Rhythmic Patterns of a Sandy Coast and Self-Organization of the Relief

The phenomenon of rhythmic patterns of the coastal-zone morphology (beach cusps, longshore sand waves, spits, and nearshore bars) is considered the result of self-organization of the relief when a positive feedback is established between sediment transport and morphology. It is shown that the dynamics of the contour and profile of the coast can be described in terms of the diffusion equation, which allows for both attenuation and growth of relief perturbations. The conditions are determined under which the self-organization mechanism is activated and the resulting perturbations grow over time. Rhythmic structures of the coastal contour are traditionally associated with waves propagating at a large angle relative to the coastal normal. However, it has been shown that the self-organization of beach cusps and megacusps also occurs in normal coast waves due to the development of circulation cells in the surf zone. The conclusion is substantiated that the formation of nearshore bars under certain conditions is also determined by the self-organization mechanism. The results obtained help to interpret the available observational data regarding the evolution of multiple bar systems.