Rhythmic morphology in a microtidal low-energy beach

Mujal-Colilles, Anna; Grifoll, Manel; Falqués, Albert

doi:10.1016/j.geomorph.2019.02.037

Cited by 7 publications

(16 citation statements)

References 46 publications

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“…In fact, the study and monitoring of cusps and other types of rhythmic bars or features has traditionally relied on remote sensing techniques, mainly video monitoring systems [22,59,60] and/or orthophotos [61]. Investigations based on the use of UAS for this purpose are still scarce in the literature, although they could be frequent in the upcoming future.…”

Section: Discussionmentioning

confidence: 99%

UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

Talavera¹,

Benavente²,

Río³

2021

Remote Sensing

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Unusual shore-normal and barred-like rhythmic features were found in Camposoto Beach (Bay of Cádiz, SW Spain) during a monitoring program using unmanned aerial systems (UAS). They appeared in the backshore and persisted for 6 months (October 2017–March 2018). Their characteristics and possible formation mechanism were investigated analyzing: (1) UAS-derived high-resolution digital elevation models (DEMs), (2) hydrodynamic conditions, and (3) sediment samples. The results revealed that the features did not migrate spatially, that their wavelength was well predicted by the edge wave theory, and that they shared characteristics with both small-scale low-energy finger bars (e.g., geometry/appearance and amplitude) and swash cusps (e.g., wavelength, seaward circulation pattern, and finer and better sorted material in the runnels with respect to the crests). Our findings pinpoint to highly organized swash able to reach the backshore during spring tides under low-energy and accretionary conditions as well as backwash enhanced by conditions of water-saturated sediment. This study demonstrates that rhythmic features can appear under different modalities and beach locations than the ones observed up to date, and that their unusual nature may be attributed to the low spatiotemporal resolution of the traditional coastal surveying methods in comparison with novel technologies such as UAS.

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Section: Discussionmentioning

confidence: 99%

UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

Talavera¹,

Benavente²,

Río³

2021

Remote Sensing

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“…Regarding the cross-shore extent of the bars, it is controlled by the depth of closure, D c , and a value of about 60-90 m is found for this site (the maximum observed is about 60 m). Finally, we should stress that the wave refraction process over the bars, which is essential to the instability, is also commonly observed at El Trabucador back-barrier beach(Mujal-Colilles et al, 2019).…”

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confidence: 88%

“…This beach is microtidal and wave energy is typically low due to the small fetch, with maximum H s ∼ 0.6 m during NW wind and T p < 3 s. Nevertheless, wave activity is intense enough to move the fine sand over all the terrace and a system of transverse finger bars is often present (Figure 1a). The alongshore wavelength is variable but the average and the most frequent is 20 m (Mujal-Colilles et al, 2019). The bars are thin and elongated with a cross-shore extent up to some 60 m and they commonly open an anti-clockwise angle of 10 • -40 • with the shore normal.…”

Section: Accepted Articlementioning

confidence: 99%

“…Datasets from El Trabucador back-barrier beach are included in the article Mujal-Colilles et al (2019). This research is part of the Spanish Government projects CTM2015-66225-C2-1-P and RTI2018-093941-B-C33 (MINECO/FEDER).…”

Section: Acknowledgmentsmentioning

confidence: 99%

“…In this situation, cross‐shore transport dominates and thereby it might trigger a destabilizing mechanism instead of a damping one. Other examples are the transverse finger bars along lake shores (Evans, 1938), estuaries (Eliot et al., 2006), barrier islands (Figures 1c and 1d) (Gelfenbaum & Brooks, 2003) and delta barrier beaches (Figures 1a and 1b), including the bar system at El Trabucador back‐barrier beach in the Ebro delta (Mujal‐Colilles et al., 2019).…”

Section: Introductionmentioning

confidence: 99%

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A New Morphodynamic Instability Associated With Cross‐Shore Transport in the Nearshore

Falqués

Ribas

Mujal-Colilles

et al. 2021

Geophysical Research Letters

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The existing theory for shore-transverse rhythmic sand bars relies on morphodynamic instabilities involving the wave-driven longshore current and rip currents. Intriguingly, transverse finger bars are common on coasts with sediment excess, something not related to those currents. Here we show that, if the actual beach profile is above the equilibrium profile, cross-shore transport can induce an instability triggered by onshore transport together with wave refraction by the emerging bars. We use a numerical model that filters out the dynamics associated to longshore and rip currents but includes a simplified version of cross-shore transport and is able to reproduce the formation of shore-transverse bars. The alongshore spacing scales with the wavelength of the incident waves and the cross-shore extent is approximately equal to the distance from shore to the depth of closure. The modelled bars compare qualitatively well with observations at El Trabucador back-barrier beach (Ebro delta, Western Mediterranean Sea).

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A new morphodynamic instability associated to the cross-shore transport in the nearshore

Falqués¹,

Ribas

Mujal-Colilles

et al. 2020

Preprint

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Beach morphology dynamics are driven by the interaction of water motion and sediment over a geological substratum. Coastal sediment transport is still poorly understood so that model representations largely rely on simplifications and parameterisations (Amoudry & Souza, 2011). At length scales comparable to the surf zone width or larger (>10-100 m), sediment transport can be conceptually decomposed into two main components. Longshore transport is driven by the surf-zone longshore current generated by breaking waves if they approach obliquely to the coast. Cross-shore transport is the main cause of the cross-shore beach profile sloping up onshore, sometimes with shore parallel sand bars. The main sources of cross-shore transport are onshore transport driven by wave asymmetry and skewness, offshore transport due to undertow (bed-return current) and downslope transport due to gravity (Fernández-Mora et al., 2015). An equilibrium bed profile is achieved if the three components are in balance. Finally, there are more contributions to sediment transport that do not fall into the longshore or cross-shore categories (e.g., those associated to rip current circulation or to low frequency motions).On sandy coasts, beach morphology is rarely alongshore uniform. Typically, the shoreline has undulations and the nearshore sea bed features shallows and deeps alongshore. Transverse bar systems (Ribas et al., 2015) are a well-known example, encompassing a series of shallows or bars separated by deeps called rip channels (Figure 1). These systems are not only fascinating but also relevant from a scientific point of

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Rhythmic morphology in a microtidal low-energy beach

Cited by 7 publications

References 46 publications

UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

UAS Identify and Monitor Unusual Small-Scale Rhythmic Features in the Bay of Cádiz (Spain)

A New Morphodynamic Instability Associated With Cross‐Shore Transport in the Nearshore

A new morphodynamic instability associated to the cross-shore transport in the nearshore

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