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Morphodynamic equilibrium is a widely adopted yet elusive concept in the field of geomorphology of coasts, rivers and estuaries. Based on the Exner equation, an expression of mass conservation of sediment, we distinguish three types of equilibrium defined as static and dynamic, of which two different types exist. Other expressions such as statistical and quasi-equilibrium which do not strictly satisfy the Exner conditions are also acknowledged for their practical use. The choice of a temporal scale is imperative to analyse the type of equilibrium. We discuss the difference between morphodynamic equilibrium in the “real world” (nature) and the “virtual world” (model). Modelling studies rely on simplifications of the real world and lead to understanding of process interactions. A variety of factors affect the use of virtual-world predictions in the real world (e.g., variability in environmental drivers and variability in the setting) so that the concept of morphodynamic equilibrium should be mathematically unequivocal in the virtual world and interpreted over the appropriate spatial and temporal scale in the real world. We draw examples from estuarine settings which are subject to various governing factors which broadly include hydrodynamics, sedimentology and landscape setting. Following the traditional “tide-wave-river” ternary diagram, we summarize studies to date that explore the “virtual world”, discuss the type of equilibrium reached and how it relates to the real world. [ABSTRACT FROM AUTHOR]

A new methodology is developed to extract tidal network from hydrodynamic conditions, and use data derived from numerical modeling or field observations to test the hypothesis that tidal networks are characterized by scale-invariant properties. Different tidal network configurations have been obtained from long-term numerical simulations in an idealized basin. These simulations show the influence of hydrodynamic conditions (tidal range, TR) and sediment (grain size sediment, D50) on the final configuration of the network. One of the signatures of scale-invariant behavior is related to the presence of a power law relationship in the probability distribution of geometrical characteristics. For each model configuration and field site, the probability distribution of drainage area and the drainage volume has been calculated, and in both cases tidal networks show scale-invariant characteristics. After assessing the sensitivity of the results, an energy expenditure analysis shows that tidal basins evolve toward a state with less morphodynamic activity, with a lower energy expenditure compare with the initial state. [ABSTRACT FROM AUTHOR]

Abstract: Tidal network morphodynamics is an active field of research and advances achieved over the last decade, particularly with respect to laboratory experiments and numerical modelling, have lead to fundamental insight about their functioning. We address how these advances have specifically contributed to the understanding of tidal network functioning, including the interaction between physical and biological processes. We discuss how the prediction of the long-term evolution of tidal networks is still limited and we focus on how it is hampered by three specific challenges. We first discuss the approach to long-term predictions, then focus on the coupling between physical and biological processes, and finally attempt to introduce the role of anthropic drivers in the evolution of these systems. [Copyright &y& Elsevier]

Abstract: Tidal embayments are characterized by a wide variety of landscape features, often including either complex tidal channel networks or extensive flood-tidal deltas. The origin of these features and the influence of hydrodynamic drivers and initial geological setting on their long-term characteristics are essentially unexplored. A model was applied to simulate the long-term morphological evolution of tidal embayments, with the purpose of providing insight into the environmental conditions that lead to the differences in tidal embayment morphology. Numerical simulations indicated that the interaction between hydrodynamics, sediment transport, and the evolving topography gives rise to the formation of channel networks. The tidal range and the depth of the initially unchannelized tidal basin controlled the way in which the morphology evolved and determined the timescale over which channels and intertidal areas developed. Channel network formation occurred more rapidly when the tidal range increased and/or when the initial basin depth decreased. Tidal basins with a large initial depth showed the development of a flood-tidal delta and for these deep basins channel incision could remain absent over long timescales. Both tidal range and initial bathymetry affected final basin hypsometry and channel network characteristics, including the channel density and the fraction of the basin occupied by the channels. All the simulated morphologies, with different combinations of the tidal range and depth of the basin, evolved towards a state of less morphodynamic activity for which the relative intertidal area was proportional to the ratio of tidal amplitude to basin depth. [Copyright &y& Elsevier]

An introduction to a special issue on what is termed the morphodynamics of estuaries and coastlines is presented, including topics such as in situ measurement devices, empirical equilibrium models, and the behavior of coastal systems.

Bank retreat in coastal tidal flats plays a primary role on the planimetric shape of tidal creeks and is commonly driven by both flow-induced bank erosion and gravity-induced bank collapse. However, existing modelling studies largely focus on bank erosion and overlook bank collapse. We build a bank retreat model coupling hydrodynamics, bank erosion and bank collapse. To simulate the process of bank collapse, a stress-deformation model is utilized to calculate the stress variation of bank soil after bank erosion, and the Mohr-Coulomb failure criterion is then applied to evaluate the stability of the tidal creek bank. Results show that the bank failure process can be categorized into three stages, i.e., shear failure at the bank toe (stage I), tensile failure on the bank top (stage II), and sectional cracking from the bank top to the toe (stage III). With only bank erosion, the planimetric shapes of tidal creeks are funneled due to the gradually seaward increasing discharge. In contrast to bank erosion, bank collapse is discontinuous, and the contribution of bank collapse to bank retreat can reach 85%, highlighting that the expansion of tidal creeks can be dominated by bank collapse process. The planimetric shapes of tidal creeks are funneled with a much faster expansion rate when bank collapse is considered. Overall, this study makes a further step toward more physical and realistic simulation of bank retreat in estuarine and coastal settings and the developed bank collapse module can be readily included in other morphodynamic models. [ABSTRACT FROM AUTHOR]

We develop a consolidation module and merge it into a morphodynamic model to assess the role of consolidation on estuarine morphodynamics. We test the model using two settings: point models without hydrodynamic forcing to validate against two benchmark experimental datasets; and a profile model to simulate a mudflat restoration. The modelled self-weight consolidation influences the simulations by gradually reducing the bed level and decreasing the bed erodibility (i.e., increasing the critical bed shear stress). Both effects modify sediment transport processes on mudflats, leading to long-term morphodynamic effects. Depending on the initial bathymetry, the hydrodynamic forcing and the soil properties, the simulated morphological change of the restored mudflat may differ considerably with and without considering consolidation. The consolidation model developed can be utilised to assess the medium to long term effects related to estuarine development (e.g., wetland restoration) and aims to be a publicly available tool. [ABSTRACT FROM AUTHOR]

Radial sand ridges (RSRs) spread in a fan-shaped pattern over the seabed of the southern Yellow Sea along the Jiangsu Coast (China) with pronounced differences between the northern and southern channel-shoals (indicated as “NCS” and “SCS”, respectively). A depth-averaged nested numerical model is employed to analyze the tidal hydrodynamics and sediment dynamics of the RSRs. Model results show considerable regional differences in terms of tidal current vector ellipses: “eight-shaped” ellipses are typical in the NCS where rotational tidal waves dominate, while the SCS are characterized by “egg-shaped” ellipses because of the dominance of progressive tidal waves. Sand ridges tend to be elongated and straight in the NCS while short and dincontinuous in the SCS. Numerical Lagrange particles released near the ridges in the NCS tend to travel across the crest of the ridges rather than move around them as observed in the SCS. Consistently, the patterns of sediment convergence and divergence indicate that suspended sediments converge over the whole Xiaoyinsha Ridge located in the NCS, while converge to the two sides of the Hetunsha Ridge located in the SCS. Model results also highlight the importance of initial sediment compositions in determining patterns of the net sediment fluxes. The suspended sediment concentration (SSC) time series during one tidal cycle may show single or double peaks depending on the grain size and the related lag effect. Bed load transport is much smaller than suspended load. So sediment compositions should be carefully considered when simulating the SSC field, and the hydrodynamic of the morphological evolution of the RSRs at the regional scale. Overall, this study suggests that the NCS and SCS may be treated as two relatively independent geomorphological systems characterized by different tidal flows and sediment compositions. [ABSTRACT FROM AUTHOR]