The Study Of The Transport Timescale In The Changjiang Estuary

The transport timescale of an estuary characterizes the nature of the dynamics of the estuary. Using transport timescale to study transport dynamics have been successful and proven to be an efficient tool. However, research on bifurcation estuary has very limited. The study on the transport timescale in Changjiang Estuary will provide new knowledge to understand the underlying dynamics of the estuary. It is not only a new endeavor in the Changjiang Estuary, but also a representative of bifurcation estuary in the world estuary. The study was carried out using a three-dimensional numerical model. The three-dimensional hydrodynamic model was developed for the Changjiang Estuary and adjacent coastal area using the Environmental Fluid Dynamic Code (EFDC). The model was fully calibrated with measured elevation, current and salinity. The results show that the model has the capability to reproduce the hydrodynamic characteristics of the estuary, which establishes the foundation for further quantitative investigation on the material transport and estuarine circulation in the Changjiang Estuary. The concept of the water age, a newly developed theory in recent years, is introduced to study the transport processes in the Changjiang Estuary. The decompositions of axis current and salinity approach based on physical processes were used to further investigate the underlying dynamic processes, in particularly, the mechanism controlling the flux of up-estuary salinity in the Changjiang Estuary. The impact of large estuarine projects on the circulation and transport timescale is discussed with respect to the Deep waterway project. Some main conclusions are summarized as follow:The impact of the river runoff, tide, wind, Coriolis force is investigated systematically. The Changjiang river runoff is the dominant factor controlling the estuarine transport timescale. Using Xuliujing as a reference location, the water parcel takes twice as much time during dry season comparing to that during the flood season to be transport out of the estuary. It takes 36 days and 20 days, respectively under high flow conditions. The vertical transport timescale in the Changjiang Estuary resembles stratification pattern as density. It has significant stratification during flood season and neap tide comparing with dry season and spring tide. A noticeable region correspond to the Turbidity Maximum is found where the water parcel moves slower compared to both the upstream and downstream. It is found that the periodic tidal mixing plays a key role in modulating the transport timescale. The runoff flushes water oceanward by intensifying mixing due to tidal oscillation. The interaction between the tidal asymmetry and bathymetry determines the spatial structure of age of in the Changjiang Estuary. Under the influence of wind, the intensification of the mixing increases the transport rate and decreases the difference of transport timescale between the spring and neap tide. The wind can change the transport timescale by altering the freshwater distribution among channels in the multi-bifurcation estuary, besides affecting the stratification. Coriolis force leads to the fluctuation of transport pathway during flood and ebb period. It has an important implication to change direction of water parcel movement, temporal and spatial pattern of the transport timescale.The impact of the Deep waterway project on the estuarine hydrodynamic is explored from new perspective of the transport timescale. The mean sea level at the upstream of the estuary rises, but barotropic pressure decreases and flow distribution between channels changes due to friction increases after building of the structure. The rate of freshwater discharge through the North Channel increases while decreases through the South Channel. The transport time decreases in the North Passage, by 16% at the peak value of the water age. The transport time increases in the south Passage, up to 41%. The distribution of salinity and water age become more stratified in both the North Passage and the South Passage after the completion of the project. The mean salinity gradient increases and the gravitational circulation intensify. The bottom transport time increases while surface decreases, although the vertical-averaged mean water age decreases. This transformation is unfavorable to flush sediment and material accumulated on the bottom. The horizontal water exchange between the North Passage and the North Passage reduces significantly due to the Deep waterway project. The water transport out the South Passage has the tendency to flow out along the dike. Part of the water transported out from the North Passage returns to the South Passage. High age zone forms in the lower part of the Jiuduan Shoal.The mechanism determining landward salinity flux in the Changjiang Estuary is investigated using decomposing axis current and salinity. Analysis on the exchange flow, salinity structure, steady shear dispersion and tidal oscillatory salt flux was conducted using mechanism decomposition method. It is found that the tidal oscillatory salt flux is the dominated factor controlling up-estuary salt flux during the spring tide. The tidal oscillatory salt flux determines the magnitude and direction of the total salinity flux. It is comparable between the steady shear dispersion and the tidal oscillatory salt flux during neap tide. In general, the lateral shear dispersion exceeds the vertical shear in the steady shear dispersion, indicating lateral circulation contributes the along axis salt transport.