Modeled Surf-Zone Eddies on a Laboratory Scale Barred Beach With Varying Wave Conditions

Abstract

Transient rip currents drive cross-shore transport of nutrients, larvae, sediment, and other particulate matter. These currents are driven by short-crested wave breaking, which is associated with rotational wave-breaking forces (vorticity forcing) that generate horizontal rotational motions (eddies) at small scales. Energy from small-scale eddies is transferred to larger-scale eddies that interact and enhance cross-shore exchange. Previous numerical modeling work on planar beaches has shown that cross-shore exchange increases with increasing wave directional spread, but this relationship is not established for barred beaches, and processes connecting the wavefield to cross-shore exchange are not well constrained. We investigate surf-zone eddy processes using numerical simulations (FUNWAVE-TVD) and large-scale laboratory observations of varying offshore wave directional spreads (0 to $\sim 25°$) and peak period (1.5–2.5 s) on an alongshore uniform barred beach. We find that mean breaking crest length decreases, while crest end density (number of crest ends in a given area) increases, with increasing directional spread. In contrast, vorticity forcing, offshore low-frequency rotational motion, and cross-shore exchange peak at intermediate directional spreads $\left(\sim 10°\right)$. Distributions of the strength of vorticity forcing per crest and across the surf zone suggest that the peak in vorticity forcing at intermediate spreads results from a combination of a larger total breaking area and relatively long crests with large forcing, despite a lower total number of crests. However, low-frequency rotational motion within the surf zone does not peak at mid-directional spread, instead plateauing at directional spreads greater than $\sim 10°$. Results suggest that eddy-eddy interaction, the transformation of vorticity across the surf zone, and influence of bathymetry are fruitful topics for future work.