Land Subsidence and Pluvial Flooding Compounding Risks in Urban Environments - A Modelling Case Study of Gothenburg, Sweden

Abstract

Cloudburst events are expected to increase in frequency and intensity due to climate change, posing growing challenges for urban flood management. While cities invest in hydrodynamic models and mitigation measures to address this risk, current as sessments generally assume a stable ground surface, an assumption that may not hold in cities subject to ongoing land subsidence. Gothenburg, Sweden, is domin ated by deep deposits of soft post-glacial clay that cause continuous land subsidence, yet subsidence is not currently included in hydrodynamic models or cloudburst risk assessments. This thesis investigates the impact of long-term subsidence on pluvial flood analysis through a case study in Gothenburg. Two 2D overland flow models were developed in MIKE+, simulated with a 100-year rainfall event. One model rep resents current terrain conditions, while the other incorporates subsidence-adjusted terrain projected 100 years into the future. By comparing the results of the two models, the influence of subsidence on flood outcomes was assessed. In parallel, a simplified risk assessment based on openly available data was conducted and eval uated as a potential tool for early identification of areas at risk, prior to a more detailed assessment being warranted. The results demonstrate that subsidence in fluences the spatial distribution of flooding, water depths, and flow paths. Existing flood-prone areas generally expand and deepen, while some areas show reduced ex posure. The relationship between subsidence and flood response is non-linear and complex, governed primarily by the spatial variability of ground movement and its interaction with local topography rather than subsidence magnitude alone. The simplified risk assessment showed partial spatial correspondence with the modelling results, but subsidence magnitude alone proved an insufficient screening criterion. A more reliable assessment should examine the coincidence of low-lying terrain, deep clay deposits, and spatially variable subsidence. Taken together, the findings demonstrate that long-term subsidence is a relevant but currently overlooked factor in urban pluvial flood planning, and that integrating it into hydrodynamic models produces meaningful differences in simulated flood outcomes.