Navigating Extreme Rainfall: Adaptive Strategies for Flooding in Upper Bengawan Solo Sub-Watershed, Indonesia

Abstract

Fluvial flooding poses an increasing threat to downstream cities within the Upper Bengawan Solo Sub-watershed, particularly in Surakarta. This study investigates how extreme rainfall, land use change, and catchment characteristics contribute to tributary-driven flood events and proposes adaptive strategies to reduce risk at the catchment level. A mixed-methods approach was employed, combining HEC-HMS hydrological modeling (using 25 years of rainfall and streamflow data) with qualitative insights from expert interviews. The model simulated flood discharges for return periods from Q1.01 to Q1000, identifying the Dengkeng, Pepe, and Samin tributaries as dominant contributors (>50% of peak discharge at Jurug Station under Q100 scenarios). Random Forest analysis and land use assessments revealed that urban expansion and soil degradation have exacerbated runoff, with larger catchments generating the highest flood peaks. To complement the technical findings, four expert interviews were conducted with stakeholders from government agencies and river basin authorities. These interviews explored the feasibility of adaptation strategies and highlighted both institutional enablers and barriers. Experts consistently emphasized the need for catchment-specific measures, including upstream retention, floodplain restoration, nature-based solutions (NBS), and improved spatial planning. The research identifies Room for the River (RfR)-style interventions and Interlocking Permeable Revetment (IPR) as promising approaches, though their implementation faces challenges due to political interventions and funding issues. The study concludes that effective flood risk reduction requires targeted action in high-risk tributaries, grounded in hydrological evidence and supported by institutional coordination. It recommends a shift toward source-control strategies that slow runoff at the catchment level rather than relying solely on downstream infrastructure. Additionally, the integration of real-time rainfall data and Dynamic Adaptive Policy Pathways (DAPP) is proposed for future planning. By linking detailed flood modeling with actionable adaptation planning, this thesis provides a replicable framework for addressing fluvial flood risk in complex and data-scarce river basins. It supports more resilient, forward-looking flood governance in line with Indonesia’s watershed management goals and the Sendai Framework for Disaster Risk Reduction.