Exploration of Decadal Tidal Evolution in Response to Morphological and Sedimentary Changes in the Yangtze Estuary

Estuarine tidal dynamics are influenced by changes in morphology and friction. In this work, we quantified changes in tidal damping in the Yangtze Estuary and explored the impact of morphology and friction using a numerical model. In-depth analyses of tidal data reveal a strong reduction in tidal damping from 1990 to 2010, followed by a slightly enhanced damping from 2010 to 2020 in the South Branch. The reduced tidal damping in the South Branch from 1990 to 2010 is controlled by sediment decline which induces an increase in water depth (erosion), thereby strongly amplifying tides. However, the effective bottom roughness (Manning coefficient) is increased by 60%, which is probably related to the (Formula presented.) 80% decrease in the suspended sediment concentration (SSC). Such an effect may enhance tidal damping, which counteracts the contribution of water depth increase on amplifying tides by (Formula presented.) 75%. From 2010 to 2020, the tides in the South Branch became more damped, suggesting a dominance of the decrease in SSC over the morphological changes. In the mouth zone, tidal dissipation is enhanced from 1997 to 2010, which is mainly caused by an overall increase in effective bottom roughness. Local structures dominate the increase in effective bottom roughness; however, fluid mud formation may contribute to a decrease after 2010. Overall, we argue that estuarine morphological and sedimentary changes in response to riverine sediment decline and local engineering works control the tidal evolution in the Yangtze Estuary, which is important for evaluation of human activities and estuarine management.