The fate of tidal flats under reduced sediment supply and human activities in the bifurcated Yangtze Estuary

Abstract

Tidal flats provide essential ecosystem services but are increasingly threatened by reduced sediment supply and human activities, requiring close monitoring and understandings in estuaries. We focus on the four tidal flats with a total area of 1800 km²in the Yangtze Estuary and systematically evaluate their morphodynamic evolution based on consistent bathymetry data over 60 years (1958–2022). While fluvial sediment supply has declined since the mid-1980s, all four tidal flats in the estuary sustained accretion until 2010, demonstrating a lag of 20–30 years in estuarine morphological response to sediment decline. However, note that accretion primarily occurs on higher parts of the shoals, whereas erosion dominates in the subtidal zones. This is mainly attributed to the combined impact of saltmarsh expansions, reclamation, and channel scour and dredging. It suggests that part of the eroded sediment from channels deposits on adjacent shoals, leading to a regional sediment budget balance, particularly in the central channel-shoal complex with the navigation channel. Moreover, the initiative of removing Spartina from the shoals, a fast-spreading invasive species that benefits shoal accretion but not native species, might disrupt the ongoing accretion of high shoals and induce overwhelming erosion and sediment loss. One management strategy to counteract these impacts and restore tidal flats is to make beneficial use of the dredged and trapped sediment from the North Passage, an annual amount of approximately 50 million m³, to the adjacent shoals, though how to sustainably manage the sediments remains another concern.