Engineering mitigation of flood inundation and riverbank instability under climate variability in the Bomo Watershed, Indonesia

Abstract Climate variability has intensified extreme flows in tropical watersheds, exacerbating flood inundation and riverbank instability. This study assesses flood hazard and riverbank slope stability in the Bomo Watershed, Indonesia, using an integrated engineering approach. HEC-RAS 1D unsteady simulations were run for 1-, 5-, and 10-year return periods. Under the 10-year scenario, inundation affected > 28.6 ha of floodplain, with floodplain depths up to 1.8 m near settlements and local velocities > 1.5 m/s along constricted reaches; 21% of the inundated area was classified as High risk. In parallel, FEM analysis of four critical banks showed BH2 to be marginally stable (FS = 0.96) under saturated conditions, with plastic-strain bands aligning with observed tension cracks and toe erosion; a mesh-sensitivity check indicated ± 1% variation in FS. Integrating hydraulic and geotechnical outputs identified dual-hazard zones for priority intervention. Cantilever retaining walls with toe protection are recommended as a design option for high-hazard segments (Zone A), while riparian buffers and setbacks are recommended for moderately vulnerable slopes (Zone B). For the critical BH2 reach, the design target is FS ≥ 1.30 under high stage, to be achieved through structural support and toe protection. Implementation should follow conservative design checks and be supported by site-specific geotechnical confirmation and operational maintenance triggers, providing a risk-informed basis for river-corridor management under observed short-term variability.