Abstract
The process of exchange and transport between salt and fresh water is affected by not only convection due to the density gradient but also turbulence. In this study, a high resolution mathematical model of the intrusion movement of saltwater in an estuary area was established and compared with a physical flume experiment. The model, whose minimum horizontal grid cell of the model is 0.05 m, and minimum vertical grid cell is 0.0005 m, simulated well the salt transport mechanism at the interface between salt and fresh water. The salt transport processes of estuarine saltwater intrusion were obtained under the conditions of no runoff and no tide, no runoff and tide, and runoff and tide. The results suggest that under the joint action of runoff and tide, the mixing process in the interface of salt and fresh water is relatively gentle, the change in the velocity gradient is small, and the advance distance of saltwater intrusion will reach a dynamic equilibrium state after a period of time.
Highlights
In estuaries, under the action of dynamic factors such as river runoff and tide, fresh water moves seaward in the upper layer due to its relatively low density, while salty seawater moves landward in the lower layer due to its higher density
To thoroughly understand the engineering problems associated with this process and formulate numerical simulation results that accurately fit the reality, numerical investigation on saltwater intrusion has gradually developed from two-dimensional (2D) to three-dimensional (3D), which can analyze the change rules of many dynamic factors in all directions
There was a notable velocity gradient change at the interface between the salt and fresh water, resulting in salt dispersion, while a local circulation occurred at the front of the salt intrusion head, making the salt and fresh water mixing more intense at the front of the salt tide intrusion
Summary
Under the action of dynamic factors such as river runoff and tide, fresh water moves seaward in the upper layer due to its relatively low density, while salty seawater moves landward in the lower layer due to its higher density. The hydrodynamic characteristics of different water layers can be simulated more finely by vertical stratification, models with high grid resolution and many layers cause a high computational demand. To thoroughly study the hydrodynamic changes at the interface between salt and fresh water of the saltwater intrusion movement, it is necessary to reduce the vertical scale to realize the small-scale fine numerical simulations of the saltwater intrusion movement. The relative water depth of the minimum vertical grid cell was 0.001667, and the time step was set to 0.0005 s, which can simulate well the process of the mixing interface layer of salt and fresh water, small circulation, and eddy currents. To represent the hydrodynamic characteristics of the interface between salt and fresh water caused by saltwater intrusion, the density difference was considered, and the vertical resolution of the model was refined. It can be used to study the salinity change in estuary at a small scale, the interface of the mixing of salt and fresh water, and the convection and diffusion of salt between water and sediment
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