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\textbf{Experiment description}\newline
At 5-6 December 2013, the northwestern storm at the North Sea called ‘Storm Xaver’ or the ‘Sint Nicholas storm (Sinterklaasstorm)’ with relatively low wind speeds and a long fetch resulted in high surge levels in combination with moderately high waves \citep{trouw2015}. This resulted in beach and dune erosion along among others the Belgian coast on the order of 8 m3/m \citep{lanckriet2015}. The Coastal Division of the Flemish Government collected 122 cross-shore profiles of the beach and (first) dune row during a dGPS survey 2-4 days before this storm and based on an airborne lidar survey 4 days after this storm \citep{trouw2015}. A selection of these pre- and post-storm profiles (\autoref{fig:overview_vlaanderen}) is being used for the validation of the morphodynamics in the BOI-version of the dune erosion model XBeach, focusing on dune erosion volumes and retreat distances.  

${tools.image(os.path.join(data_path,'Vlaanderen'), 'overview_vlaanderen.png', 'Overview of the Belgian coastline with the location of the analyzed cross-shore profiles measured before and after the Sint Nicholas storm and the water level (WL) and wave measurement locations used for this case.' ,label='overview_vlaanderen', width=145)}

Based on the data of the Vlaamse Banken, the offshore storm surge levels reached up to 6.0 m +TAW1 (5 min average) during the Sint Nicholas storm at measurement pole ‘A2’ and ‘Scheur Wielingen’ in front of the Belgian coast (bottom depth respectively about -7 m LAT2 and -10 m LAT). This was the result of a combined high storm setup and spring tide. At the harbor of Ostend, storm surge levels reached 6.19 m TAW \citep{trouw2015}. The observed water levels were highest since 1953 and were estimated to have a return period of about 40 year in Belgium \citep{IMDC2005}. Besides, significant wave heights reached up to 3.8 m (20 min average) at the offshore wave buoy ‘ZW-Akkaert’ at -20 m LAT (Figure 6). The highest peak occurred during rising tide; at the peak water level, the wave height was already about 0.5 m lower. The peak wave period during the storm was about 8 s with a peak to 15 s at the end of the storm at the same location. This corresponds to a return period in the order of 1 year along the Belgian coast \citep{IMDC2005}. The waves approached approximately shore-normal from the northwest. After more than one day of high waves, the wave height gradually reduced, together with the storm surge. 

The Belgian coast has gently sloping, dissipative beaches that are slightly steeper towards the east. They are mainly composed of fine to medium sand \citep{degraer2003}. The tidal regime along the Belgian coast is semidiurnal and macrotidal, with a of 3.7-3.9 m neap tidal range and 4.5-5 m spring tidal range which slightly decrease from west to east \citep{degraer2003}. To prevent the beaches from erosion by the strong tidal currents, a large part of the coastline is protected by groynes. Moreover, a concrete dike along the most of the coastline must protect the inland from flooding \citep{degraer2003}. In the areas with natural beach-dune transitions between Ostend and the Dutch-Belgian border, 15 profiles are selected for this case study (\autoref{fig:overview_vlaanderen}). The beaches in the selected profiles have a slope of on average 1:45 to 1:65 from the dune toe to the relatively flat zone at -5 m TAW (westernmost profiles) to -7 m TAW (easternmost profiles). Just below mean sea level, 2-4 bars are present in all profiles except the easternmost profiles (117-121). The relatively flat plateau extends more than 10 km into the sea and contains some sand banks. Further offshore, seaward of the profiles, the bathymetry is characterized by large sand banks at intermediate to deep water. 

\textbf{Results}\newline		
The dune erosion volumes and retreat distances at 7m + TAW, based on the measured pre- and post-storm profiles and the XBeach simulations for all 15 profiles are tabulated in \autoref{tab:Vlaanderen}. As an example, the pre- and post-storm profile for profile nr. 119 based on the measurements and on the XBeach simulation are presented in \autoref{fig:Vlaanderen_T_119}.

${tools.table('Vlaanderen/', 'skill.tex', 'Dune erosion volume [$m^3/m$] and retreat distance [m] at 7 m +TAW for the Sint Nicholas storm in the XBeach simulation and the measured profiles for the 15 selected profiles along the Flemish coast.', 'Vlaanderen')} 

${tools.image('Vlaanderen/T_119', 'BL_BeforeAndAfterStorm_T_119.eps', 'Cross-section profile nr. 119 before and after the Sint Nicholas storm based on the measurements and XBeach, including dune erosion volumes and retreat distances. Bottom: entire XBeach profile, top: zoom of beach and dune profile.' ,label='Vlaanderen_T_119', width=145)}