Nearshore circulation over transverse bar and rip morphology with oblique wave forcing

Abstract

There is a paucity of field data to describe the transition in nearshore circulation between alongshore, meandering and rip current systems. A combination of in‐situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at Pensacola Beach, Florida in the presence of relatively low energy oblique waves. Current speeds vary in response to the relative wave height ratio (Hs/h), which defines the degree and extent of breaking over the shoal. In the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves. As waves begin to break across the shoal (0.2<Hs/ h<0.5) the nearshore circulation is characterized by a meandering alongshore current. As conditions became more dissipative (Hs/h>0.5), the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone. The increase in wave dissipation is associated with an increase in very low frequency (VLF) variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow. The unsteady nature of the nearshore circulation is responsible for 55% of all surf zone exits under these more dissipative conditions. In contrast, only 29% of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore‐meandering current. While the currents had a low velocity (maximum of ~0.4 m s‐1) and would not pose a significant hazard to the average swimmer, the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low‐energy rip circulation systems under oblique wave forcing. Copyright © 2013 John Wiley & Sons, Ltd.

Wave breaking patterns control rip current flow regimes and surfzone retention

“…The association between increased wave breaking and increased vorticity has been observed before (Houser et al, 2013), where higher intensity breaking narrowed the available area for the rip channel, which has previously been linked to increased flow speeds (MacMahan et al, 2006). In terms of circulatory patterns, a closed rip here behaves in a similar fashion to the low 'tide factor' rips in the study of Scott et al (2014), where currents were fed predominantly by one alongshore direction.…”

Hydrodynamic and morphodynamic responses to surfzone seafloor perturbations

“…Circulation patterns that include features of both alongshore flows and rip currents have been observed in field [Sonu, 1972;Aagard et al, 1997;MacMahan et al, 2010;Houser et al, 2013;Winter et al, 2014], laboratory Borthwick and Foote, 2002], and modeling studies [Svendsen et al, 2000;Yu and Slinn, 2003;Kumar et al, 2011;Wilson et al, 2013;. Drifter tracks from several field studies suggest that transitions from rip currents to alongshore flows occur with increasing wave angle and decreasing wave breaking Houser et al, 2013], and can be modulated tidally [Winter et al, 2014].…”

“…Drifter tracks from several field studies suggest that transitions from rip currents to alongshore flows occur with increasing wave angle and decreasing wave breaking Houser et al, 2013], and can be modulated tidally [Winter et al, 2014]. In laboratory studies with fixed channeled beds, rip currents occur for shorenormal waves, whereas meandering alongshore flows occur for obliquely incident waves Borthwick and Foote, 2002].…”

“…Wave breaking on alongshore-nonuniform beaches can drive rip current circulation patterns and spatial variations in breaking-wave-driven alongshore flows [Sonu, 1972;MacMahan et al, 2010;Houser et al, 2013;Winter et al, 2014]. The alongshore gradients in wave breaking and setup that drive rip currents may be generated by local bathymetric variations [Bowen and Inman, 1969;Brander and Short, 2001;Haas et al, 2003;MacMahan et al, 2005Bruneau et al, 2009;Austin et al, , 2014, offshore bathymetric variations [Long and Özkan-Haller, 2005], wave-wave interactions [Dalrymple, 1975;Peregrine, 1998;Buhler and Jacobson, 2001], or wave-current interactions [Dalrymple and Lozano, 1978].…”

Morphological coupling in multiple sandbar systems – a review

“…Under continuing low waves the horns of the crescentic bar weld to the shore, causing the initially alongshore continuous trough to disappear and the bays to evolve into distinct cross-shore troughs (rip channels) with strong currents (up to 2 m s −1 ) (e.g. Brander, 1999;Houser et al, 2013). On the other hand, the straightening of an alongshore variable sandbar, called an upstate sequence (Wright and Short, 1984) or morphological reset, has traditionally been associated with b a high-energy, erosive-wave conditions, without an actual account of which processes lead to straightening.…”