Simulating wave-surge interaction in a non-tidal bay during cyclone Gudrun in January 2005

Cyclone Gudrun (Erwin) crossed the Baltic Sea in 8-9 January 2005. The maximum sustained wind speed measured over the Baltic Proper reached 28 m/s (gusts 37.5 m/s) and prior the high surge (2.75 m in Pärnu city), wind blew mainly from the sector SW-WSW. The hydrodynamic consequences, coastal damages and wave conditions in Baltic Proper and Gulf of Finland resulting from windstorm Gudrun have been analyzed previously. Lacking was the knowledge of wave-surge interaction and the role of wave induced setup. The aim of this paper is to study the effects of surge upon surface wave field dynamics and to reconstruct the possible wave induced set-up at a natural beach by means of numerical modeling. Modeling system consisting of a spectral wave model SWAN and non-hydrostatic depth-averaged free surface flow model SWASH was implemented. Spectral model was implemented to describe wave conditions in the Baltic Sea during the passage of cyclone and for providing boundary data to SWASH model, which in turn is used to calculate setup and inundation. Modeling relies profoundly on the quality of modeled wind fields -hence the accuracy of downscaled ERA40 wind fields during cyclone Gudrun is analyzed. We conclude an overall good level of agreement between modeled winds and observations and suggest using it in further modeling studies. Significant wave height in Pärnu bay increases up to 1 m by taking into account the additional deepening of water due to surge. The transformation of waves over the swash zone results in wave induced setup of 0.51 m and additional inundation of 130 m.