Resolving the turbidity paradox: Tide-weighted remote sensing reveals how morphodynamic siltation overwhelms flow attenuation in a macro-tidal bay

Study region Sanmen Bay, a macro-tidal embayment, Zhejiang, China. Study focus This study investigates the physical mechanisms driving decadal turbidity escalation under a regime of widespread hydrodynamic attenuation. Decadal GF-1 satellite observations were processed using a tide-weighted compositing method to eliminate tidal phase biases. A 3D hydrodynamic model (MIKE 3 FM) with decoupled scenarios was applied to isolate the individual and cumulative impacts of natural bathymetric shoaling and engineering structures (reclamations, wind farms, photovoltaic arrays) on regional flow and sediment transport. New hydrological insights for the region From 2014–2024, the regional mean suspended sediment concentration (SSC) increased from 311.1 to 348.8 mg/L, concurrent with a basin-wide mean flow velocity reduction of 0.05–0.15 m/s. Decoupled numerical experiments reveal that large-scale bathymetric shoaling dictates this regional hydrodynamic attenuation, while coastal engineering structures generate localized flow diversion. The turbidity escalation under diminished tidal forcing is controlled by cohesive bed-property feedbacks. Persistent offshore sediment importation and subsequent deposition continually renew a highly porous surficial mud layer, reducing the critical erosion shear stress ( τ ce ). Consequently, the attenuated tidal currents maintain sufficient energy to trigger frequent sediment resuspension. This deposition process, mechanically amplified by water column volume reductions over accreted shallow flats and flood-dominant tidal asymmetry, establishes a morphodynamic loop that chronically traps sediments within the bay, offering key insights for sediment management in anthropogenically altered coastal hydrological systems.