Understanding Compound Pluvial–Fluvial Flood Hazards Under Climate Change: A Case Study of the Lower Tone River Basin, Japan

ABSTRACT Under climate change, Japan is increasingly exposed to tropical cyclones and their resultant flooding. Coastal watersheds, in particular, are more vulnerable due to the compounding effects of precipitation, runoff, and sea‐level rise. The objective of this study is to examine extreme precipitation induced by tropical cyclones and the subsequent inundation in a Japanese coastal watershed under a probable worst‐case global warming scenario, SSP5‐8.5. We propose an integrated and parsimonious modelling framework for compound pluvial‐fluvial flood inundation assessment in coastal watersheds, combining scenario‐based analysis with the computationally efficient SFINCS model. Using the Lower Tone River Basin as a case study, we configured the model with state‐of‐the‐art high‐resolution topobathymetric data. On the basis of this setup, we first validated the model against Typhoon Hagibis (2019) and then conducted scenario‐based simulations to evaluate flood hazards from current conditions to the near‐future period (2031–2060) and far‐future period (2071–2100). The findings indicate that precipitation and sea‐level rise will increase over time, and the associated flood inundation will intensify in terms of extent, depth, and duration. Agricultural lands exhibit greater hazards from flood inundation relative to built‐up areas, owing to their typical location in low‐lying and low‐gradient areas. This study not only advances understanding of compound flood dynamics in this specific region but also demonstrates a transferable modelling framework applicable to other similar coastal watersheds worldwide for rapid flood hazard assessments under climate change.