A Small-Scale Field Experiment for the Validation of a Theory on Reflection of Nonlinear Short-Crested Wave Groups

Abstract

A closed-form solution of free surface displacement and velocity potential for nonlinear short-crested (3D) wave groups interacting with a reflective sea wall is presented. The theory is applied to investigate the fluctuation wave pressures when an exceptionally high crest or deep trough occurs on the reflective wall. A characteristic behaviour has been observed. Due to non-linearity, on one hand, great reduction of the highest crest and enhancement of the deepest troughs on wave pressures are realized; on the other hand, the profiles show always a strong asymmetry between the absolute maxima and the minima and sometimes the formation of characteristic humped wave pressures corresponding to the impact on the structure of the highest wave crest. All the theoretical results have been validated through a smallscale field experiment carried out at the Natural Ocean Engineering Laboratory (N.O.E.L., www.noel.unirc.it) of Reggio Calabria (Italy).Keywords: nonlinear sea wave, reflection, short-crested wave groups, wave pressure, sea wall, upright breakwater INTRODUCTIONOne of the most basic issues of coastal engineering has been the estimation of the wave pressures and wave loads acting on a vertical sea wall.In the scientific literature, it is possible to find several theories and models, based on different approaches, formulated with the aim, on one hand, to describe as the most correctly as possible the wave mechanics of sea waves interacting with a reflective structure in the absence of overtopping and without considering breaking waves generating impulsive pressure, and on the other hand, to find the most accurate solution for the calculation of wave pressure distribution acting on the structure for practical engineering applications. Many experimental investigations have been also conducted to validate theoretical solutions.First order solutions of periodic or irregular waves, are important since they are the basic approach for the prediction of wave pressure. More accurate results turn out by considering higher order contributions. Through the periodic wave theory, solution of second-order was obtained by Saintflou (1928), and by Svendsen and Jonsson (1976); of third-order by Tadjbakhsh & Keller (1960); of fourthorder by Goda (1967Goda ( , 1999.Moreover, numerical models have been developed to validate analytical results and to investigate effects of higher order components. Numerical solutions with perturbation approximations up to thirdorder in wave steepness have been given by Jeng (2002), up to the 27th order by Roberts (1983a,b) and up to the 35th order by Marchant and Roberts (1987). Numerical solutions with Fourier approximation for short-crested waves up to 10th order have been derived by Roberts and Schwartz (1983), by Lin et al. (1987) and by Tsai et al. (1994).More recent numerical study with experimental comparison have been done by Kimmoun et al. (1999) and by Prabhkar and Sundar (2001).The problem of instability for the reflection of sea waves with the related effects and phenomenon was dealt, in...