Effects Of The Deep Waterway Project On Hydrodynamics,Sediment Dynamics And Morphological Evolution In The North Passage Of The Yangtze Estuary

To improve navigability of the major access channel into Shanghai Harbor, a large-scale Deep Waterway Project (DWP) was carried out in the North Passage (NP) of the Yangtze Estuary. Training walls, groins and channel dredging directly transformed the natural geometry of the estuary. Besides, the engineering constructions greatly hindered the exchanges of water and sediment between the NP and adjacent bar areas. The large-scale interventions inevitably affected the morphodynamic processes in the NP, and even in the YE. In this thesis, we investigate how the navigational works affected morphological evolution, hydrodynamics and sediment dynamics of the NP.Morphological changes were assessed by analyzing digitized bathymetric data of the NP prior to and after the execution of the engineering works. The qualitative relations between these changes, along with the hydrodynamic changes as a result of the construction of engineering works, were subsequently investigated. The results reveal that training walls and groins increased the friction of the NP, resulting in decreased ebb transport in the upper reach of the NP, which led to intensive siltation in the distributary inlet. The frictional effects of the groins on the flow caused intense accretion of sediment in groin areas. All together, these changes caused a16%decrease in water volume in the NP and, hence, the further decrease of the flow diversion ratio of the NP. The main channel of the NP experienced significant erosion in the upper reach and lower reach, caused by the construction of training walls and groins that concentrated ebb flow in the main channel, while continuous siltation occurred in the middle reach where the estuarine turbidity maximum developed also with large amount of sediment input from the Jiuduan Shoal during flood.Changes in the characteristics of tidal and sediment dynamics were accessed by statistical analysis on the data of flow, salinity and suspended sediment concentration (SSC), which were collected simultaneously at several locations and at different depths along the main channel of the NP prior to and after the engineering works. The results show that the decreased ebb flow diversion from the South Channel and the overbanking flood tidal currents from the Jiuduan Shoal caused an increase in the flood tidal volume and an decrease in the ebb tidal volume in the distributrary inlet and upper reach, while the concentrating of flow into the main channel by the training walls and groins caused an increase in the ebb tidal volume in the lower reach. In dry seasons, the decrease in the ebb dynamics in the distributrary inlet and upper reach was more significant, while in wet seasons the increase in the ebb dynamics in the lower reach was more significant.The change in the tidal dynamics resulted in the change in the sediment transport during flood and ebb. The ratio of the ebb tidal sediment transport over the flood and ebb tidal sediment transports significantly decreased in the distributrary inlet and upper reach, while it greatly increased in the lower reach. Prior to the DWP, the sediment transport driven by ebb tide dominated over the transport by flood tide, while after the DWP, the transport by flood tide is dominating in some reaches, viz. middle reach in the wet season, as well as distributrary inlet and upper reach in the dry season. After the DWP, along-channel variation in the net sediment transport in a spring tidal cycle reveals that the sediment mainly accumulated in the middle reach in the flood season but in the distributrary inlet in the dry season.Further, the along-channel flow structure, suspended sediment distribution and its transport along the main channel were investigated, focusing on explaining the changes in residual sediment transport in terms of physical mechanisms, by combination of harmonic analysis and theoretical results of an analytical model. The results reveal that residual flow, M2tidal pumping and Stokes transport were dominant forcing agents in along-channel sediment transport in the NP. Among the main residual components, density driven flow caused landward sediment transport, while river discharge and Stokes return flow caused strong seaward sediment transport, and tidal rectification resulted in the convergence of sediment transport after the DWP.The structures of residual flow and its sediment flux were significantly altered by navigational works. Prior to the DWP, the subtidal flow structure was that of a gravitation circulation in both seasons which caused the convergence of sediment near the bottom and seaward sediment flux in the upper layer. After the DWP during the wet season there was seaward flow throughout the channel, which caused overall seaward sediment transport, whilst during the dry season the subtidal flow structure remained that of a gravitation circulation, but there was an upstream shift of zones with landward flow at the bottom. During wet seasons, the magnitudes of the main net sediment transport components almost doubled, and net sediment transport due to M2tidal pumping changed from a convergent pattern, with seaward transport in the upper reach and landward transport in the lower reach, to a strong overall landward transport.The distribution of SSC revealed that the estuarine turbidity maximum (ETM) was intensified due to the interventions. In wet seasons, the peak value of mean SSC doubled, and the ETM shifted and extended upstream. The increase in the mean SSC value was due to the increase in the tidal current velocity due to the engineering works, which caused stronger re-entrainment of sediment from the bed. The main reason for the upstream shift of the ETM was that the M2tidal pumping of sediment resulted in a stronger landward transport component, while the upstream extension of the ETM was caused by the decrease in the seaward transport component due to residual flow in the upper reach, which was caused by the decrease in the river discharge there. In dry seasons, the ETM had an upstream extension, also due to the decrease in the seaward sediment transport by river discharge in the upper reach, which was overwhelmed by landward sediment transport caused by gravitational circulation in the lower layer.The physical mechanisms leading to along-channel sediment trapping in the NP were systematically investigated by developing a two dimensional (vertical-longitudinal) semi-analytical model. The model simulates the water motion and sediment transport driven by a prescribed density gradient, river inflow and a semi-diurnal tide (M2) both at the seaward and landward boundary. The physical mechanisms involved are gravitational circulation, tidal rectification and tidal pumping. Two new sediment trapping mechanisms related to asymmetric tidal mixing and spatial settling lag effects were included, which are confirmed to be important contributors in sediment transport in the NP.Results show that the gravitational circulation causes sediment trapping in the upper reach. Residual flows due to tidal rectification cause a small seaward shift of the ETM. Here, the Stokes return flow and asymmetric tidal mixing are the two main forcing factors, driving seaward sediment flux and landward sediment flux, respectively. The M4tidal pumping mechanism causes a small amount of seaward sediment transport, mainly due to asymmetric tidal mixing. The M2tidal pumping mechanism causes a significant landward pumping of sediment throughout the channel, leading to an upstream shift of the ETM and a wider ETM zone with a smaller peak of the suspended sediment concentration (SSC). The temporal settling lag effect pumps sediment towards the head of the estuary, and the spatial settling lag effect widens the ETM zone. Sediment trapping is mainly caused by gravitational circulation, M2tidal pumping and asymmetric tidal mixing.The model results reveal that the sediment is trapped in the distributary inlet in the dry season, in the upper reach with mean discharge and middle reach in the wet season. This corresponds well with the locations of core areas with strongest back-siltation in the deep waterway of different seasons, indicating that the trapping of sediment in the ETM zone is the essential cause for the strong back-siltation in the waterway. In the dry season, the upstream shift of the core area is mainly due to the significant increase in the landward sediment pumping caused by spatial settling lag effects. The shift is also related to the decrease in the freshwater discharge and upstream shift of salt intrusion. In the wet season, the downstream shift of the core area is due to the increase in the seaward sediment transport by temperal settling lag in the distributary inlet and upper reach, as well as the the stronger seaward sediment transport induced by strong freshwater discharge.