Sediment dynamics in the mudbank of the Yangtze River Estuary under regime shift of source and sink

Abstract

The southeastern portion of the Yangtze River Estuary (or Yangtze Estuary) was considered to be the deposition center and the mudbank of the Yangtze River Delta. As the fluvial sediment supply began to decline in the 1980s and the reduction accelerated after the completion of the Three Gorge Dam in 2003, more fluvial sediment was trapped decreasing the suspended sediment concentration (SSC) environment in the river mouth area. Moreover, the accretion rate of the mudbank has slowed down in recent decades. In fact, the mudbank shrank and has faced a regime shift from sediment sink to source. A better understanding of the tidal-scale dynamics and spatial variability of the system is essential to explore the conversion of the sediment sources and sinks in the Yangtze Estuary affected by natural variations and human activities. Flow velocity, salinity, and suspended-sediment concentration during spring and neap tides were measured at three sites on the mudbank in July 1982 and July 2013. The variation in flow was not significant at all the sites measured in the study area from 1982 to 2013. However, the sediment dynamics changed remarkably over these three decades. The temporal distribution of the SSC increased in the bottom layer. The SSC was much larger during the early flood tide period in 2013. The tidal range increased by nearly 10% and the flood dominance increased in the study area from 1982 to 2013. The salinity dynamics underwent a transition from a stratified system in 1982 to a well-mixed system in 2013. The landward sediment budget increased remarkably from 1982 to 2013. The decreased fluvial sediment supply, increased flood dominance, well-mixed salinity, and increased tidal range were directly responsible for the larger landward sediment budget and more severe erosion in the mudbank in 2013. The current results reveal the flow and sediment dynamics during the conversion of the sediment sink to source. Furthermore, it was determined that the sediment-starved process in the estuarine environment, which occurred due to the reduction in the fluvial sediment, leads to an increased landward transport of sediment. The current study provides a clear understanding of the mechanisms governing the delta system transition in the mudbank of the Yangtze Estuary, which is useful for delta protection in the future.