5.2 Secondary circulations and curvature-induced helical flows
According to classic flow fields observed in sinuous channels, secondary
(i.e., cross-sectional) circulations are observed in our study bend,
both during high-amplitude (HAT) and low-amplitude (LAT )
tidal cycles (Figures 8,9,10,11). These secondary circulations are more
pronounced during overbank stages, their intensity increasing as the
water depth increases within the studied channel. Indeed, secondary
circulations tend to be stronger for HAT than LAT cycles
(Figures 8,9,10,11). They also appear to be mostly related to flood
flows, which is in agreement with the generally flood-dominated
character of tidal flows observed in the studied bend (Figure 5e,f,g).
In some cases, the orientation of secondary circulations is reversed
compared to classic flow models such as, for example, at the seaward
bend inflection as well as at the meander apex (Figure 8 and Figure 10),
where secondary circulations are directed toward the inner and outer
bank at the top and bottom of the water column, respectively. Secondary
currents can trigger cross-sectional sediment transport processes such
that fine-grained deposits are transported up to the point bar from the
channel bed, giving rise to fining upward trends due to the progressive
upbar weakening of secondary currents (Bathurst et al., 1977;
Blanckaert, 2011; Dietrich, 1987; Termini & Piraino, 2011). This is
supported by the fining upward trends that are consistently observed
from sediment cores collected at different sites along the studied bend
(Figure 3).
Interestingly, secondary circulations are more pronounced at the meander
inflections than at the apex, where they should be stronger owing to
higher channel curvature. This could however depend on the surveying
strategy we used, since we only monitored the velocity profile in
correspondence to the channel axis rather than across the entire
cross-section. Previous studies have demonstrated that secondary
circulation cells do not necessarily occupy the whole channel
cross-section (e.g., Blanckaert, 2009, 2011; Finotello, Ghinassi, et
al., 2020). Particularly, hydrodynamic nonlinearities can arise in sharp
bends characterized by radius-to-width ratios\(R/\overset{\overline{}}{W}\ \)lower than 2-3, and flow separation may
occur either at the inner or outer bank, respectively, immediately
upstream or downstream of the bend apex (Blanckaert et al., 2013;
Finotello, Ghinassi, et al., 2020; Hickin, 1978; Hickin & Nanson, 1975;
Hooke, 2013; Parsons et al., 2004; Rozovskiĭ, 1957). Flow separation,
which is common in tidal meanders owing to the high curvature values
that they typically display (Ferguson et al., 2003; Finotello, D’Alpaos,
et al., 2019), can effectively reduce the portion of the channel that is
hydrodynamically active and confine curvature-induced secondary
circulations to the nonrecirculating portion of the primary flow
(Finotello, Ghinassi, et al., 2020; Leeder & Bridges, 1975; Parsons et
al., 2004). Our studied meander bend is characterized by a\(R/\overset{\overline{}}{W}\)=2.2, and the formation of flow sepeartion
is therefore highly likely. Direct measurement of tidal flows across the
entire channel cross section would be necessary to settle the dispute,
but such data are hard to collect because channel banks at our studied
site are flooded by more than 3 m of water at high tides, thus making
field measuring campaigns complicated. Nevertheless, we can still
estimate the chance for flow separation at the apex of our studied
channel by comparing our data with the results obtained by Leeder and
Bridges (1975) for intertidal meanders in the vegetated Solway Firth
(Scotland). According to Leeder and Bridges (1975), the chances for flow
separation in tidal meander bends can be expressed as a function of bend
tightness (\(\frac{R}{W}\)) and Froude number (Fr). Although
extending the results of Leeder and Bridges (1975) to unvegetated
mudflats might not be entirely appropriate, results would still offer
useful insights on the possible occurrence of flow separation,
especially for below-bankfull stages when tidal flows are confined
within the channel. Since our measurements include several consecutive
tidal cycles, we were able to calculate how the \(\frac{R}{W}\) changes
according to varying water depths. Specifically, we assumed that \(R\)does not vary significantly with changing water elevation, and we
computed the channel width \(W_{Y}\) corresponding to different water
depths (\(Y\)) based on topographic data of the meander-apex
cross-section (Figure 4b). Plotting of \(\frac{R}{W_{Y}}\) againstFr shows that flow separations at the bend apex site are
likely to occur at near-bankfull stages (Figure 12). This is clearly
related to the morphology of the studied bend, which is characterized by
a relatively low width-to-depth ratio (\(\beta\)), whereby \(W_{Y}\)increases rapidly as \(Y\) increases, thus producing progressively lower\(\frac{R}{W_{Y}}\) in the range from 8 to 2. In addition, flow
velocities at the below-bankfull stage generate a modest Frvalue of 0.2~0.3, which can possibly induce flow
separations (Leeder & Bridges, 1975). In contrast to our observations,
Figure 12 suggests that flow separation will be suppressed at overbank
stages, likely because of the observed flow velocity reduction at\(Y\)>\(Y_{B}\). Care should be however given when
extending the results proposed by Leeder and Bridges (1975) to
situations where tidal flows do not remain confined within channel
banks. Regardless, our analyses support the idea that reduce secondary
circulations observed at the meander apex could be ascribed to flow
separation, which makes secondary circulations hard to identify through
localized flow measurements.
Regardless of flow separation, it is worthwhile noting that secondary
circulations are stronger during overbank stages, when flow confinement
within channel banks is significantly reduced and, as a result, primary
velocities (\(V_{P}\)) are small. Thus, there seems to be a phase shift
between peaks of primary (\(V_{P}\)) and secondary velocity (\(V_{S}\)),
such that \(V_{P}\) is maximum when \(V_{S}\) is low, and vice versa.
Such a shift would effectively limit the advection of cross‐stream
circulations operated by the primary flow, thus hampering the formation
of characteristic curvature‐induced helical flows (e.g., Blanckaert,
2011; Blanckaert & de Vriend, 2003; Dinehart & Burau, 2005; Ferguson
et al., 2003; Frothingham & Rhoads, 2003). Moreover, we notice that
primary velocities at overbank stages are sometimes characterized by
reverse direction relative to below-bankfull stages, that is, \(V_{P}\)are directed seaward (landward) during flood (ebb) tides (see for
example Figure 8c). This would further limit the transfer of secondary
circulation by primary velocity along the meander bend, thus hampering
the formation of helical flows even further. Such behavior has not been
observed in tidal channels flanked by vegetated intertidal plain,
wherein \(V_{P}\) and \(V_{S}\) maxima are approximately in phase and
correspond roughly to near-bankfull water stages (e.g., Fagherazzi et
al., 2008; Finotello, Ghinassi, et al., 2020; Kearney et al., 2017).
Additionally, secondary circulations also appear poorly developed at the
confluence site. It is well known that complex circulation patterns can
arise at channel confluences (e.g., Lane et al., 2000; Leite Ribeiro et
al., 2012; Rhoads & Kenworthy, 1995; Schindfessel et al., 2015), which
are likely to suppress curvature-induced secondary flows. Nonetheless,
one should appreciate that channel confluences in intertidal mudflat
channel networks are somehow less frequent than in networks carving
vegetated intertidal plains, owing to the lower drainage density that
characterizes bare intertidal areas (e.g., Kearney & Fagherazzi, 2016).
Therefore, flow disturbances and helical flow disruption due to channel
confluences and bifurcations are not likely to have a significant
limiting effect on meander morphodynamics in intertidal mudflats.
Overall, the results we illustrated so far suggest poor development of
curvature-induced secondary flows in intertidal mudflat meander bends.
The implications of this hydrodynamic peculiarity, as well as those
highlighted in Section 5.1, for the morphodynamics of intertidal mudflat
meanders, will be discussed in the next section.