A Morphodynamic Modeling Study on the Formation of the Large‐Scale Radial Sand Ridges in the Southern Yellow Sea

Abstract

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 M 2 tidal constituent, the Coriolis forcing, 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 driver in 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. Key Points:• The unique giant fan-shaped morphology of the radial sand ridges (RSRs) is numerically simulated • The formation of the RSRs requires delicate conditions governed by currents originating from the rotational, progressive tidal wave system • The mechanism underlying the asymmetric spatial pattern of the RSRs is revealed Supporting Information:• Supporting Information S1

Future Response of the Wadden Sea Tidal Basins to Relative Sea-Level rise—An Aggregated Modelling Approach

“…For tidal waters like the Wadden Sea, process-based models have been successful in simulating short-term changes and investigating physical processes and mechanisms for morphological changes [46][47][48][49]. Since the pioneering study of Hibma et al [50] many long-term morphodynamic simulations have been reported in the literature (see e.g., [17,[51][52][53][54][55][56][57][58]). However, most of the simulations are carried out for simplified geometry and initial bathymetry, and they mainly concern the morphology at the end of the simulation.…”

“…The single element ASMITA model yields (see [58]): The adjacent coastal areas, which can exchange sediment with the inlet system, are considered as an open boundary, 'the external world'. For the present analysis the single-element model for the tidal basin with the water volume below highwater V as state variable will be used.…”

“…The single element ASMITA model yields (see [58]): The power n should be the same as that in a power law relationship between sediment transport rate s and flow velocity u in a process-based model [72]. Its value is an indication of the non-linearity of the relationship.…”

On the morphology of radial sand ridges

“…Su et al (2015) also pointed out that the initial bathymetry of the RSRs influences the magnitude of the tidal flow and is crucial to the formation of channels. Tao et al (2019) carried out morphodynamic simulations of the RSRs and showed that rhythmic topography can be qualitatively reproduced starting from a smooth sloped bathymetry with M2 tide. However, the understanding of how the incision of channels occurs and why the final configuration of the RSRs is achieved is still lacking.…”

Dynamics of a Tidal Current System in a Marginal Sea: A Case Study of the Yellow Sea, China

“…The "radial tidal current" is considered to be independent from the local geomorphology, which has been demonstrated by Zhu and Chang (2001) through several numerical experiments (e.g., using a flat and a linear sloped topography replacing the "radial sand ridges, " respectively). Recently, Tao et al (2019) has demonstrated that a series of radial-pattern ridges can be shaped by the "radial tidal current" starting from a linear topography by using morphodynamic modeling. Besides, simulation of the Paleo-tidal current field reveals that this special tidal current pattern existed since 7,000 years ago and therefore is expected to play a crucial role in the formation and development of the "radial sand ridges" (Uehara et al, 2002;Uehara and Saito, 2003;Zhu and Chen, 2005).…”