WIT Press

Nearshore Mixing And Macrovortices


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M. Brocchini, A. Piattella, L. Soldini & A. Mancinelli


Horizontal mixing of shallow coastal flows is studied with a specific focus on the role played by large-scale horizontal eddies (macrovortices). Within the classic depth-averaged Nonlinear Shallow Water Equations (NSWE) framework, generation of such macrovortices can be described through one single mechanism for which lateral gradients of shock-type solutions introduce vorticity in the flow. This mechanism is intensely activated when waves break over discontinuous topographic features like natural longshore sand bars or man-made submerged breakwaters. Description of macrovortex-induced mixing is given on the basis of numerical solutions of the NSWE and interpreting the results of specifically-designed laboratory experiments. Deterministic results concerning the generation/evolution of macrovortices are obtained and statistics of passive tracers are used to interpret the overall dynamics in terms of 2D turbulence theory. Preliminary results indicate differences in the mixing features of flows induced by isolated and arrays of submerged breakwaters. A discussion is also proposed on possible approaches for improving our knowledge/modeling of such type of mixing. 1 Introduction The object of this paper is the analysis of large-scale features of shallow-water turbulence which characterizes the flows of nearshore waters. The latter evolves as shallow-water flows in which the horizontal scale is much larger than the vertical scale and are most often analysed in terms of depth-averaged properties like in the case of the classic Nonlinear Shallow Water Equations (NSWE) on which we base our subsequent, quantitative analysis. The importance for shallow coastal flows of horizontal, large-scale eddies (macrovortices hereinafter) has been widely reported [1–6]. Large-scale, horizontal mixing of coastal flows is greatly promoted by macrovortices which are generated because of a spatially-nonuniform breaking