Mangrove-induced landward sediment transport owing to sediment flux asymmetry and bed shear stress

Abstract

Mangrove-induced tidal asymmetries lead to complex patterns of sediment transport. This paper aims to detail mangrove-induced landward sediment fluxes by numerical modelling. Specifically, using rigid cylinders to generalize mangrove forests and account for geometric and morphometric characteristics, mangrove-induced asymmetries in duration, velocity and sediment flux are explicitly considered. A case study is devised for Shenzhen Bay considering its unique status as the only mangrove nature reserve located in the hinterland of the modern metropolis in China. The results under the idealized bay model reveal that mangrove forests induce asymmetric tidal durations, i.e., shorter flood tides and longer ebb tides. The tidal duration asymmetry induces tidal velocity asymmetry, characterized by faster flood tides and slower ebb tides, leading to differences in bed shear stress. The peak difference in bed shear stress contributes to landward-dominant sediment fluxes. Higher bed shear stress during flood tides transports sediment landward, whereas lower bed shear stress during ebb tides fails to induce sediment resuspension. During flood tides, the maximum suspended sediment concentration exceeds that during ebb tides by ∼0.676 kg/m³ in the middle of the bay, ∼0.789 kg/m³ outside mangrove forests and ∼0.015 kg/m³ inside mangrove forests. Sediment accumulates at the fringe and also inside mangrove forests, eventually raising bed elevation over time. In the meantime, increasing water depth attenuates tidal asymmetry in duration, velocity and sediment flux, whereas the landward-dominant asymmetry persists. Overall, this paper disentangles the integrated process of mangrove-induced landward sediment fluxes by considering sediment flux asymmetry and bed shear stress.