Figure 14. Conceptual sketch depicting the major differences of hydrodynamic processes observed in tidal channels dissecting vegetated (i.e., salt marshes, left columns) and unvegetated (i.e., mudflats, right columns) intertidal plains. (a, b) Channel hydrodynamics during below-bankfull water stages, with particular reference to early-flood and late-ebb stages; (c, d) Channel hydrodynamics at the bankfull stage; (e, f) Channel hydrodynamics during overbank stages.
6 Conclusions
This study contributes to the understanding of hydrodynamic flow structures, and related morphodynamic evolution, in meandering channels wandering through unvegetated tidal flats. Hydroacoustic measurements were carried out, for several tidal cycles, at distinct locations along a mudflat meander bend found within the macrotidal Yangkou tidal flat (Jiangsu province, China).
The main conclusions of this research can be summarized as follows:
  1. Stage-velocity relationships in mudflat channels are different from those observed in channels wandering through vegetated intertidal plains (i.e., salt marsh and mangrove forests). Specifically, while in the latter case both ebb and flood velocities tend to be higher for above-bankfull water stages, in our study case we observed significantly larger velocities when tidal flows remained confined within the channel banks. This is likely because, in vegetated intertidal plains, both frictionally-dominated flow propagation and higher elevation of channel banks (relative to tidal excursions) ensure flow confinement and high in-channel velocities even for above-bankfull stages. In contrast, in unvegetated intertidal mudflats, similar flow resistance within and outside channels and lower elevation of channel banks produce widespread sheet flow at above-bankfull stages and limit in-channel velocities due to reduced flow confinement;
  2. Secondary currents appear to be mostly related to flood flows, and are generally stronger during overbank stages. In some cases, however, the orientation of secondary circulations is reversed compared to classic flow models in meander bends. Poorly-developed secondary circulations are observed at the bend apex. However, primary flow separation, coupled with localized flow measurements that did not include the entire channel cross-section, have likely limited our ability to detect secondary circulation cells during our field measurements.
  3. Field data collectively suggest limited control of curvature-induced helical flows on meander morphodynamics. This is most likely due to a consistent phase lag between maxima of primary (i.e., streamwise) and secondary (i.e., cross-sectional) velocities. Such a lag effectively limits the landward (seaward) transfer of secondary flows during the flood (ebb) phase, thus hampering the formation of coherent helical flow structures along the entire meander bends. These findings support the results of earlier studies that suggested that, in stark contrast with both river and salt-marsh meandering channels, meander morphodynamics in intertidal mudflats are poorly related to bankfull hydrodynamics, in general, and curvature-induced helical flows in particular.
  4. We suggest that other morphodynamic processes drive the evolution of intertidal mudflat meander bends. Late-ebb tidal flows likely exert strong control on meander morphodynamics due to sustained velocities and pronounced seepage flows, which determine significant sediment transport as well as both bank undercutting and collapses. These effects are also possibly amplified by the absence of vegetation both within and outside the channel, as well as by significant bioturbation of the channel banks, which reduces bank resistance to erosion and enhances seepage flow. In addition, storm waves and both episodic and seasonal increases in discharges due to heavy rainfalls (e.g., related to the monsoon season) and melting snows can compound the morphological effects of late-ebb flows, producing abrupt morphologic changes and pronounced channel migration.
Additional field and modeling efforts would be required to corroborate the inferences presented in this study and to investigate how different tidal ranges and channel-bank elevations (relative to characteristic tidal oscillations) affect mudflat meander hydrodynamics and the related morphodynamic evolution. Particularly, cross-sectional measurements of tidal flow fields are needed to directly assess the scarce development of curvature-induced helical flows, whereas repeated measurement of flow fields during normal conditions and heavy rainfall events, coupled with morphological monitoring of channel bank evolution, would help clarify the relative importance of astronomic and meteorological forcings on the morphodynamics of intertidal mudflat meanders.
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (U2240220, 41625021), the Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-05-E00027), the China Scholarship Council (CSC) scholarships (202106190084) and Jiangsu Special Program for Science and Technology Innovation (JSZRHYKJ2021006). We thank Zhenqiao Liu, Wei Feng, Jianxiong Sun, and Dongyun Wei for their help in field work. Special acknowledgments are given to Shibing Zhu for his help in grain size analysis.
Open Research
The data sets generated and/or analyzed during the current study are freely available athttps://doi.org/10.6084/m9.figshare.20161733.v2
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