Bed shear stress and sediment entrainment potential for breaking of internal solitary waves

Abstract

We investigate the interaction of strongly non linear internal solitary waves (ISWs) with boundaries having different slopes by means of high-resolution 3D large Eddy Simulations (LES). Releasing a volume of fresh water into a stratified ambient fluid, three different breaking mechanisms are produced: plunging, collapsing and surging breakers. The different shoaling dynamics affect the ISW evolution over the sloping boundary, inducing different effects on the bottom. In order to investigate the effects of the ISW breaking on the inclined surface, we calculate the bed shear stress and estimate the local flux of sediments entrained from the bed. We analyze the relationship between the breaking criteria and the related effects on the sloping surface. Although plunging breakers are expected to induce significant effects within the fluid, causing larger amount of mixing and fluid entrainment, the effects on the bottom are totally opposite. The collapsing breaker mechanism, indeed, generates boundary layer separation, which in turn induces whirling instabilities. Results show that the ISW interaction with the inclined surface occurs in its close proximity for collapsing breaker mechanism, which explains why the largest bed shear stresses and sediment re-suspension are predicted in the simulation where a collapsing breaker mechanism is observed.