Chapter 6 Behavior of the turbidity plume relating to a density current in a tidal river

Abstract To clarify the formation and transport regimes in a tidal river, the turbidity plume in the Shira River estuary, Japan, was studied. The fieldwork took place at two stations in the Shira River on February 17–18, 2003, February 23–24, 2004 and June 3–4, 2004. Vertical velocity profiles, turbidity, salinity and floc diameter were measured. Moreover, acoustic imaging of the density current by a head rotating side scan sonar was carried out. From the acoustic imaging and the observation of the turbidity and salinity, it was revealed that the turbidity plume in the Shira River behaved as a turbid density currents. The density current resuspends the tidal river sediment in the Shira River. The origin of the suspended solids (SS) in the density current varied in a longitudinal direction. However, in the head of the density current, the origin of the SS was thoughts to be the surface sediment from the tidal riverbed; in the rear of the density current, SS of marine origin gradually increased. The sedimentation rate increased with the passage of the density current.