A self-similar scaling for cross-shelf exchange driven by transient rip currents

Transient rip currents, episodic offshore flows from the surf zone to the inner shelf, present a recreational beach hazard and exchange material across the nearshore ocean. The magnitude and offshore extent of transient rip-current-induced exchange and its relative importance to other inner shelf exchange processes are poorly understood. Here 120 model simulations with random, normally incident, directionally spread waves spanning a range of beach slopes and wave conditions show that the transient rip current exchange velocity is self-similar. The nondimensional exchange velocity, surf zone flushing time, and cross-shore decay length scale are scaled by beach slope and wave properties, depending strongly on wave directional spread. Transient rip-current-driven exchange can be compared to other cross-shelf exchange processes. For example, transient rip-current-driven exchange is stronger than wave-induced Stokes-drift-driven exchange up to six surf zone widths from shore.