Combination of Extreme Water Levels and Waves in a Semi-enclosed Sea: Reconstruction of the Baltic Sea 2023 Storm Surge (Babet)

The October 2023 storm surge, denoted Babet, caused severe flooding and unprecedented damage along the southwestern Baltic Sea coasts. The beginning of October 2023 was dominated by several westerly low-pressure systems over the Baltic Sea region. The abundance of westerly winds pushed water into the Baltic Sea basin through the Danish straits resulting in elevated water levels of +50 cm above normal in the basin. Around the 18^th of October the wind direction shifted from west to north and subsequently to east while also increasing in intensity, with mean wind speeds of 25 m/s and gusts exceeding 30 m/s. This gave strong wind setup in the southwestern part of the Baltic Sea and the strong winds generated large waves as well as increasing the water levels further. Notably, the area experienced the most extreme storm conditions in over a century with the storm peak was reached during the night between the 20^th to 21^st October.

We present a study with results of the numerical model simulation using SWAN set up for the southwestern part to the Baltic Sea basin. The simulation combines the wave conditions derived from wind forcing and observed water levels from a network of observation gauges. These levels are compared to historical event statistics from an existing long-term hindcast model of wave climate conditions for the region. Finally, the results of storm surge levels are assessed in relation to observed flooding and erosion impact on natural coastal areas and impact to existing coastal protection.