Besides well-controlled laboratory cases, the model is also applied to the field. The first example concerns the morphodynamic response of sandy dunes to extreme storm impacts at Assateague Island, Maryland, USA, which was analyzed before by Jiménez et al (2006). Two consecutive northeasters attacked the barrier island during late January and early February, 1998. The bathymetry was measured using LIDAR in September 1997 and again February 9th and 10th, 1998 after the two storms had subsided.
Three types of dunes were identified by Jiménez et al (2006), shown in \autoref{fig:Assateague_Island_profiles.eps}. Profile A (upper left panel) is initially characterized by a steep faced dune, where the maximum run-up exceeded the dune crest height and the mildly sloped back of the dune. The morphological response is characterised by profile lowering, decrease of the beach face slope and landward barrier displacement, while retaining barrier width.
Profile type B is a double-peaked dune profile and has two different shapes. Profile B1 (upper right panel) is initially characterized by a primary and secondary dune, both of which are lower than the maximum run-up height and which are separated by a valley. Profile B2 (bottom left panel) initially has two peaks of which the seaward one is lower. The backside of the barrier of either type is therefore either characterized by a secondary dune line (profile B1) or a taller crest of the dune (profile B2) which prevents the eroded sand from being transported to the backside of the dune. The main morphological response for these profile types is a decrease of the beach face slope, outer shoreline retreat and narrowing of the barrier.
The height of the dune crest of profile C (lower right panel) exceeds the maximum run-up height and so little overwash is observed. The morphological response of this type of profile is crest lowering due to slumping, decrease of the beach face slope and retreat of the outer shoreline. The width of the barrier is seen to decrease.
The storm impact of the two North Easters on Assateague Island were modelled with XBeach for the four profiles described by Jiménez et al. (2006). The profiles were extended with a shallow foreshore and a 1:100 slope in seaward direction till a water depth of 9 m below NAVD88. As XBeach has not been shown to accurately simulate morphological change during very long storm durations, the simulations were run for a total of 20 hours. The measured wave and surge conditions were parameterized for each storm by a constant surge level and a constant wave spectrum (Pierson-Moskowitz) (see \autoref{tab:Assateague_Island_conditions.tex}). This approach assumes that two 72 hour storms with varying surge and wave conditions can be approximated by two 10 hour simulations with constant maximum surge and wave conditions following a similar approach as Vellinga (1986). This approach also facilitates further sensitivity studies into the effect of varying hydraulic forcing conditions. The calculation grid size varies from 18 m at the offshore boundary to 2 m on the islands. A morphological acceleration factor of 5 is applied. The final simulated bed profiles are shown in \autoref{fig:Assateague_Island_profiles.eps}.