It is of significant importance for longshore current instability to nearshore hydrodynamic and sediment transport. To further study the mechanism of longshore current instability, this paper obtains different instability growth modes and its reasons caused by the different velocity profile characteristics of slope 1 : 100 and 1 : 40 based on the experimental study of longshore current instability by using the linear shear instability model. The results show that:
The instability of longshore current only exists backshear mode for slope 1 : 40 and may exist frontshear and backshear modes for slope 1 : 100.
The peaks of linear instability growth mode for slope 1 : 100 correspond to three cases, the dominant peak is formed by the joint action of frontshear and backshear or by backshear alone.
The results of oscillation period, wavelength and spatial variation obtained by analysis of linear shear instability are agreed with the experimental results.
The wave tranmits obliquely toward the coast and produces current along the coast, that is, longshore current. It involves the mechanical problems which cannot be completely resolved today such as wave breaking, wave boundary layer and turbulence, and strong nonlinear fluid motion. The movement law is complex, and it is also one of the main dynamic factors of coastal pollutant movement, sediment transport and coastal deformation(Burchard et al.,2008;Wang and Zou,2015;Chen,2005). Fleming and Swart(1982) indicated that 10% of the longshore current forecast errors may lead to 70% longshore sediment transport errors. For the study of waved-driven currents, Longuet-Higgins and Stewart (1964) proposed the concept of radiation stressand stated that longshore current is mainly caused by the radiation stress and the uneven distribution of wave setup induced by the uneven distribution of wave height. With the deepening of the understanding of longshore current, Oltman-Shay et al. (1989) found that the longshore current velocity vector exists left and right swing, which is proved to be an unstable movement finally. Miles et al. (2002) pointed out that sediment transport in the vertical direction of 16% and along the coast of 12% are caused by the instability of longshore current. The study of Sanchez et al. (2003)showed that instability along the coast plays a major role in the formation of coastal rhythmic terrain. All of these indicated that the instability of longshore current has an important effect on sediment transport and shoreline evolution.Since the unsteady movement of longshore currents observed by Oltman-Shay et al. (1989), many studies have been made on its characteristics.