Modeling investigation of asymmetric tidal mixing and residual circulation in a partially stratified estuary

A three-dimensional hydrodynamic model was applied to the Danshuei River estuarine system in northern Taiwan to investigate the influence of flood-ebb, spring-neap tidal cycles, and salinity distribution on tidal mixing, residual circulation, stratification, and tidal asymmetry. The model was validated using observational data collected in 2008. The results from the model agreed well with observations of water surface elevation, tidal currents, and salinity. It was found that the depth-averaged tidal current during flood tide is weaker with a shorter duration than that during ebb tide in the estuary, which was attributed to tidal asymmetry. Vertical profiles of salinity, flow, eddy diffusivity, and Richardson number also showed a marked asymmetry between flood and ebb tides. Bottom boundary stresses were higher during flood tides than during ebb tides, resulting in more mixing occurrence and consequently decreasing the Richardson numbers. The tidally averaged salinity was more stratified during neap tides than during spring tides because the presence of the stronger vertical diffusivity and turbulent kinetic energy during spring tides. The modeling results also confirmed that the residual circulation was stronger during neap tides than during spring tides.