Modeling estuarine morphodynamics under combined river and tidal forcing

Estuarine morphodynamics are of broad importance to estuaries’ functions related to navigation, human settlement and ecosystems. Inspired by the Yangtze River estuary (YRE), this study aims to explore the impact of river discharge, tides and their interaction on long-term estuarine morphodynamics. Use is made of 1D and 2D process-based models. In first instance we focus on purely hydrodynamic characteristics in a 560 km long basin. We analyze the non-stationary river tides in the YRE by harmonic analysis and continuous wavelet transformation which reveals a wide range of subtidal variations and non-linear modulation by varying river discharges. An intermediate river discharge could be defined at which the amplitudes of the internally generated overtides and compound tides reach maxima. Based on these hydrodynamic insights we conduct long-term (millennia time scale) morphodynamic simulations in schematized long basins. Vanishing spatial gradients in tidal residual sediment transport indicate an approach towards morphodynamic equilibrium. Morphodynamic equilibrium is also reached in case of a seasonally varying river discharge, which is reflected by a balance between erosion and accretion during low and high river discharge periods, respectively. River flow supplies sediment, accelerates ebb currents, and alters tidal asymmetries. Each of these processes has its own effects on tidal residual transport and morphodynamics. For example, the interaction between a mean flow (i.e., Stokes return flow or river flow) and tidal currents induces significant tidal residual sediment transport which explains net ebb transport dominance in the presence of a flood tidal asymmetry. A larger river discharge does not necessarily lead to deeper equilibrium bed profiles. An intermediate river discharge is found which induces largest residual sediment transport gradients along the estuary leading to deepest equilibrium bed profile. Quantification of this medium river discharge is case dependent because of the non-linearities involved. The 2D model approach applied in a large scale fluvio-deltaic system reveal river, estuarine and deltaic types of morphodynamic features, such as alternating sand bars, meandering channels inside the estuary and more elongated sand bars and distributary channels in the mouth zone and delta. The cross-sectionally averaged depth of the 2D model responds in a similar way to increased river discharge as a 1D model. Furthermore, a high river discharge induces ebb transport dominance, restricts development of flood channels and prolongs meander wavelength. In summary, this research unveils the fundamental effects of tidal asymmetries, river discharge, and river-tide interaction in governing residual sediment transport and associated long-term estuarine morphodynamics under combined river and tidal forcing.Hydraulic EngineeringCivil Engineering and Geoscience