Sedimentation in the Outer Hangzhou Bay, China: The Influence of Changjiang Sediment Load

Most estuaries in the world are facing the risk of subsidence due to insufficient sediment supply and relative sea-level rise. In the case of the world's fourth largest river, the Changjiang (aka Yangtze River), its river sediment load has been decreasing sharply, especially since the construction of the Three Gorges Dam. Due to the complexity in the estuarine dynamics, human activities, and the sediment supply, discerning whether the Changjiang Estuary is facing erosion or subsidence is a controversial issue. Immediately south of the mouth of the Changjiang is Hangzhou Bay. On the centennial time scale, accretion on the southern flank of the Changjiang deltaic plain has caused the seaward extension of the north bank of the Qiantang Estuary, leading to the formation of the macrotidal, funnel-shaped Hangzhou Bay. In this study, we used multiyear geographic information system (GIS) rectified bathymetric charts of Hangzhou Bay for bathymetry and sediment-capacity change analysis. Our major findings show that the outer Hangzhou Bay has gone through the following stages: relatively steady sedimentation (1931-1997), erosional-depositional transition (1997 2002), and rapid erosion (2002-2008) at the rate of 58.5 x 10(6) m(3)/y. Our findings also corroborate measurements of suspended sediment discharge at the Datong station in the lower reaches of the Changjiang. Our findings confirm the dependence of the sedimentation in the outer Hangzhou Bay on the sediment supplied by the Changjiang. The severity of the erosion of the outer Hangzhou Bay and the sediment load decrease of the Changjiang might serve as a warning for the impending erosion of the Changjiang Estuary.