Vortical VLF Motions Under Shore-Normal Incident Waves

Abstract

Field observations and numerical model simulations are examined to investigate the magnitude of vortical very low frequency (VLF) velocity fluctuations (i.e. large scale surfzone eddies) under different offshore wave forcing. Observations of vortical VLF motions under shore -normal wave incidence at Duck, NC, USA are re-analyzed and compared with the characteristics of the incident wave spectrum. Long wave periods and narrow frequency spread incident waves were found to coincide with stronger vortical VLF motions. Numerical model simulations investigating the effect of the incident wave parameters in a more isolated way confirm the observed effect of frequency spread and wave period on the magnitude of VLF motions. Variations in incident wave spectrum resulted in changes in the vortical VLF magnitude of the same order as the magnitude of the vortical VLF velocity fluctuations themselves. These results imply that under shore-normal incident waves strong vortical VLF velocity fluctuations in the surfzone are more likely under swell conditions and at swell dominated coasts.

Three‐dimensional transient rip currents: Bathymetric excitation of low‐frequency intrinsic variability

“…If we define our VLF (very low frequency) band for the periods between 5 and 100 min, the EKE contained in the VLF explains about 70% of the total EKE. The modeled intrinsic EKE in the VLF band is quantitatively comparable to that of the rotational VLFs observed on the same beach [e.g., MacMahan et al ., ; de Schipper et al ., ], clarifying that bathymetric variability is the main source of the intrinsic VLFs. In addition, the VFLs due to group waves [e.g., Long and Özkan‐Haller , ] and individual breaking waves [e.g., Spydell and Feddersen , ], both of which are neglected in the present model, are anticipated to further excite the forced VLFs.…”

“…A quantitatively similar result was reported by de Schipper et al . [], who extended the analysis of MacMahan et al . [] for the much longer period between August and December 1997.…”

“…Our focus is on the intrinsic part of the VLFs, yet the forced VLFs usually contain equivalent turbulence intensity [e.g., Reniers et al ., ] or enstrophy [e.g., Long and Özkan‐Haller , ] with that in the intrinsic VLFs. We further demonstrate rip currents are spontaneous for spatially variable bathymetry with alongshore undulations, resulting in unstable motions that contain significantly energetic intrinsic VLFs, comparable to those measured during the SandyDuck field campaign [ MacMahan et al ., ; de Schipper et al ., ] without wave groups and random wave breaking. Supplemental simulations are carried out to examine influences from three‐dimensionality, alongshore variability in the bathymetry, wave‐current interaction (i.e., CEW), and different forcing conditions on the intrinsic VLFs.…”

Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone

“…Although σ L = σ E can be assumed, the dependence of σ on incident wave conditions is not well understood. However, recent research has indicated that for weak alongshore currents, σ E is related to the offshore significant wave height [ MacMahan et al ., ] and period [ de Schipper et al ., ]. For stronger V , surfzone σ E is related to V [ Noyes et al ., ].…”