Hydrodynamic, Depositional And Morphological Evolution Of Tidal Flat In The Yangtze Estuary For The Last30Years

For several thousands of years, huge amounts of sediment has been carried by the runoff of Yangtze River and deposited in the estuary, which constructed the large area of the Yangtze Delta. The quite big deposition rate of sediments in the Yangtze Estuary remained a tendency until1980s. However, the erosion of the tidal flat was shown to intensify in response to the decrease in the sediment supply from the catchment and the large area of land reclamation in the estuary. The present dissertation aims to examine the hydrodynamic, depositional and morphological evolution of tidal flat in the Yangtze Estuary for the Last30Years.The hydrodynamic parameters as tidal current velocity, suspended sediment concentration (SSC), bed sediment component and geometry during spring and neap tide in dry and flood season on the tidal flats along the North Channel and South Nanhui including the intertidal and sub tidal profiles are measure and assimilated historical data of them are collected. And also,11nautical charts of two typical tidal flats with the channels from1980to2012are digitized. The analytical results show as follows:1. A significant decrease in the tidal current velocity and SSC in the North Channel during flood season appear since2004, while an obvious increase tended to occur in the tidal current velocity and SSC during2007and2012. Meanwhile, an increase in the flood tidal current velocity with SSC and decrease in the ebb tidal current velocity with SSC exist in flood season, while a significant decline of flood and ebb tidal current velocity in dry season show on the Nanhui tidal flat.2. An coarsening tendency of sediments are obviously shown on the sediment cores on the tidal flats near the engineering as the bridges of Changjiang across the North Channel and Qingcaosha fresh water Reservior, Deep Waterway in the North Passage in the Yangtze estuary, and the bridge connected the mainland to the Yangshan Harbor. It implies the significant local hydrodynamic and depositional change by the strong local human interventions.3. A sharp narrowing width of intertidal flats of the Yangtze Estuary and a steepening slope of the flats are present, which shows concave profile being as the erosion feature along the coastline with artificial coastline for the last30years. The erosion tendency of tidal flat profile may attributed to the continuing decline of sediment discharge from the Yangtze river and large area of land reclamation in the estuary.