The impact of vegetation on hydrodynamics in Port Phillip Bay

This study investigates the impact of submerged aquatic vegetation (SAV) on hydrodynamics and sediment transport in Port Phillip Bay (PPB), a large tidal embayment in southeastern Australia. A high-resolution coupled hydrodynamic–wave–sediment model (SCHISM-WWMIII) was developed and validated against tide-gauge and buoy observations. Two scenarios, with and without seagrass, were compared to quantify SAV effects. Results show that including SAV improves model accuracy, especially for water levels, with correlation coefficients exceeding 0.9 and biases reduced to below 0.1 m. Modest improvements are also observed in wave height simulations at vegetated sites. SAV notably enhances the accuracy of current simulations, reducing velocity bias by up to 27.9 % at the bay entrance. It significantly reduces current speed (up to 0.2 m/s) at vegetated areas. These changes result in reduced and redistributed bedload transport, with peak variations exceeding 110 % under extreme conditions. Time series at vegetated and non-vegetated sites show that seagrass reduces currents and sediment transport during energetic conditions and helps stabilise the wider bay. This study highlights the regulatory role of seagrass in modifying intra-bay hydrodynamics, such as water levels, wave heights, currents, and sediment transport. The findings provide a scientific basis for nature-based approaches to managing coastal hydrodynamic processes. • Coupled wave–hydrodynamic–sediment model with explicit SAV drags for PPB. • Including SAV improves model skill for water levels (R > 0.9, bias <0.1 m) and currents (bias reduced by up to 27.9 %). • SAV reduces extremes, with currents decreasing by up to 0.5 m/s and SWH by up to 0.1 m (∼5 %) along the entrance pathways. • SAV modify nearshore sediment pathways, with peak bedload transport differences exceeding 110 %.