MONITORING FLOW RATE AND SALINITY INTRUSION IN A TIDAL FLOODWAY USING FLUVIAL ACOUSTIC TOMOGRAPHY

摘要

Six-year record of tidal current and sound speed has been collected in a tidal diversion channel using Fluvial Acoustic Tomography System (FATS) with a couple of 30 kHz broad-band transducers. The FATS was located around 9 km upstream far from the mouth of Ota River diversion channel, Japan. The reciprocal sound transmission that was performed between the two acoustic stations, located on both sides of the channel, enabled us to measure range-averaged sound speed and water velocity along an oblique transmission line. The channel is a shallow tidal-forced river with a maximum tidal range of 4 m at the mouth. The freshwater runoff into the channel is regulated by the array of sluice gates, located 270 m upstream of the observation site. The saline water was flushed out by the gate operation for flood events. Thus, the flow and the salinity intrusion presented strong unsteady nature. The salinity intrusion was investigated from the mean sound speed measured by FATS. The recovering time of the salinity ranged from 9.5 days to 27 days after the gates got back. The tidal velocity amplitude and the outflow velocity associated the river discharge basically controlled the recovery of salinity intrusion.