A self‐similar scaling for cross‐shelf exchange driven by transient rip currents

Suanda, Sutara H.; Feddersen, Falk

doi:10.1002/2015gl063944

Cited by 50 publications

(71 citation statements)

References 56 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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“…Similarly, other potential exchange mechanisms such as winds [e.g., Fewings et al, 2008], tides [e.g., Lentz and Fewings, 2012], or internal waves [e.g., Sinnett and Feddersen, 2014;Suanda et al, 2014] are expected to generally have much larger alongshore length scales and longer time scales than those observed. Farther offshore of the surfzone, Stokes-drift-driven exchange [e.g., Lentz et al, 2008;Suanda and Feddersen, 2015] or other inner-shelf processes may become important. The above differences in exchange velocity magnitude, vertical structure, and time and alongshore length scales indicate that on this day with moderate waves, the observed surfzone to inner-shelf cross-shore dye transport is dominated by rip current ejections.…”

Section: Surfzone/inner-shelf Exchange Mechanismsmentioning

confidence: 99%

“…The observed ejection events are episodic and brief (O(1) min) and occur at random alongshore locations, indicating that these rips are transient and are not bathymetrically controlled, consistent with the approximately alongshore-uniform bathymetry at Imperial Beach (Figure 1). The magnitudes of u Ã and T x SZ=IS are related to the surfzone eddy field and the frequency and intensity of these transient rip current events, which depend on the incident wave field and beach slope [Johnson and Pattiaratchi, 2006;Suanda and Feddersen, 2015].…”

Section: Surfzone/inner-shelf Exchange Mechanismsmentioning

confidence: 99%

Surfzone to inner‐shelf exchange estimated from dye tracer balances

et al. 2015

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Surfzone and inner‐shelf tracer dispersion are observed at an approximately alongshore‐uniform beach. Fluorescent Rhodamine WT dye, released near the shoreline continuously for 6.5 h, is advected alongshore by breaking‐wave‐ and wind‐driven currents, and ejected offshore from the surfzone to the inner‐shelf by transient rip currents. Novel aerial‐based multispectral dye concentration images and in situ measurements of dye, waves, and currents provide tracer transport and dilution observations spanning about 350 m cross‐shore and 3 km alongshore. Downstream dilution of near‐shoreline dye follows power law decay with exponent −0.33, implying that a tenfold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner‐shelf dye mass balances close, and in 5 h, roughly half of the surfzone‐released dye is transported offshore to the inner‐shelf. Observed cross‐shore transports are parameterized well ( , best fit slope ) using a bulk exchange velocity and mean surfzone to inner‐shelf dye concentration difference. The best fit cross‐shore exchange velocity is similar to a temperature‐derived exchange velocity on another day with similar wave conditions. The magnitude and observed inner‐shelf dye length scales, time scales, and vertical structure indicate the dominance of transient rip currents in surfzone to inner‐shelf cross‐shore exchange during moderate waves at this alongshore‐uniform beach.

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“…). Reflective beaches are at the other extreme and are characterized by narrow surf zones, steep shore slopes, coarse sand if a beach and an absence of bathymetrically controlled rip currents, although transient rip currents can be present (Wright and Short , Suanda and Feddersen , Hally‐Rosendahl and Feddersen ). Transient rip currents form in all surf zones.…”

Section: Introductionmentioning

confidence: 99%

“…At sites without bathymetric rip currents, transient rip currents act like large patches of turbulence, which can eject water from the surf zone (Hally‐Rosendahl et al. , Suanda and Feddersen , Hally‐Rosendahl and Feddersen ). To compensate for this loss of water, water must enter the surf zone; transient rip currents cause the exchange of surf zone water with offshore water.…”

Section: Introductionmentioning

confidence: 99%

Alongshore variation in barnacle populations is determined by surf zone hydrodynamics

Shanks

Morgan

MacMahan

et al. 2017

Ecological Monographs

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Abstract. Larvae in the coastal ocean are transported toward shore by a variety of mechanisms. Crossing the surf zone is the last step in a shoreward migration and surf zones may act as semipermeable barriers altering delivery of larvae to the shore. We related variation in the structure of intertidal barnacle populations to surf zone width (surf zone hydrodynamics proxy), wave height, alongshore wind stress (upwelling proxy), solar radiation, and latitude at 40 rocky intertidal sites from San Diego, California to the Olympic Peninsula, Washington. We measured daily settlement and weekly recruitment of barnacles at selected sites and related these measures to surf zone width. Chthamalus density varied inversely with that of Balanus, and the density of Balanus and new recruits was negatively related to solar radiation. Across the region, long-term mean wave height and an indicator of upwelling intensity and frequency did not explain variation in Balanus or new recruit densities. Balanus and new recruit densities, daily settlement, and weekly recruitment were up to three orders of magnitude higher at sites with wide (>50 m), more dissipative surf zones with bathymetric rip currents than at sites with narrow (<50 m) more reflective surf zones. Surf zone width explained 30-50% of the variability in Balanus and new recruit densities. We sampled a subset of sites <5 km apart where coastal hydrodynamics such as upwelling should be very similar. At paired sites with similar surf zone widths, Balanus densities were not different. If surf zone widths at paired sites were dissimilar, Balanus densities, daily settlement, and weekly recruitment were significantly higher at sites with the wider, more dissipative surf zone. The primary drivers of surf zone hydrodynamics are the wave climate and the slope of the shore and these persist over time; therefore site-specific stability in surf zone hydrodynamics should result in stable barnacle population characteristics. Variations in surf zone hydrodynamics appear to play a fundamental role in regulating barnacle populations along the open coast, which, in turn, may have consequences for the entire intertidal community.

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A self‐similar scaling for cross‐shelf exchange driven by transient rip currents

Cited by 50 publications

References 56 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

Surfzone to inner‐shelf exchange estimated from dye tracer balances

Alongshore variation in barnacle populations is determined by surf zone hydrodynamics

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