Optimization of jetty layout design for sedimentation mitigation at a floodway mouth using a hydrodynamic–wave–sediment transport model

Floodway mouths often face severe sedimentation, causing channel closure and disrupting fishing vessel navigation. This study optimizes the jetty layout at the Batang Kandis floodway mouth, Indonesia, using a fully coupled MIKE 21/3 numerical approach integrating hydrodynamic, spectral wave, and sand transport modules. The model utilized a non-cohesive sediment formulation (d50 = 0.15 mm) driven by hourly time-series of tides, winds, and waves to ensure realistic environmental forcing. Five design scenarios were evaluated, ranging from the baseline (no jetties) to an optimized asymmetrical elbow configuration. Results indicate that Scenario 5, featuring a 150-m 45°-bent left jetty and an 80-m right jetty, is the most effective in intercepting the dominant southwest littoral drift. While this layout significantly reduces sedimentation, initial dredging to a 3.5 m depth remains necessary for traditional bagan vessels. Post-dredging simulations under extreme flood discharge (454 m3/s) confirmed a natural flushing mechanism that effectively maintains channel depth. These findings offer practical design guidance for jetty layouts in sediment-prone floodway mouths facing similar hydrodynamic challenges.