Drag Reduction Induced By Salinity Stratification In An Idealized Estuary

A numerical modelling investigation based on FVCOM was conducted for an idealized semi-closed partially mixed estuary.The scenarios with and without salinity were compared to analyse the effect of salinity stratification on water flow.T The effect of tide amplitude and river runoff on the drag reduction was also studied.The main conclusions of this paper are as follows.The influence of salinity stratification on the hydrodynamic dynamics is embodied in three aspects: the vertical average velocity,the velocity profile and the friction characteristic.The vertically averaged velocity is increased by the salinity stratification and the change of flow velocity lags behind.During most of the time in a tide period,the flow velocity near bottom decreases and flow velocity near surface increases because of the existence of salinity stratification.The velocity shows a reasonable estuarine circulation induced by the existence of salt wedge before and after the ebb in some stations,while there is no circulation in the case without considering salinity stratification there is no circulation.Besides,the roughness length and friction velocity are both reduced by the salinity stratification except when there is estuarine circulation.As for spatial distribution,the delay of vertically averaged velocity,the increase of the vertical mean velocity and the degree of drag reduction in ebb of each station is positively correlated to its stratification parameter,which the larger the stratification parameter is,the more obvious is the drag reduction effect.Drag reduction can be interpreted through that vertical eddy viscosity coefficient and turbulent kinetic energy are both reduced by salinity stratification,then energy dissipation is weakened and shear flow is developed,which shows drag reduction.As far as the effect of tide amplitude and river discharge on the drag reduction concerned,the larger the tide amplitude is,the stronger the drag reduction is and in larger range the drag reduction exits.The larger the river discharge is,the stronger the drag reduction is and the range of drag reduction moves seaward.