4.1 Flow magnitudes, tidal asymmetries, and stage-velocity
relationships
Overbank flows invariably occurred for all the tide cycles on record.
The high-tide water depth (\(Y_{H}\)) reached a maximum value of 4.2 m
on Oct. 18, whereas a minimum value of \(Y_{H}\)=2.2 m was observed on
Oct. 14. The latter was still higher than bankfull water depth
(\(Y_{B}\)), which is about \(Y_{B}\)=1.20 m (Figure 5a). There are no
significant differences in horizontal velocity magnitudes
(\(|\overrightarrow{v}|\)) between the four monitoring stations, with
peak velocities in the order of \(|\overrightarrow{v}|\)=0.96-0.99 m/s
consistently observed during the rising limb of the tide (Figure
5a,b,c,d). Pronounced differences in flow velocity are observed for
water stages above and below the bankfull depth. Specifically, higher\(|\overrightarrow{v}|\) values are typically observed for
below-bankfull water stages, when tidal flows are conveyed entirely
within the channel, both during flood and ebb tides. In contrast,
comparably lower \(|\overrightarrow{v}|\) values are found for water
levels exceeding the bankfull depth \(Y_{B}\), although relatively large\(|\overrightarrow{v}|\) values occur when the high-tide water depth
(\(Y_{H}\)) exceeds a critical value of about 3.2 m, that is, for tidal
oscillations akin to spring-tide conditions (Figure 5a). When this
happens, the vertical velocity profiles display significant variations,
with reduced (enhanced) velocities found at water depths lower (higher)
than \(Y_{B}\).
The computed values of tidal flow asymmetries (\(\rho_{v}\) and\(\rho_{d}\)) can be plotted as a function of the high-tide water depth
(\(Y_{H}\)) observed during each monitored tidal cycle (Figure 5e,f,g).
Made exception for the first monitored tidal cycle, flow velocities are
found to be consistently flood-dominated for below-bankfull depths
(\(Y\)<\(Y_{B}\), Figure 5e). In contrast, ebb dominance
becomes more common considering water depths above the bankfull
(\(Y\)>\(Y_{B}\)), even though most of the data still falls
within the flood-dominated domain (Figure 5f). Changes in high-tide
water depth (\(Y_{H}\)) seem to not significantly affect peak flow
asymmetry, although both data scattering and flood dominance appear to
decrease slightly as \(Y_{H}\) increases, both for above- and
below-bankfull water depths (Figure 5e,f). Conversely, variations in\(Y_{H}\) significantly affect tide duration asymmetry (\(\rho_{d}\)).
The collected data suggest the persistence of flood-dominated conditions
in our study channel during the entire monitoring period, with\(\rho_{d}\) increasing proportionally to \(Y_{H}\) in a statistically
significant fashion (Figure 5g).
Stage-velocity diagrams for all the measuring stations display
pronounced variations in the observed depth-averaged velocity
(DAV ), with DAV maxima typically occurring immediately
below the bankfull water depth both for flood and ebb tides (Figure
6a,b,c,d). Flood DAVs are observed to decrease significantly once
water depths exceed the bankfull stage. On the contrary, ebb DAVsrapidly increase once water depths become lower than the bankfull water
depth. Although the peak DAVs are typically higher during the
flood phase, which is in agreement with our previous observations
regarding flow asymmetries, ebb DAVs attain near-maximum values
for comparably longer times at water stages lower than bankfull (Figure
6a,b,c,d).
Similar to the DAV patterns, the rate of water-level change
(\(\dot{Y}\) = dY/dt ) peaks around the bankfull stage during the
flood, whereas ebb peaks of \(\dot{Y}\) are observed for water stages
well above the bankfull (Figure 6e,f,g,h). Notably though, \(\dot{Y}\)attains a nearly-constant value during most of the ebb phase, whereas
much more pronounced changes are observed during the flood. A
statistically significant, positive linear correlation is found between\(\dot{Y}\) and DAV for below-bankfull stages during both the ebb
and the flood (Figure 6i,j,k,l), although such correlation is more
robust for ebb flows (Figure 6i,j,k,l). For overbank stages, significant
correlations between \(\dot{Y}\) and DAV can only be obtained for
the flood phase, whereas ebb DAVs are not significantly
correlated to \(\dot{Y}\) (Figure 6m,n,o,p).