5.3 Implications for meander morphodynamics
Since the generation and propagation of helical flow are hampered,
questions arise regarding what are the chief morphodynamic processes
driving meander evolution in unvegetated intertidal mudflats. Previous
studies suggested that mudflat meanders can form and develop without
significant secondary circulations. For example, the evolution of small
mudflat meandering channels (about 1 m wide) in the Westerschelde
estuary (Netherlands) was found to be primarily driven by late-ebb
flows, which determined the erosion of channel bed due to
backward-migrating steps generated by hydraulic jumps, which in turn
promoted channel bank erosion due to bank undercutting and pronounced
flow separation in sharp bends (Kleinhans et al., 2009). In our studied
channel, sustained velocities at low water stages (Figures
5,6,8,9,10,11), together with direct visual inspections of sustained
flow velocities near the end of ebb tides (see Figure 4), support the
idea proposed by Kleinhans et al. (2009) that the morphodynamics of
intertidal mudflat meanders is strongly controlled by late-ebb flows
rather than by classic bar‐hugging helical flow produced by
curvature-induced secondary flows at high-water stages. Reduced control
of helical flows on channel morphodynamics is also testified by the
symmetric, V-shaped form of the studied channel cross-sections (Figure
4b), which attests to the scarce development of secondary (i.e.,
cross-sectional) flows and contrasts with the asymmetrical U-shaped
cross-sections displayed by meandering channels in vegetated tidal
marshes (Finotello, Ghinassi, et al., 2020; Zhao et al., 2022).
In contrast to late-ebb flows, we speculate that tidal flows at
early-flood stages are not likely to have significant effects in terms
of bank undercutting and sediment transport because velocities increase
more slowly than during late ebb, and rates of water depth change
through time (\(\dot{Y}\)), though sustained, do not produce significant
variations in DAVs (Figure 6). Our analyses indeed confirm
that tidal flows tend to be ebb-dominated at low water depths (Figure
5e,f), and also highlight that at \(Y\)<\(Y_{B}\) ebb
velocities attain values close to the maximum for much longer periods
than during the flood (Figure 6), thus likely enhancing the
morphodynamic control of late-ebb flows on channel evolution. Moreover,
late-ebb flows are likely to occur even for tidal oscillations lower
than those we monitored here, whereas pronounced overbank flows and
related secondary circulations require significant tidal oscillations to
be formed. Because intense late-ebb flows act at every tidal cycle and
operate for extended periods, the total morphodynamic work they produce
is in all likelihood much more significant than that produced during
other tidal phases, further supporting the hypothesis that late-ebb
tidal stages are the most morphodynamically relevant for mudflat meander
evolution.
The above-described morphodynamic control of late-ebb stages is likely
to be even more relevant compared to vegetated tidal landscapes due to
the absence of vegetation not only on intertidal plains but also within
tidal channels. In fact, previous studies focusing on salt-marsh
channels demonstrated how in-channel aquatic vegetation can enhance
bottom roughness and flow turbulence (e.g., Finotello, Ghinassi, et al.,
2020; Folkard, 2005), further limiting tidal flow velocities at low
stages, especially in relatively small channels with widths comparable
to the characteristic size of vegetation patches. The presence of
aquatic vegetation would clearly prevent significant morphodynamic work
to be performed by late-ebb tidal stages, which is likely not the case
in the unvegetated mudflat channel we investigated here.
In addition to the above, meander morphodynamics in unvegetated
intertidal flats can also be driven by episodic and seasonal increases
in discharges due to heavy rainfalls and melting snows (Choi et al.,
2013; Choi & Jo, 2015). Choi (2011, 2014) observed that these episodic
events are likely to cause abrupt morphologic changes, pronounced point
bar migration, frequent meander-bend cutoff, as well as channel
abandonments. Particularly, significant rainfall-induced runoff during
low tides would mimick late-ebb flows, thus further increasing the
morphodynamic relevance of seaward-directed, shallow, in-channel flows.
New field measurements will however be required to support this
hypothesis in the Yangkou tidal flat, since the data presented in this
study were collected in October, which is outside the monsoon season.
Storm waves could also induce bank collapses in unvegetated tidal
channels (Choi, 2011; Choi & Jo, 2015; Ghinassi et al., 2019), thus
critically affecting meander morpho-sedimentary evolution. In spite of
the absence of vegetation that can help stabilize banks and prevent
erosion, no collapsed slump blocks were observed within our study
channel (Gabet, 1998; Hackney et al., 2015), although such blocks could
be easily disgregated and removed, once formed, by sustained in-channel
velocities combined with the absence of additional cohesion given by
vegetation roots.
Bank collapses can also form due to significant tidal oscillations and
pore-excess pressure between channel and banks driven by rapid changes
in water levels (Zhao et al., 2022; Zhao et al., 2019), which generate
significant seepage flows (e.g., Gardner & Wilson, 2006; Wilson &
Morris, 2012). Seepage flows during late-ebb tides, also favored by
extensive bioturbation due to fiddler-crab and mudskipper burrowing
(Harvey et al., 2019; Ishimatsu et al., 1998; Perillo et al., 2005; Xin
et al., 2022), are likely responsible for the widespread bank slumps
that we observed at the middle and lower portions of channel
cross-sections in the studied channel (Figure 13). Notably, strong
seepage flows can also help explain why sustained velocities are
observed over nearly the entire duration of the ebb phase (Figure 6
a,b,c,d), and further support the idea that the ebb-late phases exert a
strong control on the morphodynamics of intertidal mudflat meanders.