Risk evolution of valley cities under compound flooding: a case study of Yingde City, China

This paper investigates the risk evolution of valley cities under compound flooding, which arises from the interaction between riverine overflow and intense local rainfall. Using Yingde, a flood-prone valley city in the Pearl River Basin of southern China, as a representative case, we integrate historical flood records with hydrodynamic simulations to analyse the long-term temporal evolution and topography-controlled spatial variability of compound flood risk within the valley. Comparative analyses with mountainous and plain cities are further conducted to isolate the role of valley topography in shaping flood risk evolution. The results show that, for valley cities: (1) compound flood risk shows a persistent upward trajectory over time that becomes increasingly decoupled from hazard severity, and is projected to intensify further under future climate change, with human instability risk growing far faster than economic damage risk; (2) spatially, risk follows a distance-decay rule with river distance, where human instability risk exhibits a steeper near- to mid-river gradient than economic damage risk, while hazard severity is topography-dominated and non-monotonic, forming a distinct V-shaped pattern; (3) compound flood risk shows strong spatial inequality with pronounced near-river concentration and is markedly more polarised in valley cities than in mountainous and plain cities.