A numerical model study of the transport timescale and change of estuarine circulation due to waterway constructions in the Changjiang Estuary, China

A three-dimensional hydrodynamic model was developed for the Changjiang Estuary and adjacent coastal sea to study the transport timescale and change of estuarine circulation due to human activates. The model was calibrated with measured tidal current and salinity forced by observed freshwater discharge and tides. The tracer age was introduced to measure the transport timescale and evaluate the influence of the man-made construction on estuarine circulation through a series of numerical experiments under various hydrodynamic conditions. The results show that it takes about 23 and 35. days, respectively, for dissolved substances to be transported from the Xuliujing to the mouth of the Estuary (122°30' E) under high and low discharge conditions. The transport time increases significantly in the upper portion of the North Passage and the South Passage due to the diversion of the freshwater discharge from the upstream North Channel and the increase of the friction downstream. However, the rate of increasing transport time along the estuary decreases farther towards the mouth of the Estuary, presumably resulting from the enhancement of gravitational circulation. Two major physical mechanisms that contribute to the transport timescale, tide and river discharge are analyzed and quantified. The results indicate that freshwater discharge is one of the dominant factors controlling the transport timescale in the Changjiang Estuary. The man-made construction has a significant impact on both horizontal estuarine circulation and gravitational circulation, which ultimately influences the estuarine transport processes. The transport time increased about 50% by man-made constructions, especially near the turbidity maximum. The simulation results provide useful information for understanding the change of transport process and circulation caused by the man-made construction in the Changjiang Estuary.