Monitoring Tidal Bores using Acoustic Tomography System

Abstract

Kawanisi, K.; Zhu, X.-H.; Fan, X., and Nistor, I., 2017. Monitoring tidal bores using acoustic tomography system.Continuous measurements of flow velocities and suspended sediments were carried out in the Qiantang River (China) with extreme tidal bore conditions. Fluvial and coastal acoustic tomography systems (FATS/CATS) were used in the field studies simultaneously with acoustic Doppler current profilers (ADCPs) and optical backscatter (OBS) equipment. A couple of broadband transducers were installed diagonally across the river around 90 km upstream of the mouth. The length of the sound transmission line was 3050 m. Cross-sectional averaged velocities (V) collected by FATS/CATS enabled the authors to estimate important characteristics of the tidal bores (bore height and celerity). The changes in V recorded during the upstream movement of the bore ranged from 1.35 to 1.76 ms−1. The height and celerity of the bore varied from 1.0 to 1.32 m and 7.59 to 8.29 ms−1, respectively. Since a point/vertical measurement cannot represent a river's cross section, the ADCP data for velocity and water level (pressure) near the riverbank underestimated the bore height by 22% and 16%, respectively. The maximum suspended sediment concentration (SSC) was observed to have occurred approximately 1 h after the bore's arrival; the time lag between the maximum SSC and the bore front is considerably larger than the time lags in previous works. In the case of larger bores, the section mean SSCs, which were deduced from the sound attenuation of FATS/CATS due to suspended sediment, appeared to be appropriate. The time history of the velocity, measured by ADCPs, during the passage of the bore deviated from the normal velocity profile, i.e. the velocity magnitude decreased with an increasing height above the bed for the relaxation time of a few minutes following the arrival of the bore.