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.