Application of XBeach to model a storm response on a sandy spit at the southern Baltic

The process-based XBeach model has been used to simulate changes in beach and dune morphology in terms of influence of the significant storm event on the sandy Dziwnow Spit, located in the western part of the Polish coast. The research was carried out as part of the SatBałtyk project and represents the first stage of XBeach model application to create a system for recording the selected effects and hazards caused by current and expected storm events. The significant storm event, registered in 2009, was used for model calibration. Ten cross-shore profiles were selected and compared against preand post-storm morphological data. Model performance was verified on the basis of BSS values for the terrestrial part of the profiles. Verification of the results was performed using two different approaches: on the basis of the highest mean BSS value for all profiles together and for one set of parameters (approach no. 1) and on the basis of the highest BSS value for each profile and most adequate sets of parameters (approach no. 2). Additionally, the observed and modelled beach and dune volume changes were calculated. The research showed that the XBeach model is well capable of simulating the dune and beach erosion caused by the storm event, but the model requires site-specific calibration. High sensitivity of the XBeach model to the facua parameter was determined; the parameter defines the wave shape and affects the sediment transport. The best fit of the profiles was obtained for BSS, ranging between 0.71 and 0.93, with the parameter hmin = 0.01 or 0.05, facua = 0.2-0.5, wetslp = 0.2-0.4 and dryslp = 1 or 1.5. The volume estimation error ranged from + 0.6 m³ m^-1 to -7.7 m³ m^-1, which represents 2.7% to 31.6%.