Morphodynamics of nearshore rhythmic sandbars in a mixed-energy environment (SW France): 2. Physical forcing analysis

Lafon, Virginie; Dupuis, Hélène; Butel, Rémi; Castelle, Bruno; Michel, Denis; Howa, Hélène; Apoluceno, D. De Melo

doi:10.1016/j.ecss.2005.06.011

Cited by 34 publications

(31 citation statements)

References 32 publications

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“…More often, inner-bar rip channels possess remarkably smaller and more variable alongshore scales than the outer bar (e.g. Barusseau et al, 1994;Van Enckevort et al, 2004;Lafon et al, 2004Lafon et al, , 2005. This would suggest that morphological coupling does not play a substantial role in the overall double-sandbar dynamics.…”

Section: Discussionmentioning

confidence: 99%

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Castelle

Ruessink

Bonneton

et al. 2010

Earth Surf Processes Landf

113

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Crescentic sandbars and rip channels along wave-dominated sandy beaches are relevant to understand localized beach and dune erosion during storms. In recent years, a paradigm shift from hydrodynamic template models to self-organization mechanisms occurred to explain the formation of these rhythmic features. In double sandbar systems, both the inner-and outerbar rip channels and crescentic planshapes are now believed to be free instabilities of the nearshore system arising through selforganization mechanisms alone. However, the occasional occurrence of one or two inner-bar rip channels within one outer-bar crescent suggests a forced, morphologically coupled origin. Here we use a nonlinear morphodynamic model to show that alongshore variability in outer-bar depth, and the relative importance of wave breaking versus wave focussing by refraction across the outer bar, is crucial to the inner-bar rip channel development. The coupling patterns simulated by our model are similar to those observed in the fi eld. Morphological coupling requires a template in the morphology (outer-bar geometry) which, through the positive feedback between fl ow, sediment transport and the evolving morphology (that is, self-organization) enforces the development of coupling patterns. We therefore introduce a novel mechanism that blurs the distinction between self-organization and template mechanisms. This mechanism may also be extended to explain the dynamics of other nearshore patterns, such as beach cusps. The impact of this novel mechanism on the alongshore variability of inner-bar rip channels is investigated in the companion paper.

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Section: Discussionmentioning

confidence: 99%

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Castelle

Ruessink

Bonneton

et al. 2010

Earth Surf Processes Landf

113

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“…The inner bar is modally rip-channelled and can go through (Sénéchal et al 2009). Mean rip spacing of the inner and outer bars is about 400 and 700 m, respectively, along the Gironde coast, with mean spacing increasing southward (Lafon et al 2005;Castelle et al 2015). Because the outer bar is deep (about 3-7 m below mean sea level) and located far offshore (about 400-500 m from the beach), outer-bar rip currents are generally inactive during the summer months and do not pose a hazard to beach users.…”

Section: Beach Morphologymentioning

confidence: 96%

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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“…Bowen and Inman, 1969;Van Enckevort and Ruessink, 2003;Holman et al, 2006) and intertidal (e.g. Kroon and De Boer, 2001;Lafon et al, 2005;Van Houwelingen et al, 2006) zones, the temporal evolution (from generation to decay) of the intertidal rip channels has received little attention, especially in microtidal settings.…”

Section: Introductionmentioning

confidence: 99%

Temporal and spatial behaviour of rip channels in a multiple‐barred coastal system

Quartel

2009

Earth Surf Processes Landf

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A 15-month data set of daily time-averaged video images (Argus) has been analyzed to describe the spatial and temporal variability of the rip channels on a multiple-barred coast at Noordwijk aan Zee, The Netherlands. The landward boundary of the intertidal bars and a proxy of the subtidal bar crest, defined as the intertidal and subtidal bar lines respectively, were derived from the Argus images. Local seaward-directed deviations of the bar lines represent the cross-shore and alongshore locations of the rip channels. The average intertidal rip spacing (l) was 243 m, but the rips were not spaced regularly (σ λ /l = 0.47). Some intertidal rips were observed to fill up during falling tide, but the majority remained open. The filled intertidal rip channels had more landward positions and migrated more slowly (2.4 versus 4.6 m/day) in the alongshore direction than the open intertidal rip channels. The number and the alongshore migration rate of open intertidal rip channels increased with the preceding wave heights (r = 0.26, p < 0.01) and alongshore component of the offshore wave power (r = 0.25, p < 0.01), respectively. The shape of the intertidal bar lines was similar to the subtidal bar line shape, suggesting that the intertidal morphology is coupled to the subtidal alongshore variability. The phase of two bar lines could vary from in phase (0°) to out of phase (180°). The phase changes gradually, due to different alongshore migration rates of the intertidal and subtidal bar lines. Figure 4. Time series of the hydrodynamic parameters H rms (black) and P l (gray) at the offshore location MPN (a), location X 1 (b) and location X 2 (c). Vertical dash-dot bars mark storm events, which induced a morphologic reset on the intertidal beach. Note the different vertical scales of P l .Figure 5. Rectified and merged Argus image taken around low tide on 27 May 2002 (a) showing the dunes, the beach (region of interest of intertidal zone marked by dashed line) and a part of the subtidal zone with the inner bar line (white solid line). The region of interest regarding the intertidal zone was produced with a higher resolution (b) and consequently classified based on the surface composition (c) with the intertidal bar line (black solid line). with the alongshore position versus time (a) including their smoothed positions (solid lines). Spatial variability of the number of days on which rip channels appeared (b) and the time-averaged cross-shore position (c), and temporal variability of the number of rips N (d) and alongshore-averaged cross-shore position (the solid line shows the filtered data with a window of 14 days) (e). Except for (a), the distinction of IRC type was made: fIRCs are represented by black-filled bars or dots, and oIRCs by unfilled, white bars or dots. The extreme storm events are numbered and marked by horizontal dash-dot lines. with the alongshore position versus time (a) including their smoothed positions (solid lines). Spatial variability of the number of days on which rip channels appeared (b) and the time-av...

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Morphodynamics of nearshore rhythmic sandbars in a mixed-energy environment (SW France): 2. Physical forcing analysis

Cited by 34 publications

References 32 publications

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

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

Temporal and spatial behaviour of rip channels in a multiple‐barred coastal system

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