Coastal Response to Sea Level Rise in Ystad Municipality

Abstract: The globally accelerating sea-level rise poses a problem in coastal areas through, e.g., the erosion of sandy beaches. Due to this, buildings, infrastructure, and other values can be lost, making the future shoreline position important to project in coastal management. One location with historical erosion problems is Ystad municipality in southern Sweden, where the erosive problems are probable to worsen with accelerated SLR. To quantify these problems, calculation methods for projecting the future shoreline change are of high importance, and a commonly used method is the Bruun Rule. The Bruun Rule estimates the shoreline recession due to changes in forcing conditions during sea-level rise. However, the method is coupled with restrictive assumptions and has been widely criticised. Due to this, several modifications of the Bruun Rule exist in the literature, but no other has seen global application in coastal management. In this thesis, Bruun Rule-based methods are identified, developed, and then qualitatively compared to the Bruun Rule to evaluate the effects of alternative calculation methods in Ystad municipality. First, to detect coastal processes affecting the studied coastline and calculation methods corresponding well to the conditions in Ystad municipality, a literature study was performed. To compare the methods, four beach profiles in areas with different states of historical erosion were chosen for application, and data was compiled. Finally, the future shoreline changes for the beach profiles in 2150 were estimated by each method, and the results qualitatively compared. In addition, a simplified Monte Carlo analysis was performed to visualise the data uncertainties. What could be concluded from the comparison was that including the effect of the background erosion/accretion, assumed to be constant over the studied period, in some profiles overshadowed the effect from the SLR-induced erosion by the Bruun Rule. Modifying the Bruun Rule to include the dune system, assumed not to migrate inland, gave lower recession with the given dune dimensions, but still in the same order of magnitude as the Bruun Rule. Furthermore, the low availability of sand along the coastline of Ystad municipality resulted in the conclusion that the Bruun Rule is not applicable on the majority of the coastline. The results from the Monte Carlo simulation showed that the large uncertainties in data resulted in probability intervals of up to 90 m for the projected shoreline change using the Bruun Rule. Thus, when calculating shoreline change in Ystad municipality, methods applicable on non-sandy beaches are needed, in addition to the Bruun Rule, better access to detailed data is essential, and the underlying erosion is an important factor to consider for coastal management purposes.