Nonlinear modelling of surf zone morphodynamical instabilities

The surf zone of sandy beaches exhibits morphological patterns which are rhythmic along the coast. Numerous numerical models simulate the formation of rhythmic features but none of them could describe their long term behaviour. The 2DH numerical model MORFO55 based on a wave and depth averaged nonlinear shallow water equations solver with wave driver, sediment transport and bed updating is developed. It is applied to investigate the generation and the long term evolution of surf zone rhythmic features on initially alongshore uniform beaches. In particular, it is found that shore-attached transverse or oblique bars, crescentic bars and ridge and runnel systems can emerge by self-organisation of the coupling between topography, waves and currents. In each case, a dynamical equilibrium state may be eventually reached. A physical explanation for their formation, their evolution and the saturation of their growth is given. For the first time, a quantitative description of rhythmic features is reproduced with a numerical model. This is a step forward in the nearshore modelling approach and helps to bridge the gap between modellers and geoscientists.