Compound effects of sea level and flow on river-induced flooding in coastal areas of southern Sweden

Rönne River, Säve River, and Höje River, Sweden. River-induced flooding in coastal areas results from a multitude of drivers interacting in complex ways. The primary drivers are sea level (SL) and river flow (Q) that often exhibit coherent behavior to be considered in flood risk management. To describe and quantify the compound effects of SL and Q on flooding, a methodology was developed involving hydraulic simulations with long time series of data yielding statistical properties of output quantities such as river water level and flooded areas. Dominance analysis was conducted to quantify the relative influence of SL and Q on river water level along reaches. Also, simplified, empirically based equations were derived to predict the river water level at any location based on SL and Q . The long-term simulations revealed that the relative influence of SL and Q on the river water level changes significantly from the coast to upstream. For example, at the Rönne River, influence of SL decreases from 90 % to 20 % between 1 km and 11 km from the coast. Meanwhile, influence of Q increases from 10 % to 80 % over the same distance. The simplified equations derived to predict the water level can be used by stakeholders to forecast flood events or in risk assessment where many alternatives need to be considered. • A methodology was developed to quantify the compound effects of sea level and flow on river water levels and flooding. • The methodology was validated with long time series of input data for several rivers in south Sweden. • The methodology is applicable in areas with similar forcing conditions correctly representing input data statistics. • Dominance analysis was performed to quantify the relative importance of sea level and flow on river water levels. • A method was proposed to derive simplified equations that yield river water levels based on sea level and river flow.