Joint Occurrence of Extreme Rainfall and Storm Surge along the Croatian Coast: Exploring Seasonal Variations

High storm surges, extreme sea waves, high river discharges and intense short-term rainfall are flood drivers that make densely populated coastal areas especially vulnerable to flooding. An additional increase of flood hazard and risk in coastal areas is expected due to changes in storminess, mean sea level rise, land subsidence and urbanisation. The simultaneous or consecutive occurrence of two or more flood drivers can lead to an event known as compound flooding.

In Croatia, compound flooding caused by the co-occurrence of high river discharges and storm surges is the only combination of compound flood drivers investigated to date. Consequently, other combinations of compound flood drivers remain unexplored. This study aims to address this gap by conducting further research on compound flooding in Croatia, specifically investigating the co-occurrence of extreme storm surges and rainfall along the Croatian coast. This study will provide insights into the compound flood potential due to extreme storm surges and rainfall at 42 locations along the Croatian coast. The rainfall data was obtained from rain gauge stations and the sea level data was obtained from Coastal Extremes in the Mediterranean Sea reanalysis and has been corrected by tide gauge data using machine learning.

High storm surges and heavy rainfall are flood drivers that often originate from the same weather system. Neglecting their seasonality can lead to a significant underestimation of the dependency and consequently the underestimation of the compound flood potential. With its pronounced seasonality, the Croatian coast is a great example for investigating seasonal correlation and co-occurrence of storm surges and rainfall. By disaggregating the time series into the individual seasons and analysing them separately, we gained a more detailed insight into the co-occurrence patterns of these flood drivers through maps with assigned correlation coefficients and a number of co-occurrences for each location.

As a result of this analysis, we will be able to identify the vulnerable areas with the highest probabilities of co-occurrence of high storm surges and intense rainfall more precisely. The selected locations will be eligible for a more detailed analysis of compound flood risk at the local level in future studies.