Transport Mechanism of Suspended Sediments and Migration Trends of Sediments in the Central Hangzhou Bay

Abstract

Based on the 2013 field survey data of hydrology, suspended sediments and bottom sediments in the Central Hangzhou Bay, this paper explores the dynamic mechanism of suspended sediments in Hangzhou Bay by employing material flux decomposition. Meanwhile, the migration trends of bed sediments are also investigated by analyzing grain size trends. The results show that during an ebb or flood tide, the hydrograph of suspended sediment concentration of Hangzhou Bay is dominated by an M shape (bimodal), which is attributed primarily to the generation of a soft mud layer and a separate fluid mud layer. Laterally, the distribution of suspended sediment concentration is high in the south and low in the north. From a macroscopic perspective, the net sediment transport in the study area displays a “north-landward and south-seaward” trend, presenting a “C”-shaped transport mode. That is, the sediments are transported from the bay mouth to the bay head on the north side and from the bay head to the bay mouth on the south side. The sediment transports by advection and tidal pumping are predominant, while the sediment transport by vertical circulation makes little contribution to the total sediment transport. Moreover, the sediment transport in the center of the reach area is dominated by advection, whereas that near both sides of the banks is controlled by tidal pumping. The asymmetry of the tides, i.e., flood-dominance in the north and ebb-dominance in the south, is the primary cause of the dynamic mechanism for the overall “C”-shaped transport mode in Hangzhou Bay. Additionally, coupled with the narrow-head wide-mouth geomorphology, Hangzhou Bay remains evolving by south shore silting and north shore scouring.