Vertical velocity structure distribution in the Sansha area of the Yangtze Estuary, China

The vertical velocity structure (VVS) of estuarine water columns provides an objective basis for investigating shear turbulent flow. In this study, based on VVS data measured during the 2004 flood season and 2005 dry season, and on the ratio of the tidal range to water depth, the VVS of the Yangtze Estuary was classified into the following three types: S-shaped, W-shaped (i.e., switch shaped), and U-shaped. During flood tides, the VVS was found to be S-shaped when the ratio was greater than 0.20, W-shaped when the ratio was between 0.15 and 0.20, and U-shaped when the ratio was less than 0.15. During neap tides, the VVS was S-shaped when the ratio was greater than 0.15, W-shaped when the ratio was between 0.10 and 0.15, and U-shaped when the ratio was less than 0.10. These results indicate that the relative strengths of the tides and runoff control the VVS in the estuary, which tends to be U-shaped when runoff is high, but S-shaped when the tide increases.