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