Morphodynamic modeling of a large inside sandbar and its dextral morphology in a convergent estuary: Qiantang Estuary, China

Abstract

We investigate the evolution of a large‐scale sand body, a unique type of sandbars in a convergent estuary. Specifically, we analyze and simulate the sand deposition system (defined as an inside bar) in the Qiantang Estuary (QE) in China. The deposit is 130 km long and up to 10 m thick and is characterized by a dextral morphology in the lower QE. Numerical simulation is carried out using an idealized horizontal 2‐D morphodynamic model mimicking the present QE settings. Our results indicate that the morphological evolution is controlled by the combination of river discharge and tides. The seasonal and interannual cycles of river discharges play a major role on the inside bar evolution. The bar is eroding during high river discharge periods, but accretion prevails during low river discharge periods. Meanwhile, the highest part of the sand body can move downstream or upstream by several kilometers, modifying the seasonal sediment exchange patterns. We also show that the Coriolis force plays an important role on the dextral morphology patterns in wide, convergent estuaries. It induces a significant lateral water level difference and a large‐scale gyre of residual sediment transport. Subsequently, the seaward tail of the inside bar shifts southward to help create a condition for the development of tidal flats in the lower reach of the estuary. The lateral bed level differences induced by Coriolis force are up to several meters. Coriolis effects also modify the behavior of flood and ebb tidal channels.

Oblique sand ridges in confined tidal channels due to Coriolis and frictional torques

“…However, tidal bars occur in natural systems (e.g., Western Scheldt) where the Coriolis force is a first-order term in the momentum balance. The importance of Coriolis on the hydro-morphodynamics in tidal channels is supported by several other studies, e.g., Valle-Levinson (2008), Winant (2008), Xie et al (2017), and Olabarrieta et al (2018). Furthermore, 2D morphological simulations similar (but now with and without the Coriolis effect) with those performed by Hibma et al (2004) show clear differences between the initial formation of bottom patterns with and without the Coriolis effect taken into account (see Fig.…”

Morphodynamics of the Qiantang Estuary, China: Controls of river flood events and tidal bores

“…4b). The serious bed degradation can be explained by the fact that the discharge during a river flood event is much larger than the normal discharge which is around 1000 m 3 /s (Chen et al, 2006;Xie et al, 2017a). In previous studies, the cross-sectional area and the cross-sectionally averaged depth, have been found as a power function of the river discharge (Leopold and Maddock, 1953;Smith, 1974;Han et al, 2009).…”

“…Based on sedimentological surveys, it has been revealed that the sediment of this large sedimentary system is from the adjacent Changjiang River (Chien et al, 1964;Chen et al, 1990;Zhang et al, 2015). Recent modeling by Xie et al (2017a) found that the deposits would grow unlimitedly under normal discharge condition due to the continuous sediment import by flood dominance, and the growth can be constrained by high discharge. Xie et al (2017b) also analyzed the morphological response of the Qiantang Estuary-Hangzhou Bay system to the reduction of sediment load from the adjacent Changjiang Estuary and the large-scale embankment within the estuary.…”

“…(c) Bathymetry of the estuary measured in April, 2015.Fig. 1a and 1bwere modified fromXie et al, 2017a.…”

Resilience of River Deltas in the Anthropocene

“…The resolution is relatively weak in the shallow coastal zone below 0 m, yet this is precisely where significant bathymetric change should be expected. These topobathymetric DEMs form an essential boundary condition for many hydrodynamic models, including hurricane storm surge (Hope et al, ; Xing et al, ), and flood and tidal modeling (Gaweesh & Meselhe, ; Vinh et al, ; Xie et al, ). While such models are validated to varying degrees, it is likely that modeling can be improved, particularly in very shallow regions, with improved DEMs.…”