TRANSVERSE SALINITY DYNAMIC IN A SHALLOW TIDAL CHANNEL

Abstract

In this study, the variation in vertical and transverse salinity in stratification in the main branch of the Ōta River system was investigated over a tidal cycle at both spring tide and neap tide. The quantity and depth of stratification layers as well as the surface-to-bottom salinity difference deduced from the transverse distribution of salinity were used to evaluate the stratification/de-stratification degree in a shallow and narrow estuary. Two Compact Conductivity-Temperature-Depth sensor campaigns to collect salinity data sets were run across the river at two bridges, with a longitudinal distance of approximately 850 m between the two bridges. It was found that salt wedges moved from the left side to the right side of the riverbank during both neap flood tide and spring flood tide, resulting in lateral salinity distribution, likely due to the difference in the advection of the longitudinal salinity gradient, and the effects of cross-channel bathymetry variation. Besides, the stratification developed gradually from the surface to the bottom of the water column over the ebb tide period primarily caused by the tidal straining; tidal time scale, and freshwater discharge. The re-stratification process at the end of the flood phase is likely due to the combination of the lateral baroclinic pressure gradient and complex bathymetry.