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\textbf{Experiment description}\newline
The field dataset of Saint Trojan in France is used for hydrodynamic validation of the BOI-version of dune erosion model XBeach, in particular regarding the infragravity-wave behavior. The field campaign took place in February 2017 on the gently sloping Saint-Trojan Beach during an energetic storm. \citet{bertin2020} compared observations to a 2DH XBeach model and presented detailed analysis on infragravity-wave behavior.  

The field site is a dissipative sandy beach located in the central part of the French Atlantic coast (\autoref{fig:Overview_ST}), along the southwestern coast of Oléron Island. The continental shelf in front of the study area is about 150 km wide, with a very gently sloping shoreface, the 20 m isobath being some 10 km away from the shoreline. The beach slope typically ranges from about 0.0015 at the shoreface to 0.015 in the intertidal area, and the beach is mainly composed of fine and well-sorted sands (D$_{50}$ = 0.18–0.22 mm). The tidal regime in this region is semidiurnal and macrotidal, with a tidal range varying between about 1.5 m during neap tides and 5.5 m during spring tides. Tidal currents are weak at the studied beach, and the impact of tides on short waves remains mostly restricted to water level variations.  

The storm Kurt generated very long swell waves that reached the coast between the 2nd and 3rd of February 2017. At the deep-water buoy of Biscay (\autoref{fig:Overview_ST}a), the mean wave period increased from 8.0 to 13.0 s, and Hs rapidly increased from 3.0 m to almost 10.0 m, which corresponds to a return period on the order of 1 year \citep{lerma2015}. The wave hindcast described in \citet{guerin2018} suggests that the peak wave period Tp exceeded 20 s.  

${tools.image(os.path.join(data_path,'Saint_Trojan'), 'overview_ST.png', 'Location of the Saint Trojan study area in the Bay of Biscay, with the Biscay buoy (×) and Chassiron Meteorological station (×). (b) Detailed bathymetry of the study area (m relative to mean sea level), showing the location of the offshore ADCP1 (×) and the instrumented cross-shore profile (dashed line). The coordinates are in meter (Lambert-93 projection). Source: \citet{bertin2020} ', label='Overview_ST',width=145)}

The measurement period of the field campaign encompassed four tidal cycles, from the 1st of February 2017 to the 3rd of February 2017, and is characterized by a tidal range of 3.5 to 4 m. An Acoustic Doppler Current Profiler (ADCP1) equipped with a pressure sensor was deployed about 3 km offshore (\autoref{fig:Overview_ST}b). In the intertidal zone, nine pressure transducers (PT) sampling at 4 Hz were deployed, as well as a second ADCP at the location of PT3. For each sensor, bottom pressure measurements were first corrected for sea level atmospheric pressure measured at the nearby meteorological station of Chassiron. The entire record was then split into consecutive bursts of 30 min, and only the bursts in which the sensor was continuously submerged were considered. PT9 was never continuously submerged for more than 30 min and data from this PT was therefore discarded. Bottom pressure power density spectra Ep(f) were computed using Fast Fourier Transforms, with 10 Hanning windowed,s 50\% overlapping segments (20 degrees of freedom). These pressure spectra were subsequently converted into elevation spectra considering linear wave theory. The spectral significant wave height (H$_{m0}$) was computed as $Hm0 = 4\sqrt{m0}$ where the upper threshold frequency was set to 0.4 Hz. The threshold frequency between the high-frequency waves and the lower infragravity waves is time-varying and defined following \citet{Roelvink1989} and \citet{hamm1997wave} as half the continuous peak frequency fp. It is important to note that in processing of the observations the infragravity-wave frequency band was not delimited by a lower threshold, and therefore also covers the VLF (very low frequency) waves. For more information on the field campaign and data processing please refer to \citet{bertin2020}.

\textbf{Results}\newline		
The observed and predicted water level, short- and long-wave heights during the peak of the storm are presented in \autoref{fig:Hydro_at_peak}. The local timeseries of observed and modeled water level and wave-height variations of measurement locations PT3 and PT7 are depicted in \autoref{fig:Hydro_at_PT3} and \autoref{fig:Hydro_at_PT7} respectively. Goodness-of-fit indicators are presented in each subplot (per location), and additionally assembled for all locations in \autoref{tab:Saint_Trojan}. An aggregated scatter plot of observed versus modelled water levels and wave heights is presented in \autoref{fig:ST_scatter}. 

${tools.image('Saint_Trojan', 'cross-shore-at-peak.png', 'Cross-section at the transect location during storm peak (3rd of February 2017 at 8 AM) of the water level (upper panel), short-wave height (middle panel) and low-frequency (IG and VLF) wave height (middle panel). In blue the model results, and the observations are represented by circles. In grey the predicted minimum and maximum short-wave heights during the half hour around the peak of the storm.' ,label='Hydro_at_peak', width=145)}

${tools.image('Saint_Trojan', 'Hydro_at_PT3.eps', 'Modeled (blue line) against observed (circles) water depth (top), short-wave height (middle) and low-frequency wave height (bottom) at PT3.',label='Hydro_at_PT3', width=145)}

${tools.image('Saint_Trojan', 'Hydro_at_PT7.eps', 'Modeled (blue line) against observed (circles) water depth (top), short-wave height (middle) and low-frequency wave height (bottom) at PT7.',label='Hydro_at_PT7', width=145)}

${tools.table('Saint_Trojan/', 'skill.tex', 'Goodness-of-fit (GoF) indicators for the modelled water depth [m] and the high- and low frequency wave heights [m] compared to the measurements at the PT1 to PT8 (seaward to landward side of measurement transect).', 'Saint_Trojan',small=True)} 

${tools.image('Saint_Trojan', 'scatter_hydro.eps', 'Observed versus predicted (left) water levels, (middle) short-wave height $H_{m0,HF}$ and (right) low-frequency wave-height $H_{m0,LF}$.',label='ST_scatter', width=150)}