Observations and modeling of alongshore variability in dune erosion at Egmond aan Zee, the Netherlands

• Calibration of the XBeach dune erosion model with pre- and post storm topography measurements of a dune-erosion event.
• Hydrodynamical calibration and validation of XBeach with hydrodynamical data of intertidal zone along a cross-shore transect.
• The calibrated model reproduced the measured pattern in alongshore dune erosion variability.
• The alongshore variation is caused by the dune topography and not by beach and nearshore topography.

Dunes can erode within a few hours when exposed to high storm surge levels and large waves. If the dunes are the primary defense, such as in the Netherlands, this could result in flooding of the hinterland when dunes breach. Models are often used to analyze how dunes will respond to extreme conditions, but they can also be applied to study milder events, and with that gain more insight in the processes of dune erosion. There is, however, growing demand for validating existing models under field conditions. Here, we calibrate the XBeach dune erosion model with pre- and post-storm topography measurements of the dune-erosion event in January 2012 at Egmond aan Zee, The Netherlands. Furthermore, hydrodynamical data of the intertidal zone along a cross-shore transect in the same area collected 3 months prior to the storm event are used to hydrodynamically calibrate and validate XBeach. During the January 2012 storm the dune erosion was variable alongshore, from the erosion of embryo dunes, the forming of a dune scarp to considerable slumping of the entire dune face. This caused the erosion volume V to range from 5 to 40 m3/m. The calibrated model reproduces the alongshore variability in V with reasonable accuracy. Additional simulations show that the alongshore variation in V is due to variability on the initial dune topography (primarily steepness), and that variability in beach and nearshore bathymetry was of secondary importance.