wcm.03.2025.481.489

The Semarang–Demak coastline of northern Java, Indonesia, faces severe erosion and land subsidence driven by tidal forcing, sediment transport, and human interventions. This study evaluates the impacts of the Semarang–Demak Sea toll road on hydrodynamics, sediment dynamics, and seabed morphology. Field data were collected through sediment sampling, tidal and current measurements, and monitoring of suspended sediments. A coupled hydrodynamic–sediment transport model (MIKE 21/3) was developed to simulate tidal currents, sediment dispersion, and bed level change. Results indicate a mixed, predominantly diurnal tidal regime, dominated by O1 and K1 constituents. Current simulations have revealed significant spatial variability, with spring ebb tides generating seaward flows exceeding 0.6 m/s, which enhance sediment resuspension and export, while neap tides promote fine sediment deposition. Suspended sediment concentrations were notably higher during spring tides, particularly in mid-reach zones of convergent flow. Bed level change modeling identified critical erosion near Tanjung Emas Port and Bedono, as well as depositional zones offshore of Sriwulan and Demak. Importantly, the toll road corridor intersects dynamic sediment redistribution areas, posing risks of localized scour, differential settlement, and altered flow pathways that could compromise structural stability and increase coastal hazards. These findings underscore the importance of integrated sediment management, continuous monitoring, and adaptive engineering approaches in mitigating infrastructure impacts and fostering long-term coastal resilience.