On the morphology of radial sand ridges

Abstract

We use a hydrodynamic model applied to an idealized fan‐shaped basin to explore the morphology and dynamics of radial sand ridges in a convergent coastal system. A positive morphological feedback between channel incision and flow redistribution is responsible for the formation of the channel‐ridge pattern. The selection mechanism of bottom wavelength is associated with flow concentration in the deeper part of the channels. Our results are compared to sediment and hydraulic dynamics in the radial sand ridges (RSRs) in China. In a convergent, sloping basin the tangentially averaged tidal velocity peaks at 47 km from the apex. This distance is similar to the arc distance, 62 km, where the RSRs are most incised. An offshore shift in tidal phase results in stronger flows near the north coastline, explaining the presence of asymmetric channel patterns. A numerical stability analysis indicates that small radial oscillations with a wavelength of 10° to 15° maximize the velocity in the troughs. This oscillation wavelength also emerges in the RSRs, which display a peak in spectral energy at a radial wavelength between 25° to 37.5°. High‐resolution numerical simulations in the RSRs confirm that flow concentration occurs in the deeper part of the channels, keeping them flushed. We therefore conclude that the RSRs display morphometric characteristics similar to other tidal incisions, like tidal inlets and intertidal channels. This result further supports the dominant role of tidal prism and related peak velocities in incising coastal landscapes. © 2020 John Wiley & Sons, Ltd.

On the Role of Anthropogenic Activity and Sea‐Level‐Rise in Tidal Distortion on the Open Coast of the Yellow Sea Shelf

“…Prediction from numerical models (e.g., K. Chen et al., 2019; C. X. Li et al., 2001) have suggested that the morphology of the Jiangsu coastal area is entering a period of quasistability. The general profile is now settled except for some local variations, which has been suggested to be periodic (Zhang et al., 2020). Zhang deemed that in a warm epoch, the RSR would experience erosional formations, which is consistent with the evolution of the TDA under the 1‐m SLR scenario shown in this study.…”

“…Therefore, a minimum of three σ ‐layers in the vertical direction were set to enable the functionality available with the 3D mode of FVCOM. To simplify the factors to the uncertainties in estimating the evolution of TDA due to bathymetric and topographic influences, meteorological forcing and oceanic currents were not included in the model, following previous analyses focusing on the tide‐morphologic interactions (Xing et al., 2012; Zhang et al., 2020; Zhu et al., 2016). Because there is no large river flow or estuarine system along the Jiangsu coast, no river flow was considered in the model setup.…”

“…This is due to the limitation on the detailed bathymetry of the nearshore region for the Jiangsu coastal domain. The phase profiles and amplitudes of the overtides or compound tides are quite sensitive to the nearshore bathymetry (Zhang et al., 2020; Zhu et al., 2016). Whereas, because the sign and magnitude of TDA relies mainly on the phase lag or amplitude ratio between the tidal constituents of the tidal interactions, the biases of a given harmonic constituent can be offset by another.…”

“…At the apex of the RSR, the finest grid horizontal resolution is approximately 100 m. The water depth varies from −9 (land cell) to 77 m on the offshore boundary of the Jiangsu coastal domain. The bathymetric data are a collection from multiple sources, including nautical chart data, official investigations conducted by the government of Jiangsu Province and a few updates with field measurements until 2014 (Feng et al., 2019; Zhang et al., 2020). The model was driven by the predicted tidal level obtained from the NAO.99b tidal prediction system (Matsumoto et al., 2000) from the open boundary of the large‐scale domain, with 18.6‐year nodal variation included.…”

Geomorphic Evolution of Radial Sand Ridges in the South Yellow Sea Observed from Satellites

“…Tao et al [17] constructed an ideal morphological dynamics model of RSRs and predicted the geomorphological evolution trend over a long timescale. Zhang et al [18] used a hydrodynamic model applied to an idealized fan-shaped basin to explore the morphology and dynamics of radial sand ridges in a convergent coastal system. Due to the limitations of grid resolution and measured seabed topography data, at present, the numerical model method mostly involves an ideal model and mechanism analysis [19].…”