Evolutions of hydrodynamics and sediment transport pattern in the Qiantang Estuary (China) in response to multidecadal embankment constructions

Since the late 1960s, large-scale coastal embankments have been constructed along the Qiantang Estuary (QE, China) covering both the Qiantang River (QR) and the Hangzhou Bay (HZB) to reduce the tidal prism and thus stabilize the channel. While most previous studies have reported their relevant impacts on hydrodynamic and sediment transport in the HZB, a more comprehensive investigation covering the whole QR + HZB system remains lacking, so as a quantitative analysis of the tidal prism and the channel stability. Therefore, numerical case studies have been conducted to examine evolutions of hydrodynamics and sediment transport of the QR + HZB system in response to different coastal embankments (i.e., no embankment as in 1958; QR embankment as in 1984; QR embankment + part of HZB embankment as in 2004; both QR and HZB embankment as in 2016). It is shown that the flood tidal asymmetry in the QR and the tidal range in the HZB are significantly increased, implying an intensified tidal distortion and thus a strengthened river role after embankments. While this intensification does not change the direction (from HZB to QR) of the net sediment transport, all of the sediment exchange rates [e.g., the landward (during flood tides) and seaward (during ebb tides) sediment transport volumes, as well as the net sediment transport rate between them] exhibits obvious decreasing trends. In addition, strong deposition is observed in the Jianshan (JS) reach, which has led to further increase of the underwater giant sandbar. The constructions of embankments have facilitated the initial goal of reducing tidal prism in most part of the QR + HZB system, except some local parts of the HZB, where increased tidal prism can be observed as the effect of the enlarged tidal range overwhelms that of embankments. Moreover, the channel stability was greatly improved as the mainstream has been mostly converged to the reach centerline and the width-depth ratio of the channel has been considerably reduced. The present investigation of the QE regulation could be indicative for the management of other estuaries in the world.