The Influence of Storms on Sand Wave Evolution: A Nonlinear Idealized Modeling Approach

Abstract

We present a new 2DV nonlinear process-based morphodynamic model to investigate the effects of storms, specifically wind-driven flow and wind waves, on finite amplitude tidal sand wave evolution. Simulations are performed on periodic domains of two lengths: (i) on a 350-m domain, comparable to the wavelength of observed sand waves, we study the evolution toward equilibrium shapes, and (ii) on a 4-km domain, we study the evolution from a randomly perturbed seabed. Our model results demonstrate that both wind-driven flow and wind waves reduce sand wave height and tend to increase wavelength. Wind-driven flow breaks the tidal symmetry, resulting in horizontal sand wave asymmetry and migration. Waves alone do not induce migration but can enhance migration induced by, for example, tidal asymmetry and wind-driven flow. On the 350-m domain, we further find that migration rates decrease with increasing sand wave height. However, in an irregular sand wave field, large sand waves tend to overtake the smaller ones, suggesting a complicated interaction among neighboring bed forms. The above results concern steady state storm conditions. However, since storms occur on an intermittent basis, we also simulated a synthetic storm climate consisting of alternating short periods of storm conditions and long periods of fair-weather conditions. Simulations reveal a dynamic equilibrium with sand wave heights significantly below those obtained for tide-only conditions, also for relatively short storm duration. Our work identifies mechanisms that explain why sand wave heights are generally overpredicted by numerical models that do not include storm processes.