Figure 13. Bank erosion along the studied channel. (a) Locations of the photographs. (b,c,d,e,f) Close-up views of bank slumps (white dotted lines) and gullies (white dashed lines) along the studied channel. All photos were taken during ebb tides.
If, on the one hand, bank collapses driven by seepage flow are also commonly documented in vegetated macrotidal settings (Cosma et al., 2022; Zhao et al., 2022) and are therefore more closely linked to sustained tidal oscillations, on the other hand, the abundant erosional gullies (Figure 13) observed at channel banks are most likely specific of unvegetated settings. The formation of such gullies, which can significantly contribute to bank erosion processes, is promoted by strong erosion at the ebb-bankfull transition and favored by the absence of vegetation cover (Guimond & Tamborski, 2021). Bank collapses and gullies can also be counterintuitively related to the presence of cohesive extracellular polymeric substances (EPS) generated by microorganisms abundant on intertidal flats. However, although EPS are widely regarded as bed “stabilizers” (Flemming & Wuertz, 2019), recent flume experiments show that they may enhance sediment mobility under wave actions, inducing liquefaction of otherwise stable bank sediment (Chen et al., 2021), with clear implications for the dynamics of meandering tidal channels.
Overall, our results support the idea that meander evolution in intertidal mudflats might not be necessarily correlated with classic curvature-induced helical flows at near-bankfull stages, and that other ecomorphodynamic factors, most likely related to tidal hydrodynamics at late-ebb stages, can be more relevant for meander morphodynamics. A synthesis of the main hydrodynamic characteristics of meandering channels developed in vegetated and unvegetated intertidal plains, and the differences thereof, is reported in Table 2. In addition, a conceptual summary sketch illustrating the major hydrodynamic and morphodynamic differences between tidal meandering channels in vegetated and unvegetated contexts is shown in Figure 14. Further analyses will be needed to corroborate the inferences presented in this study, as well as to investigate the role played by different tidal amplitudes on the processes we described here, especially in terms of distinct hydrodynamic behavior between above and below-bankfull stages. Nonetheless, large tidal ranges (relative to characteristic wind-wave heights) are needed for the development of intertidal mudflats (e.g., Friedrichs, 2011; Klein, 1985; Morales, 2022), so we argue that the processes observed in the present study are likely to be common also in intertidal mudflat channels different from the study case we analyzed here.
Table 2. Comparison of the major hydrodynamic characteristics of meandering tidal channels wandering through vegetated (e.g., salt marshes) and unvegetated (e.g., mudflat) intertidal plains