Socio-hydrological prediction of soft-path vs. hard-path in flood risk management under climate change: A case study from the Lower Kelani River Basin, Sri Lanka

Lower Kelani River Basin, Sri Lanka. The dynamics of human-flood interactions are particularly pronounced in flood-prone regions. Effective flood management requires a synergistic approach that combines “hard” with “soft” measures in these areas. However, a gap remains in the empirical evaluation of the combined efficiency of these measures in the context of a changing climate and human–flood interaction dynamics. This study introduced an enhanced socio-hydrological model that evaluates hard and soft measures. We projected future annual maximum river levels using downscaled GCMs under Representative Concentration Pathways (RCP) scenarios (2.6, 4.5, and 8.5), incorporating simulations from the HEC-HMS model. Subsequently, a socio-hydrological model (SHM) was employed to outline potential socio-hydrological trajectories from 2025 to 2100. Floodplain dynamics were assessed under four management options: business-as-usual, soft-path, hard-path, and combined hard- and soft-path scenarios. These findings underscore the critical role of soft measures in mitigating flood damage across protected and unprotected floodplains. The soft-path approach demonstrated a 10–14 % reduction in damage compared with the business-as-usual scenario across all GCM-RCP scenarios. The integrated hard- and soft-path strategies showed a damage reduction of 12–38 % compared with the hard-path alone. This study also highlighted the significant role of Flood Risk Information (FRI) in reducing flood damage, particularly in protected floodplains. • Soft measures are vital to reduce flood damage in protected and unprotected areas. • Soft-path reduced damage by 10–14 % compared to business as usual scenario. • Combined hard and soft-path showed 12–38 % damage reduction compared to hard-path.