INTERNAL SOLITARY WAVES INTERACTING WITH THE BOTTOM

Abstract

Nonlinear internal waves are observed in many parts of the world ocean. They are mostly generated by the interaction between tidal flow and the bottom topography. Internal waves are important physical phenomena on the continental shelf/slope. They are often very energetic, and their breaking provides an important dissipation and mixing mechanism, with implications for biological productivity and sediment transport. Breaking mechanisms associated with internal solitary waves include bottom boundary layer instabilities, shear instabilities in the interior of the water column, and wave overturning as they shoal. In the present work, internal solitary waves (ISWs) propagating in a two layer stratified fluid system are primarily studied by laboratory experiments. The generation, propagation, and breaking phases of large amplitude internal solitary waves of depression, propagating horizontally in a wave tank, are investigated. ISWs main features result to be dependent on the geometrical parameters that define the initial experimental setting: empirical relations between ISWs geometric and kinematic features and the initial setting parameters are obtained. The approach of the ISWs towards a uniform sloping boundary is investigated. Depending on both waves properties and slopes values, different physical processes characterize the shoaling phase, causing the development of different breaker types. Following a qualitatively analysis of the different breaking mechanisms, collapsing-plunging breakers show the larger contribution in terms of mixing. Internal wave packets propagating towards the North of the Messina Strait (Mediterranean Sea) are investigated. In this region, the analysis of both the observed internal waves, and the bathymetry suggests that plunging breakers are expected to occur. Additional laboratory experiments are carried out in order to investigate how the pycnocline thickness is affected by the breaking of plunging ISWs. The increase of the pycnocline thickness results to be non-linearly related with the Iribarren number. High-resolution 3-D Large Eddy Simulations (LES) are also used to investigate the effects of internal solitary waves breaking over a sloping boundary. Here again, the lock release method is applied in a two-layer stratified fluid system to generate the three main breaking mechanisms. The different breaking dynamics are investigated in terms of their effects on the dynamics of the ISW and the interaction of the ISW with the sloping boundary. The bulk entrainment 2 parameter and the mixing efficiency are used to characterize the effects of the different breaking mechanisms on the initially stably stratified fluid. The larger entrainment is observed for the surging case as a consequence of the large gravity current flowing upslope, while the plunging breaker shows the relatively largest amount of mixing, which is mostly induced by rear-edge overturning in the onshore direction. The bed shear stress and the local flux of sediments entrained from the bed are estimated to investigate the effects of the ISW breaking on the inclined surface. The collapsing breaker mechanism generates boundary layer separation, which in turn induces whirling instabilities. Results also 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 resuspension are predicted in the simulation where a collapsing breaker mechanism is observed. Further laboratory experiments are performed in order to investigated the role of the unsteady behavior of solitary waves interacting with a mobile bottom. The sandy bedforms triggering process induced by surface solitary waves in shallow water conditions is analyzed. The standard lock-release method is applied in a flume, producing an initial displacement between the free surfaces that are divided by the gate. The quick removal of the gate induces the generation of a single surface solitary wave which interacts with the sandy particles composing the original flat bottom. For 15 different cases, the effect on the mobile bed of 400 consecutive waves with the same features is investigated. From the boundary layer theory, it is possible to relate the wave stroke action to the bedforms triggering process. Nearby the bottom, the adverse horizontal pressure gradient results to be strongly affected by the horizontal velocity induced by the wave. It induces a reverse flow and a boundary layer separation. Depending on the wave and sand features and on the water depth, the boundary shear stress induced by the reverse flow can exceed the critical value, inducing the back motion of the sand particles. Under these circumstances, the first erosion zones form, and they gradually evolve in asymmetric dunes because of the action induced by the following waves. The breaking generation mechanism results linked to the wave features and their effect at the bottom.