Hydraulic modelling of the vulnerability for compounding high sea levels and high river discharge on the Swedish coastline

Sammanfattning: Coastal floodplains are at risk for combined high discharge and sea level events. Consequences when they occur can be several magnitudes greater than the impact from any isolated event. In this thesis the compounding of different river and coastal flood combinations at three case study sites were studied through the use of 1D/2D-hydraulic modelling software LISFLOOD- FP. The areas were; Henån, Ängelholm and Sundsvall. They have different physical conditions and were chosen to give an indication of the vulnerability to combined events at river mouths in Sweden. Results from the sites showed very few compounding effects. The exception occurred in a future scenario in Ängelholm where a combined 100-year sea level and 100-year discharge event resulted in a 14% increase of the inundation, compared to the corresponding isolated sea level event. The main conclusion drawn was that one factor is more important to the severity of a flooding at the coastal sites that were studied. Only when both the sea-level rise and the high discharge had significant consequences as isolated events was there a compounding effect when they occurred simultaneously. This is an important conclusion because it may facilitate future mapping of the vulnerability of the Swedish coastline to combined events.