On the optimum of the energy transfer efficiency in the generation of waves by subaerial landslides

This paper studies the problem of the energy transfer from a subaerial landslide to the waves it produces, in two dimensions, with a thoroughly validated multiphase Navier-Stokes model. The completeness of the model and the large domain considered give access to the time evolution of the slide and water energy components up to the far field. Considering a configuration favoring high energy transfer efficiency, namely a triangular water slide lying at the free surface, the slide volume (per unit width) is varied. The results show that the transfer efficiency is reduced by dispersion for the smaller slides and by breaking for the larger ones, leading to the existence of a local optimum for an intermediate slide size. Starting from this point, a new optimum is obtained by varying the slide density and the slope angle. Hence, with a density of 1750 kg. m−3 and a slope of 30°, the efficiency reaches a value of 87% in the near field and 73% in the far field, which may be good approximates of the related maxima in two dimensions. Compared to the previous optimum case, considering a granular rheology or slides at higher elevations drastically decreases the efficiency. The energy transfer rate from a two-dimensional subaerial landslide to the tsunami produced may be limited to a value of about 70 % in the far field, according to Navier-Stokes simulations.