Uneven Impacts of Seawall Heightening on Tsunami Risk Reduction under Rising Sea Levels: Probabilistic Scenarios from the Japan Trench

ABSTRACT Disasters exacerbated by climate change have prompted adaptation measures, including seawall heightening, which is an effective approach to protecting coastal areas. However, the combined effect of rising sea levels and tsunamis can create a compound coastal hazard, in which long‐term sea level rise amplifies the impact of tsunamis. The effectiveness of seawall heightening under such compound conditions and the spatial disparities in tsunami risk reduction benefits has not been sufficiently quantified. This study provides the first probabilistic assessment of the effectiveness of seawall heightening against increasing tsunami risks under rising sea levels. We also demonstrate that seawall heightening can introduce inequality in tsunami risk reduction through a case study of Sendai Port, Japan. To quantify tsunami losses while accounting for uncertainties, we introduce a probabilistic tsunami risk assessment (PTRA) framework. Loss estimations were conducted under varying seawall conditions, and the distributions of changes in annual expected loss (AEL) were analyzed to evaluate spatial variation in risk reduction. The absolute Gini index was applied to quantitatively assess the disproportionate effects of risk reduction. The results show that greater seawall heights lead to greater reductions in aggregate AEL under rising sea levels, although the magnitude of reduction varies substantially across return periods. Additionally, greater heightening may increase spatial disparity in risk reduction. Our findings highlight the need to include probabilistic risk assessments and structure‐level disparity evaluation to support climate change adaptation and disaster mitigation strategies. Such integration can support equal and more effective coastal management against rising sea levels.