4.2.2 Streamwise velocities and secondary circulations
In all the measuring sites, the primary (i.e., streamwise) velocity
(\(V_{P}\)) reduces significantly once the water stage reaches bankfull,
both for HAT and LAT cycle. On the contrary, \(V_{S}\)increases significantly once water depth exceeds the bankfull (see panel
b in Figures 8,9,10, and 11). To better describe flow dynamics near the
bankfull stage, we also plot \(V_{S}\) vectors overimposed to \(V_{P}\)magnitude values computed 15 minutes before and after the bankfull stage
for both HAT and LAT cycle (see panel c~f
in Figures 8,9,10, and 11).
Velocity patterns observed at the seaward inflection site (AD2CP-1) are
reported in Figure 8c~f. During the flood, once the
water depth exceeds the bankfull, \(V_{P}\) decreases suddenly and even
reverses its direction in the upper layers, where \(V_{S}\) increases
significantly and takes direction pointing toward the meander inner
bank. In contrast, during the ebb, \(V_{S}\) is very weak both for the
above- and below-bankfull stages (Figure 8c~f).
Similar results are observed at the landward inflection site (Figure 9).
Particularly, peaks in \(V_{P}\) are observed for\(Y\)<\(Y_{B}\), whereas the largest values of \(V_{S}\) are
attained when water depths exceed the bankfull
(\(Y\)>\(Y_{B}\)). During the flood, secondary velocities
(\(V_{S}\)) are generally higher than at the seaward inflection and
consistently point to the outer bank in the upper vertical layers.
Possible secondary circulations emerge during the flood for bothHAT and LAT cycles, with \(V_{S}\) directed toward the
outer bank at the water surface and near the inner bank at the channel
bottom. Overall, data from both the landward and seaward inflection
sites suggest the presence of secondary circulations for overbank stages
both for HAT and LAT cycles, with less obvious patterns
being observed during below-bankfull stages (Figure 8,9; see also
Supplementary Figure S1a,b).
Secondary circulations can be observed at the bend apex (Figure 10),
though they appear to be generally weaker than those found at the bend
inflections. During the flood, both \(V_{S}\) magnitude and secondary
circulations are very weak when \(Y\)<\(Y_{B}\) (Figure
10b~f and Figure S1c). Once Y reaches\(Y_{B}\), \(V_{S}\) increases and
secondary circulations develop, especially during HAT cycles.
However, contrary to classic (i.e., fluvial) secondary circulation
patterns where flows are directed toward the outer bank in the uppermost
portions of the water column, we observe secondary circulations
characterized by \(V_{S}\) directed toward the inner bank near the water
surface. During ebb tides, \(V_{S}\) are generally lower than during the
flood, and secondary circulations are less clearly noticeable both forHAT and LAT cycles.
Finally, at the confluence site, no significant secondary circulation is
detected during either the flood or the ebb, both for HAT andLAT cycles. During the flood, \(V_{S}\) are consistently directed
toward the outer bank where the tributary is located and increase
significantly when the water rises above the bankfull stage (Figure
11b~f; see also Supplementary Figure S1d). In contrast,
during the ebb, a chaotic distribution of \(V_{S}\) is found, with no
clear indication of relevant secondary circulations (Figure 11d,f,
Figure S1d).