On the impact of an offshore bathymetric anomaly on surf zone rip channels

Castelle, Bruno; Marieu, Vincent; Coco, Giovanni; Bonneton, Philippe; Bruneau, Nicolas; Ruessink, Gerben

doi:10.1029/2011jf002141

Cited by 21 publications

(23 citation statements)

References 68 publications

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“…For instance, topographic rips can flow against permanent topographic features such as headlands or coastal structures (Pattiaratchi et al 2009;Castelle and Coco 2013;Scott et al 2016), flash rips are transient in both time and space (e.g. Ozkan- Haller and Kirby 1999;Feddersen 2014;Suanda and Feddersen 2015), and some rip currents can also be enforced by offshore bathymetric anomalies (Long and Ozkan-Haller 2005;Castelle et al 2012). These studies showed unique flow behaviours, suggesting that safety outcomes may contrast with what has been found for existing studies on rip current escape strategies on rip-channelled beaches.…”

Section: Assessment Of Rip Current Escape Strategies: Implications Fomentioning

confidence: 89%

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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Section: Assessment Of Rip Current Escape Strategies: Implications Fomentioning

confidence: 89%

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

et al. 2015

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“…Over the last three decades, a considerable amount of research effort has gone into understanding the morphodynamic processes governing the upstate/downstate transitions between beach states (as defined by Wright and Short, 1984), using field observations (Wright et al, 1985;Brander 1999), ARGUS video imaging (Lippmann andHolman, 1990;Ranasinghe et al, 2004, Turner et al 2007Ojeda et al, 2011;Price and Ruessink, 2011), and numerical modelling (Damgaard et al, 2002;Reniers et al, 2004;Drønen and Deigaard, 2007;Calvete et al, 2007;Smit et al, 2008;Castelle et al, 2012). However, to date, there has been no concerted research effort focussing specifically on the important issue of nearshore morphological recovery time scales.…”

Section: Introductionmentioning

confidence: 99%

Quantifying Nearshore Morphological Recovery Time Scales Using Argus Video Imaging: Palm Beach, Sydney and Duck, North Carolina

Ranasinghe

Holman

Schipper

et al. 2012

Int. Conf. Coastal. Eng.

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Time scales of post-storm nearshore morphological recovery and physical processes governing these time scales are poorly understood at present. The ability to predict nearshore morphological recovery time scales based on pre-, during-or post-resetting storm conditions is an essential requirement for building and validating scale aggregated models that operate at macro-and higher spatio-temporal scales. In this study, quality controlled ARGUS video derived beach states at Palm Beach, Sydney (4 years) and Duck, NC (2 years) and concurrent wave data are analysed to quantify the nearshore morphological recovery time scales (T mr ) and to determine the physical processes that may govern T mr . The results show that T mr is of the order of 5-10 days at these two beaches. T mr is moderately positively correlated with the averaged longshore current over the 3 days immediately after the resetting storm, indicating that it might be possible to develop a predictor for T mr based on wave conditions immediately after the resetting storm. Weak correlations are present between T mr and several pre-storm, during-storm and post-storm parameters at the two sites. However, these correlations are inconsistent between the two sites. A thorough analysis employing long-term beach state and wave data at several different study sites is required to fully understand this phenomenon.

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“…Furthermore, the model is able to properly simulate pattern formation under a wide range of local wave incidence, as observed along the curved coastline of the Sand Engine. Preliminary tests at a straight coast showed that crescentic patterns could develop under higher incidence angles using the nonlinear model of Dubarbier et al () than using a nonlinear model with the basic‐state approach (e.g., Castelle et al, ; Garnier et al, ). Although linear stability models, also based on a basic state, may produce crescentic patterns under higher incidence angles as well (e.g., Ribas et al, ), such models do not include cross‐shore bar migration and thus are not suitable for this study.…”

Section: Methodsmentioning

confidence: 99%

“…Periodic lateral boundaries conditions were used. In line with Castelle et al (), the shoreline was allowed to evolve by computing the sediment fluxes at the cell centers and interpolate them at the cell interfaces. Accordingly, the sediment fluxes could transfer across the interface between dry and wet cells.…”

Section: Methodsmentioning

confidence: 99%

“…However, coasts that are concave, like embayed beaches, or convex, such as shoreline sandwaves and km‐scale nourishments, impose an alongshore variation in forcing in the surf zone due to the refraction pattern over the curved depth contours (e.g., Castelle & Coco, ; Rutten et al, ). Similarly, offshore perturbations can create an alongshore variation in forcing (offshore bathymetric anomaly or offshore island; Bryan et al, ; Castelle et al, ) and, accordingly, in bar behavior. For example, the breaker height may vary alongshore, which was suggested by Short () to generate an alongshore variation in sandbar characteristics.…”

Section: Introductionmentioning

confidence: 99%

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Alongshore Variability in Crescentic Sandbar Patterns at a Strongly Curved Coast

et al. 2019

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Sandbars, submerged ridges of sand parallel to the shoreline, tend to develop crescentic patterns while migrating onshore. At straight coasts, these patterns form preferably under near‐normal waves through the generation of circulation cells in the flow field, whereas they decay under energetic oblique waves with associated intense alongshore currents. Recently, observations at a man‐made convex curved coast showed an alongshore variability in patterning that seems related to a spatiotemporal variability of the local wave angle (Sand Engine). Here, we aim to systematically explore how coastline curvature contributes to alongshore variability in crescentic pattern formation, by introducing local differences in wave angle and the resulting flow field. A nonlinear morphodynamic model was used to simulate the patterns in an initially alongshore uniform sandbar that migrates onshore along the imposed curved coast. The model was forced by a time‐invariant and time‐varying offshore wave angle. Simulations show that the presence of patterns and their growth rate relate to the local breaker angle, depending on the schematization of the offshore angle and the local coastline orientation. Growth rates decrease with increasing obliquity as both refraction‐induced reductions of the wave height as well as alongshore currents increase. Furthermore, simulations of variations in coastline curvature show that patterns may develop faster at strongly curved coasts if this curvature leads to an increase in near‐normal angles. This implies that beaches where the coastline orientation changes substantially, for example, due to km‐scale nourishments, become potentially more dangerous to swimmers due to strong currents that develop with pronounced bar patterns.

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On the impact of an offshore bathymetric anomaly on surf zone rip channels

Cited by 21 publications

References 68 publications

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

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

Quantifying Nearshore Morphological Recovery Time Scales Using Argus Video Imaging: Palm Beach, Sydney and Duck, North Carolina

Alongshore Variability in Crescentic Sandbar Patterns at a Strongly Curved Coast

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