The radial sand ridges (denoted as “RSRs” hereafter) in the Southern Yellow Sea, China, are morphologically striking because of the remarkable size and radial planar orientation, standing out as a unique coastal geomorphology among the worldwide sand ridge systems. The formation of this giant fan‐shaped geomorphic feature requires delicate conditions and awaits in‐depth investigation. Using an idealized morphodynamic model, this study unravels the governing factors for the formation of the unique large‐scale RSRs, in comparison with other types of sand ridge systems over the world. The effects of the M2 tidal constituent, the Coriolisforcing, the bed resistance, and the initial water depths on the morphodynamic behavior of the RSRs are explored. Numerical results indicate that the tidal regime, characterized by rotational and progressive current action associated with the tidal bulge, is dependent on the eastern coastline of China as well as latitudinal effects. Through the comparison between the simulated and the measured morphology, this tidal regime is demonstrated to be the key driverin forming and maintaining the present‐day RSRs. The runs with different parameters further suggest that the asymmetric pattern of the RSRs, which shows larger northern sand ridges than the southern ones, results from both the asymmetric distribution of current activity caused by the tidal bulge and unequal sediment supply. Overall, this study highlights the delicate condition, predominantly represented by the particular currents set up by the tidal wave system and the sediment supply, required to shape the striking large‐scale RSRs in the Southern Yellow Sea.@en

This paper presents a new model, using existing consolidation theory, suitable for long-term morphodynamic simulations; we refer to the dynamic equilibrium consolidation (DECON) model. This model is applicable for muddy systems at small suspended particulate matter (SPM) concentrations, where the sedimentation rates are smaller than the consolidation rates and small fractions of sand can be accounted for. Thus, the model assumes quasi-equilibrium of the consolidating bed. It is derived from the full consolidation (Gibson) equation and is implemented in a mixed Lagrangian-Eulerian bed model guaranteeing stable and non-negative solutions, while numeric diffusion remains small. The erosion and deposition of sand and mud is accounted for, whereas internal mixing (e.g., bioturbation) is modeled through diffusion. The parameter settings for the new consolidation model (the hydraulic conductivity, consolidation coefficient, and strength) can be obtained from consolidation experiments in the laboratory. The model reproduces one-dimensional consolidation experiments and the qualitative behavior of erosion and deposition in a tidal flume. The DECON model was also applied to more natural conditions, simulating fine sediment dynamics on a schematized mud flat and in a schematized tidal basin under tide and wave forcing. The computational results of the mudflat simulations compared well with the simulations with the full Gibson equation. For the tidal basin simulations, DECON predicted the expected landward tidal transport of fine sediment during tide-dominated conditions, while the tidal basin withstood erosion during the more energetic wave-dominated periods. Computational times for the morphodynamic simulations of the tidal basin example without waves increased by a factor of 5 when consolidation was included. For the simulations with waves, this increase in computational times was only a factor of 2, as simulations with waves are always expensive. Applying a complete consolidation model would be prohibitive. The DECON model therefore serves as a useful tool to simulate fine-sediment dynamics in complex wave- and tide-dominated conditions, as well as the effects of seasonal variations.

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Simple stability relationships are practically useful to provide a rapid assessment of coastal and estuarine landforms in response to human interventions and long‐term climate change. In this contribution, we review a variety of simple stability relationships which are based on the analysis of tidal asymmetry (shortened to “TA”). Most of the existing TA‐based stability relationships are derived using the one‐dimensional tidal flow equations assuming a certain regular shape of the tidal channel cross‐sections. To facilitate analytical solutions, specific assumptions inevitably need to be made e.g. by linearising the friction term and dropping some negligible terms in the tidal flow equations. We find that three major types of TA‐based stability relationships have been proposed between three non‐dimensional channel geometric ratios (represented by the ratio of channel widths, ratio of wet surface areas and ratio of storage volumes) and the tide‐related parameter a/h (i.e. the ratio between tidal amplitude and mean water depth). Based on established geometric relations, we use these non‐dimensional ratios to re‐state the existing relationships so that they are directly comparable. Available datasets are further extended to examine theutility of these TA‐based relationships. Although a certain agreement is shown for these relationships, we also observe a large scatter of data points which are collected in different types of landscape, hydrodynamic and sedimentologic settings over the world. We discuss in detail the potential reasons for this large scatter and subsequently elaborate on the limited applicability of the various TA‐based stability relationships for practical use. We highlight the need to delve further into what constitutes equilibrium and what is needed to develop more robust measures to determine the morphological state of these systems.@en

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 todate that explore the "virtual world", discuss the type of equilibrium reached and how it relates to the real world.@en

This paper presents a fast consolidation model suitable for long-term morphodynamic simulations. This model is applicable for muddy systems where sedimentation rates are smaller than consolidation rates, assuming quasi-equilibrium of the consolidating bed. It compares to the consolidation model developed by Sanford (2008). However, in that model, a heuristic, exponential density profile was used. Instead, the current model is derived from the full consolidation (Gibson) equation. The model’s material parameters (hydraulic conductivity, consolidation coefficient and strength) can therefore be derived from soil mechanical experiments in the laboratory. @en