Analysis and modelling of compound hazards to the desalination plants on Gotland: The Hans Storm 2023 event

The island of Gotland located in the middle of the Baltic Sea is Sweden's largest island, and also a county and municipality with the population of about 60,000, employed mainly in agriculture and tourist sectors. Gotland is a very popular domestic tourist destination for mainland Swedes, reaching nearly 2 million ferry and plane passengers per year. Gotland experiences limited capacity in groundwater reservoirs combined with increased demand during the warm season when tourists visits peak leading to recurring water stress. Desalination of drinking water from the Baltic Sea is a promising alternative to complement municipal water supply. The operation of the two major desalination treatment plants becomes however disturbed by compound hazards due to extreme sea weather events (marine heatwaves, strong upwelling events) and related hydro-sedimentary and biological processes (macro- and microalgae blooms) that are predicted to intensify under the climate change. Developing an apt forecasting system for this "multi-hazard" to inform sustainable management of Gotland's water resources becomes thus a priority and is of broader relevance to other regions in Sweden.

The ALGOTL project, funded by the Swedish research council for sustainable development (FORMAS), is a collaboration between Stockholm University, the Swedish Meteorological and Hydrological Institute (SMHI) and Region Gotland to develop a novel forecast framework for natural hazard impacts on management of water resources, both short term (early warning) and long term (climate scenarios). The project aims at development of Lagrangian- and risk modelling tools based on the operational ocean state forecast at SMHI. Our stakeholders on Gotland will provide input on adverse impacts, information required for management, and feedback on the forecast framework during the project.

This contribution will summarize results from observational and modelling analysis of the oceanographic, hydrological and biological conditions due to the Hans storm event in August 2023 that led to the disturbance in operation of the desalination plants on Gotland, located at the Herrvik (eastern side) and Kvarnåkershamn (western side of the island). The storm triggered an upwelling event leading to sea temperature changes of 10 K (prompting the change from summer to winter operational mode) and high vertical and horizontal velocities and associated excessive transport of sediment and biological (algae) material disrupting the filtering process. We will also show results from Lagrangian backtracking of the source waters reaching the desalination plants and present prospects for developing of a forecast system for related events in the future.

More information about the ALGOTL project: https://www.su.se/english/research/research-projects/algotl-forecast-framework-for-algae-blooms-to-secure-water-supply-on-gotland