3.1 Acoustic measurements of flow velocities
We continuously monitored water levels and flow velocities in the study
meander bend from October 14, 2020 to October 21, 2020. Three Nortek’s
new Acoustic Doppler Current Profiler (AD2CP, Signature 1000kHz) were
placed at three different sites along the studied bend, namely the bend
apex and both the landward and seaward inflections, whereas one Teledyne
RDI ADCP1200kHz was deployed at the confluence with the small side
tributary (Figure 4a). All instruments were placed at the channel
thalweg with an up-looking orientation to record velocities and
pressures (Figure 4c,d,e,f). The fifth probe of Nortek AD2CPs can
observe the vertical velocity separately from the other four probes,
thus effectively avoiding acoustic cross-interference so that the noise
in the vertical velocity signal is significantly lower than that of
traditional ADCP instruments. The three AD2CPs were programmed to
operate at 1.0 Hz, recording velocities in 20 cm vertical bins over
timespans at 5-minute intervals. The blanking distance of the AD2CP was
set to 10 cm, so that the center of the first sampling bin is 20 cm
above the instrument. In contrast, the Teledyne ADCP was programmed to
operate at 1.0 Hz to record velocities in 5 cm bins at 5 minutes. The
blank distance of the ADCP was set to 20 cm, so that the center of the
first bin is 22.5 cm above the instrument. Details regarding the
instruments’ parameters are shown in Table 1.