Numerical predictability experiments of cross‐shore sandbar migration

Abstract

Surf zone sandbars, common features along the world's sandy coastlines, continuously change their position in response to time‐variable offshore wave conditions. Process‐based predictions of cross‐shore sandbar migration, relevant to the understanding of autonomous and artificially altered evolution of beaches, are intrinsically imprecise because of uncertainty in the model equations and, potentially, the sensitive dependence on the initial bathymetry. However, the magnitude of the resulting predictability limit and its dominant source are unknown. Here we show that cross‐shore sandbar migration on the time scale of years is deterministically forced rather than deterministically chaotic, and that the unpredictability of sandbar migration results primarily from model inadequacy during major wave events. Because the unpredictability of sandbar migration is related to the stochastic nature of the forcing, the predictability limit is not a fixed value but depends on the timing of the wave event. We anticipate that detailed experiments to understand nearshore evolution from the underlying first principles will eventually pay off by extending the range of plausible simulated behavior and, consequently, will increase our ability to understand and predict how coasts may respond to changing climate.