2D Numerical Research On Characteristics Of Tidal Shear Fronts In Zhoushan Islands Channels

Shear front is a very common phenomenon in the nature water area, it has instinct influence on the characteristics of flow field, the movement of suspended matters, the evolution of geomorphy, the living environment of creatures nearby, thus it draws attention to many scholars, they had made researches on its formation mechanism and characteristics. This essay used numerical method to research the characteristics of tidal shear fronts in three representative channels of Zhoushan islands channels.Zhoushan islands channels were chosen to be the research sea area, the landform conditions of Changbai Channel, Daishan Channel, Guanmen Channel in this area were analyzed in detail. The landform conditions of these channels were appropriate to the formation of shear front, but the difference of their landform characteristics might cause shear fronts to represent different characteristics.HD module of MIKE 21 software was used to calculate the 2D tidal current field of the research sea area, and by comparing with the matched monitoring data, it proved that the calculation well represented the characteristics of the tide and tidal current in this area. Based on the tidal current model, the flow fields in the chosen channels were analyzed, and shear fronts were found to form in these channels, their characteristics, influence and formation mechanism were as follows:(1)The common characteristics of tidal shear fronts were obtained through analyzing the distribution of velocity and flow direction in the research channels. Flow direction shear fronts normally formed during slack tide, they formed at the shallow side of the underwater place where the bathymetry shown a distinct break in slope, and disappeared at the deep side, the time they lasted was about 0.5～2 hours; velocity shear fronts were more distinct when the velocity reached its maximum during flooding and ebbing, their location were kept at the place where the bathymetry shown a distinct break in slope. Shear fronts would form, develop and disappear with the change of hydrodynamic force in a tidal circle.(2)The characteristics of shear fronts were found to be influenced by various landform conditions through analyzing the shear phenomena of flow in different locations of three chosen channels. The extent of the break slope had direct impact on the intensity of shear fronts. The shape of the coast line in the longitudinal direction and the cusp would influence the flow pattern in the longitudinal direction, and might cause stream difference during flooding and ebbing. The landform conditions of the channels' outlets might lead backwater, and cause back flow. These landform conditions would cause frontal zones and its movement, the intensity of shear to represent different characteristics.(3)Through analyzing the velocity and flow direction of the characteristic points during spring tide, mean tide and neap tide, tidal stencils was also found to be related with the characteristics of shear fronts. At the same characteristic point, velocity shear fronts were more intense during spring tide than during neap tide; the time that flow direction shear fronts lasted was longer during neap tide than during spring tide. (4)After analyzing the numerical simulation result and the research findings by other scholars, this essay made conclusion that the most important factor which induced tidal shear fronts was the slope break of the underwanter place in the lateral direction, it not only induced the discontinuity in velocity and flow direction, but also made the discontinuity remarkable in a short lateral distance. The extent of the slope break also influenced the intensity of the shear fronts and the lateral circulations in the frontal zones.