Rip currents under obliquely incident wind waves and tidal longshore currents

Winter, Gundula; Dongeren, Ap van; Schipper, Matthieu de; Vries, J.S.M. van Thiel de

doi:10.1016/j.coastaleng.2014.04.001

Cited by 38 publications

(44 citation statements)

References 29 publications

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“…This has been called into question by a number of recent studies which have found that exit rates can vary significantly between beaches (e.g. Austin et al 2013;Castelle and Coco 2013;Houser et al 2013;Scott et al 2014;Winter et al 2014;McCarroll et al 2014b), and hence floating may not be as viable a strategy as initially proposed. Extensive testing of the different bather escape strategies was first conducted by McCarroll et al (2014a) across three rip current systems comprising two open beach rip currents featuring TBR morphology, and a topographic rip current adjacent to a shore platform.…”

Section: Introductionmentioning

confidence: 93%

Examining rip current escape strategies in non-traditional beach morphologies

et al. 2015

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Rip currents are a significant hazard on global surf beaches and are a factor in hundreds of drowning fatalities each year. Contemporary rip current safety information often idealises rip currents as part of a Transverse Bar Rip (TBR) morphology with rip channels bound by shallow, shore-connected bars. Real-world conditions frequently differ from this model, with potential implications for rip current escape strategies promoted to, and undertaken by, the general public. This study describes outcomes of rip current escape strategies conducted at North Cronulla Beach, NSW, Australia, over two distinct morphologies; a mixed Low Tide Terrace/Transverse Bar Rip (LTT/TBR) and a Rhythmic Bar Beach (RBB) system lacking shore-connected bars. Swimmers attempted to escape by adopting one of three pre-determined strategies: Stay Afloat, Swim Parallel and Swim Onshore. A total of 100 escape attempts were conducted, with the RBB system experiencing longer duration ( t = 2.4 min) escapes than the LTT system ( t = 0.8 min). The RBB system was associated with a higher rate of action failure, particularly for Stay Afloat, due to a lack of shore connectivity of adjacent bars. Swim Parallel was of lower duration ( t RIP1 = 0.66, t RIP2 = 2.68 min) in both systems, but durations and distances to safety in the RBB system often exceeded swimming abilities of weaker bathers. Although Swim Onshore was more successful ( t RIP1 = 0.22, t RIP2 = 1.65 min) than Swim Parallel, promotion of such a strategy is strongly discouraged in conventional safety advice. Results suggest that contemporary rip current escape strategies may be inappropriate in non-TBR rip current systems and that alternative strategies should be considered, including Swim Onshore and a greater focus on preventative strategies, particularly in relation to bathers with limited swimming ability.

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Section: Introductionmentioning

confidence: 93%

Examining rip current escape strategies in non-traditional beach morphologies

et al. 2015

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“…Based on this new perception of rip current flow behaviour, MacMahan et al (2010) proposed an alternative ''stay afloat'' rip current escape strategy to minimize bather energy expenditure as the recirculation would carry bathers to shallower and safer depths on the order of minutes. Of note, this dichotomous discrimination between recirculating and exit flows masks a considerable variability evident by many recent field, numerical and laboratory studies that show a large variability in the ability of rip currents to flush out the surf zone (Austin et al , 2013Castelle et al 2010;Houser et al 2013;Castelle et al 2014;McCarroll et al 2014a;Winter et al 2014;Scott et al 2014).…”

Section: Introductionmentioning

confidence: 95%

Modelling the alongshore variability of optimum rip current escape strategies on a multiple rip-channelled beach

et al. 2015

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Rip currents are a leading cause of drowning on beaches worldwide. How bathers caught in a rip current should attempt to escape has been a subject of recent debate. A numerical model of human bathers escaping from a rip current flow field is applied to a 2-km-long section of the open beach of Biscarrosse, SW France. The study area comprises 4 rip channels that visually appear similar from the beach, but exhibit different morphologies. Simulations are run for 2 representative hazardous summer wave conditions. Results show that small changes in the bar/rip morphology have a large impact on the rip flow field, and in turn on the alongshore variability of the optimal rip current escape strategy. The overall flow regime (dominant surf-zone exits versus dominant recirculation), which is found to be influenced by the alongshore dimensions of the sand bars adjacent to the rip channel, is more important to rip current escape strategy than rip velocity. Flow regime was found to dictate the success of the ''stay afloat'' strategy, with greater success for recirculating flow. By comparison, the dominant longshore feeder current and rip-neck orientation determined the best direction to ''swim parallel'' towards. For obliquely incident waves, ''swim parallel'' downdrift then swim onshore with breaking waves was highly successful and can become a simple safety message for beach safety practitioners to communicate to the general public. However, in SW France where rip spacing is large ( $ 400 m), surf-zone eddies have large spatial scales of the order of 100? m, requiring a large distance (100? m) to swim to reach safety, therefore requiring good swimming & Bruno Castelle ability. This also shows that in addition to rip current intensity, rip flow regime and the depth of adjacent sand bars, rip spacing is important for defining rip current hazard and the best safety message. Our results also indicate that for normal to near-normal wave incidence, rip current hazard and best rip current escape strategy are highly variable alongshore due to subtle differences in bar/rip morphology from one rip system to another. These findings challenge the objective of developing a universal rip current escape strategy message on open rip-channelled beaches exposed to normal to near-normal wave incidence, even for seemingly similar rip channels.

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“…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].…”

Section: Introductionmentioning

confidence: 99%

“…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].…”

Section: Introductionmentioning

confidence: 99%

“…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].…”

Section: Introductionmentioning

confidence: 99%

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Hydrodynamic and morphodynamic responses to surfzone seafloor perturbations

Moulton¹

2016

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Holes and channels were excavated in the surf zone on an ocean beach near Duck, NC, and observations of the subsequent evolution of waves, currents, and the modified seafloor were used to investigate nearshore dynamics. In one set of seafloor perturbation experiments, deep holes with steeply sloping sides were excavated in the inner surfzone seafloor. Observations of the infilling holes were used to make the first field estimates of the surfzone morphological diffusivity, which describes the rate of seafloor smoothing by downslope sediment transport. To improve the temporal resolution of bathymetric estimates, a mapping method was developed to combine infrequent, spatially dense watercraft surveys with continuous, spatially sparse in situ altimeter estimates of the seafloor location. In another set of seafloor perturbation experiments, channels were dredged across the surf zone with the propellers of a landing craft. Alongshore variations in wave breaking caused by the perturbed bathymetry resulted in strong rip currents in the channels under some conditions, whereas alongshore currents bypassed the channels under other conditions. The dynamics of the circulation response for changing wave forcing, bathymetry, and tidal elevation are investigated using the observations, a numerical model, and a parameter based on wave properties and bathymetry.

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Rip currents under obliquely incident wind waves and tidal longshore currents

Cited by 38 publications

References 29 publications

Examining rip current escape strategies in non-traditional beach morphologies

Examining rip current escape strategies in non-traditional beach morphologies

Modelling the alongshore variability of optimum rip current escape strategies on a multiple rip-channelled beach

Hydrodynamic and morphodynamic responses to surfzone seafloor perturbations

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