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 ) 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 . 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 . Results suggest that eddy‐eddy interaction, the transformation of vorticity across the surf zone, and influence of bathymetry are fruitful topics for future work.
Low‐frequency surf zone eddies disperse material between the shoreline and the continental shelf, and velocity fluctuations with frequencies as low as a few mHz have been observed previously on several beaches. Here spectral estimates of surf zone currents are extended to an order of magnitude lower frequency, resolving an extremely low frequency peak of approximately 0.5 mHz that is observed for a range of beaches and wave conditions. The magnitude of the 0.5‐mHz peak increases with increasing wave energy and with spatial inhomogeneity of bathymetry or currents. The 0.5‐mHz peak may indicate the frequency for which nonlinear energy transfers from higher‐frequency, smaller‐scale motions are balanced by dissipative processes and thus may be the low‐frequency limit of the hypothesized 2‐D cascade of energy from breaking waves to lower frequency motions.
more » « less- PAR ID:
- 10374473
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 3
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 1531-1536
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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