On the mechanism behind the shift of the turbidity maximum zone in response to reclamations in the Yangtze (Changjiang) Estuary, China

Reclamation in estuaries can greatly change the channel geometry and hydrodynamic conditions and these changes may have significant impacts on spatial and temporal distribution of the turbidity maximum zone. This study focuses on the effects of a large area of reclamation built in 2007–2018 and the behavior of the turbidity maximum zone along the North Channel of the Yangtze Estuary. Data were collected of bathymetry in the North Channel, tidal elevations at Sheshan Station, river discharge at Datong Station and turbidity, retrieved from six Landsat remote sensing images in the dry season from 2006 to 2019. In-situ measured data on flow velocity and suspended sediment concentration were obtained in the dry season of 2003 and 2018. Analysis of the data revealed that reclamations, which led to narrowing (0.86–2.74 km) and fixing of the channel, caused erosion of 0.19–3.72 m in the deep channel and deposition on the tidal flats. Furthermore, it was found that the length of the turbidity maximum zone decreased: its landward boundary shifted 5 km seaward during spring tide and 17 km seaward during neap tide in the dry season. The position of the seaward boundary wandered within a range of 3 km, being further downstream during neap tide than that during spring tide. A conceptual model of changes in the borders of the turbidity maximum zone in response to reclamation is proposed. After the reclamation works, the deeper and narrower channel intensified ebb-dominance of the flow velocity. The coarsening of bed sediment weakened resuspension and decreased the bottom tidally averaged suspended sediment concentration. These changes led to a significant decline in the depth-mean of tidally averaged suspended sediment concentration and caused the seaward movement of the landward boundary of the turbidity maximum zone.