFD model. Fig. 1b illustrates the synthetic foredune profile, with foredune height (Hd varied to analyze its impact on the velocity speed-up factor. Fig. 1.c presents the speed-up factor from the crest to the toe of the synthetic foredune for different foredune heights and slope. The beach slope (S1) is 0.01, while the stoss foredune slope (S2) is set to 0.25 and 0.5. The findings indicate that speed-up increases with dune height and is largest for dune-normal wind. The speed-up is barely noticeable for dunes with a height of 6 m, while the speed-up can increase up to 11 for a dune with a height of 25 m and a slope of 0.5. Fig. 1c also shows that the stoss slope strongly affects the speed-up. According to the results, it is concluded that increasing both the dune's height and stoss slope amplifies the velocity speed-up.
Figure 1. a) velocity speed-up factor (crest to toe) for CFD validation b) synthetic foredune profile c) Velocity speed-up factor for synthetic foredune considering different values of the stoss slope (S2).
References
[1] P.A. Hesp, T.A.G. Smyth, CFD flow dynamics over model scarps and slopes, Phys. Geogr. 42 (2021) 1–24. https://doi.org/10.1080/02723646.2019.1706215.
[2] C. Schwarz, C. van Starrenburg, J. Donker, G. Ruessink, Wind and Sand Transport Across a Vegetated Foredune Slope, J. Geophys. Res. Earth Surf. 126 (2021) 1–18. https://doi.org/10.1029/2020JF005732.
