Pieter Rauwoens

A geometric multigrid solver for the free-surface equation in environmental models featuring irregular coastlines

A recently developed multigrid method (Botto, 2013), based on the concept of volume fraction, has been tested for the inversion of the Helmholz-type equation for the free surface in the environmental public domain code COHERENS. The volume fraction concept is particularly interesting for coarse grid cells that are agglomerated from both dry and wet fine grid cells at irregular coastlines. At these locations, modifying the prolongation operator and the coarse grid discretization, using the volume fraction, results in better convergence. However, as convergence deteriorates in the case of small, elongated islands that tend to disappear by the multigrid coarsening procedure, a correction is proposed, yielding good convergence rates, irrespective of the presence of small or large islands. The method is tested extensively for the inversion of the academic Poisson equation. Larger test cases, solving the Helmholz-type equation, prove the applicability for real-life applications of environmental flows.

Dynamics of Surface Moisture Content on a Macro-tidal Beach

ABSTRACT Montreuil, A-L.; Chen, M.; Brand, E.; Strypsteen, G.; Rauwoens, P.; Vandenbulcke, A.; De Wulf, A.; Dan, S., and Verwaest, T., 2018. Dynamics of surface moisture content on a macro-tidal beach. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 206–210. Coconut Creek (Florida), ISSN 0749-0208. Surface moisture content is a significant factor controlling aeolian sand transport. It is influenced by atmospheric, marine and sub-surface processes. Although several studies reported direct links in surface moisture content with the processes responsible for those variations, there is still a lack of understanding of its dynamic on a macro-tidal beach. This study aims to investigate spatial and temporal dynamics in the surface moisture content of a macro-tidal beach, and to determine the relative importance of factors controlling these. A field experiment was performed on a dissipative and non-barred beach at Mariakerke (Belgium) during an aeolian sand transport event in March 2017. Surface moisture content was measured from the backshore to the tidal zone using a video monitoring system. Simultaneous measurements of grain size, volumetric moisture content, groundwater level, atmospheric conditions, wind parameters, water level and topography were carried out. The hourly generated moisture maps indicate a clear cross-shore gradient of decreasing surface moisture content from the intertidal zone (ranging from 4–18%) to the backshore (none–8%), while it is more complex in the alongshore dimension. The backshore experienced the most rapid reduction of moisture content below 4% with a dryness rate of the surficial zone reaching 29% per hour in the late morning. It progressively continued to dry in the afternoon when sand strips, mobile aeolian bedforms, were well developed. Changes in moisture content over the beach surface reflect the atmospheric (solar radiation and wind) and marine (tidal elevation, wave and groundwater level) conditions and internal beach characteristics such as bedforms produced by aeolian sand transport and topography. Thus the continuous combinations of direct and indirect interactions between all these factors contribute to the spatial and temporal dynamics of surface moisture content. A better knowledge of the dynamics of the surface moisture content is a necessary prerequisite for the development of models and to compute budgets of aeolian sand transport.