Flash rip current driven suspended sediment flushing amplification in depth-integrated modeling framework

Abstract

Until recently, wave and hydrodynamic properties in the cross-shore direction have been linked to accretive or erosive morphological evolution. However, various uncertainties remain, hindering the full understanding and accurate prediction of the transport direction and quantity of sediment transport in nearshore areas. In this study, we showed that flash rip currents (FRCs) could significantly amplify suspended sediment flushing out of the surf zone by greatly increasing advective flux, even under accretive wave conditions in a depth-integrated modeling framework. Compared to the case without FRCs, where only one-dimensional cross-shore direction waves and currents exist, FRC-driven seaward advective sediment flux increased by approximately O(102) times under normal incoming waves on alongshore uniform beaches, at least for the tested wave and bathymetry conditions. To properly consider the role of FRCs in suspended sediment flushing, short wave related physics, such as nonhydrostatic pressure and frequency dispersion, had to be included. In addition, it was important to accurately predict not only wave properties but also local hydrodynamics that cause erosion and advection. Finally, uniform sandbars prevented beach erosion, even when FRCs occurred on uniform beaches, by dissipating the wave energy closely associated with FRC generation. Therefore, one-dimensional cross-shore direction approaches neglecting the role of FRC-driven sediment transport processes, which are closely related to short wave motions and occur in two-dimensional space, could lead to lower prediction accuracy.