Compound Flooding in Surabaya: A Multi-driver Sensitivity and Interaction Analysis

Abstract. Coastal compound flooding, caused by interactions between various flood drivers, presents serious threats to urban coastal regions. In this study, a process-based modelling framework was applied by integrating statistical analysis of historical data with reduced-physics numerical simulations to assess compound flood hazards. A very low correlation between extreme sea level and heavy rainfall events in the study area was demonstrated, which confirmed the initial assumption of flood driver independence. Single-driver and compound-driver scenarios corresponding to a 100-year return period were simulated for present-day and future conditions, accounting for relative sea-level rise and intensified extreme rainfall under SSP2-4.5 and SSP5-8.5 by 2100. Results indicate that non-linear interactions between drivers lead to a considerable amplification of flood hazards. The primary impact mechanism is increased inundation depth within existing floodplains due to backwater effects, which affects 8.6 % to 20.9 % (10.7 to 36.6 km²) of the total inundated area. A secondary effect is the expansion of the flood extent into previously unaffected areas by 0.82 % to 1.1 % (1 to 1.7 km²) of the total inundated area. Climate change is projected to exacerbate these hazards substantially. Under the high-emission SSP5-8.5 scenario, a coastal compound event with a current 100-year return period is projected to occur as frequently as every 2–3 years by 2100. This dramatic increase in frequency, along with a substantial rise in flood intensity, is primarily driven by sea-level rise.