Influence of tidal motion on discharge distribution in Yangtze Estuary

Flow division is a classic issue encountered in river dynamics. Bifurcations in subtidal estuarine environments are influenced by river discharge as well as tidal motion. Residual circulation usually occurs in estuaries owing to the effect of tidal dynamics. Flow divisions that occur at bifurcations are generally more complicated compared to those occurring in regions without tides. This study aims at quantifying tidal influence on subtidal discharge distribution in the Yangtze Estuary through application of the discharge asymmetry index. A 2-D numerical model was developed based on the bathymetry of the estuary, as measured in 2002. To decompose the effect of tidal motion, river discharge, and river-tide interactions, the model was subjected to three independent operating conditions-river discharge and tides, river discharge only, and tides only. In addition, the effect of Deepwater Navigation Channels on flow division has been qualitatively explained. Through use of the proposed model, the discharge asymmetry index at the South and North Channel bifurcations was reduced by 32.95% and 35.71%, respectively, during the dry and wet seasons. As observed, the general effect of tides is to make flow divisions more even. In the South Channel, river-tide interactions were strengthened by construction of the Deepwater Navigation Channel. Consequently, the subtidal water level was enhanced, and the dominant trend of flow allocation to the South channel was weakened.