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\textbf{Experiment description}\newline
The island of Langeoog is one of seven inhabited barrier islands situated along the East Frisian German North Sea coast (\autoref{fig:overview_langeoog}). The shoreface of Langeoog can be characterized by migrating sand shoals through the ebb-tidal delta and a breaker bar system that migrates in eastern direction, comparable to the morphological configuration for the Dutch Wadden Sea Islands. The mildly sloping and shallow shoreface extents to deeper water with a slope of ~1:300. On Langeoog approximately 3.9 million m$^3$ of sand have been nourished since 1971 to protect the inhabited areas landwards of the dunes. This coastal area can be classified as mesotidal with semidiurnal tides with a mean tidal range of about 2.7 m at the Langeoog gauge. The mean grain size (D50) for Langeoog northern beaches is in the order of 0.25 mm \citep{interreg2021}. The upper beach and dune foot area has a mean grain size of 0.20 mm, whereas in the surf zone it is in the order of 0.30 mm. 

Storm Xaver passed Langeoog from 5 to 7 December 2013, in the Netherlands referred to as ‘Sinterklaasstorm’, and led to an extreme maximum storm surge level of NHN1  +3.95 m at Langeoog gauge station. Offshore of Langeoog a maximum wave height of Hs = 7 m and period Tp = 15 s was observed. The storm caused significant dune erosion at the Northern side of the island. Two months prior to the storm, October 2013, a beach nourishment was deployed on the North-Western part of the island (\autoref{fig:overview_langeoog}). A large part of the beach nourishment was eroded due to the storm, and the observed dune erosion landwards of the nourishment was considerably less compared to the adjacent dunes. 

Topographic measurements of the beach/dunes were collected after the beach nourishment placement, on October 18th, 2013, 2 months prior to Storm Xaver. Post-storm topography was collected on December 13th, 2013 for the area around the beach nourishment and April 30th, 2014 for the Eastern part of the island. Bathymetric data of the shoreface was collected in August 2013.  

In the Interreg VB North Sea Region Building with Nature project a 2D XBeach model of Langeoog was set-up and validated for the 2013 storm \citep{Hillmann2021}. The 2D model domain is 4km in longshore and 4.5km in cross-shore direction, and covers the area where the beach nourishment was located (\autoref{fig:overview_langeoog}). The initial model bathymetry was constructed by combining the August 2013 shoreface measurements and October 18th, 2013 topographic measurements. From this 2D model bathymetry and accompanying forcing conditions, six transects were extracted to use for the morphological validation of the BOI-XBeach model and settings.    

${tools.image(os.path.join(data_path,'Langeoog'), 'overview_langeoog.png', 'Location of the study site in Langeoog, Germany and locatation of the XBeach model. Model input bathymetry (right panel) with the location of the beach nourishment and the six transects A to F. Source: \citet{Hillmann2021} ' ,label='overview_langeoog', width=145)}

\textbf{Results}\newline		
The model results are validated using the topography measurements of the beach and dunes, collected on December 13th, 2013 for the area around the beach nourishment and April 30th, 2014 for the Eastern part of the island. The dune erosion volumes and retreat distances at 5 m +NHN, based on the measured pre- and post-storm profiles and the XBeach simulations for all 6 profiles are tabulated in \autoref{tab:Langeoog}. Furthermore, the pre- and post-storm profile for profiles A and D based on the measurements and on the XBeach simulation are presented in \autoref{fig:profile_A} and \autoref{fig:profile_D} respectively.

${tools.table('Langeoog/', 'skill.tex', 'A quantitative comparison of modelled and observed erosion volumes [$m^3/m$] and dune retreat [m] at 5 m +NHN for all 6 profiles', 'Langeoog')} 

${tools.image('Langeoog/profile_A', 'BL_BeforeAndAfterStorm_profile_A.eps', 'XBeach results for cross-shore transect A at Langeoog. Initial bed levels are depicted by the black dotted line, observed post-storm bed levels by the blacksolid line and XBeach model results are presented in red. The modelled dune erosion is shown by the red shaded area.',label='profile_A', width=145)}

${tools.image('Langeoog/profile_D', 'BL_BeforeAndAfterStorm_profile_D.eps', 'XBeach results for cross-shore transect D at Langeoog. Initial bed levels are depicted by the black dotted line, observed post-storm bed levels by the blacksolid line and XBeach model results are presented in red. The modelled dune erosion is shown by the red shaded area.',label='profile_D', width=145)}


