ABSTRACT A graphical abstract illustrating hybrid flood defence strategies in the Jompo Watershed, showing the transition from existing conditions to a combination of grey infrastructure and nature-based solutions (land restoration and retention ponds) to reduce peak flow, flood depth, and inundation. Steep-gradient rivers such as the Jompo Watershed in Jember Regency, East Java, are highly prone to flash floods. This study assesses four mitigation scenarios using Hydrologic Engineering Center–Hydrologic Modeling System and Hydrologic Engineering Center–River Analysis System: (i) existing conditions, (ii) grey infrastructure, (iii) Nature-based Solutions (NbS) involving land restoration and retention ponds, and (iv) a hybrid grey–NbS approach. Among these, the hybrid scenario produced the strongest risk reduction, decreasing peak discharge by 37%, flood depth by 42%, and inundation area by 39% (from 4.81 to 2.91 km2). Grey infrastructure primarily reduced flow velocity, whereas NbS increased infiltration, storage, and runoff delay. Their combined effects generated greater attenuation than either strategy alone. The magnitude of reduction reflects the watershed's steep slopes and limited spatial availability, which constrain mitigation performance relative to wider or low-gradient basins. Overall, the results suggest that relying solely on conventional grey infrastructure is insufficient for steep tropical watersheds. Hybrid solutions offer a more cost-effective and ecologically sustainable alternative, enhancing both hydraulic performance and catchment resilience. The study provides an evidence-based framework that supports SDG 11 on resilient urban development and can be applied to similar flash-flood-prone tropical catchments facing heightened climate risks.
Hidayah et al. (2026) studied this question.