Monitoring of flow rate and salinity intrusion in a tidal floodway using fluvial acoustic tomography

Kiyoshi Kawanishi, Mahdi Razaz, Masoud BahrainiMotlagh

Thursday 2 july 2015

9:30 - 9:45h at Africa (level 0)

Themes: (T) Special session, (ST) Acoustic monitoring of flow, turbulence and river discharge

Parallel session: 10F. Special session: Acoustic monitoring of flow, turbulence and river discharge

Six-year record of tidal current and sound speed has been collected in a tidal floodway 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, Japan. The Ota River bifurcates into two main branches (floodway and another tributary) at just upstream of the observation site. 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 a ray path. The channel is a shallow tidal-forced river with a maximum tidal range of 4 m at the mouth. The tides are primarily semidiurnal, but mixed with a diurnal component. The freshwater runoff into the channel is regulated by the array of sluice gates, located 270 m upstream of the observation site. Although only one sluice gate is usually opened slightly, all sluice gates are completely opened during flood events. 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 fractional rate of the river discharge at the diversion point was found using FATS. The salinity varied in the span of 0 to 30 when the gates were normal positions; at the time, the sound speed was significantly influenced by the salinity. Thus, the salinity intrusion was investigated from the 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 controlled the recovery of salinity intrusion.