Variability of tidal current, density stratification and tidal flat sediment in a tidal river

Long-duration observations of tidal currents, stratification, and suspended sediment (SS) were conducted using moored acoustic Doppler current profilers (aDcps) and CTDs in the Ohta flood-way where flow and density conditions change significantly. Spatial variation of SS transport was examined from simultaneous measurements at longitudinal two places (Sta. A and B), which are located in about 2.8 km and 5.8 km upstream from the mouth. The deployment period of one month allowed us to examine the effect of a flood incident, wind, and both semi-diurnal and neap/spring aspects of tides. The data revealed a semi-diurnal stratification cycle driven by tidal straining of freshwater- induced horizontal density gradient at Sta. A. The tidal straining and the nonlinearity of tidal wave bring about the asymmetry of tidal current. The direction of net transport was upstream at Sta. A and downstream at Sta. B for the restricted freshwater discharge. Additional surveys were made to examine particle size distributions of the tidal flat sediment from I September 2004 to 28 January 2005.