Stage and rate of formation of a complex spit in a megatidal environment

Robin, Nicolas; Levoy, Franck

doi:10.1127/0372-8854/2007/0051-0337

Cited by 11 publications

(12 citation statements)

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“…Enhanced erosion in the downdrift sector induces a change in shoreline orientation at the distal part of the spit. This change in orientation is at the origin of the formation of a new hook spit [Robin and Levoy, 2007]. The recent morphodynamic behavior of Agon spit, between T 0+6 and T 0+10 (2008) (bar 2) is in agreement with this pattern.…”

Section: Onshore Megatidal Swash Bar Migration and Shoreline Evolutiosupporting

confidence: 64%

“…Because of low migration rates, the time of presence of the bar near the high-tide beach is much larger in megatidal environments ($7 years for bar 1 (1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984) and at least 4 years for bar 2 (2004 -2008, with a welding phase that is still ongoing) than on coasts with smaller tidal ranges (approximately 1 year). This explains the strong disturbance such bars induce in the sediment dynamics of the high-tide beach and in shoreline evolution in large tide range environments, it also explains the longer cycle of migration and welding of swash bars [Robin and Levoy, 2007]. The transverse bar morphology observed in 1977 for bar 1, and since 2005 for bar 2, also testifies to the influence of these forms on sediment transport on the high-tide beach.…”

Section: Comparison Of Megatidal Tidal Inlet Shoreline Behavior With mentioning

confidence: 74%

“…The seaward slope generally has a gentle gradient (tanb % 0.02) and is characterized by sand with a mean D 50 value of 0.5 mm and by numerous shells and gravel clasts. The bars are generally shorter than those in other tidal environments, but have nearly identical cross-shore morphometric parameters [Robin et al, 2007a[Robin et al, , 2007b. The trough is generally flat and characterized by finer sediment than the seaward slope.…”

Section: Study Areamentioning

confidence: 89%

“…Once all the shorelines and limits of the high-tide beach were obtained, the next step consisted of extracting their variations over time. This was carried out using a code developed at the University of Caen [Robin, 2007;Robin and Levoy, 2007]. To this end, 120 working profiles spaced at 20 m intervals were constructed orthogonal to the oldest line.…”

Section: Analysis Of Bar and Shoreline Variationsmentioning

confidence: 99%

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Short‐term to decadal‐scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta

Robin¹,

Levoy²,

Monfort³

et al. 2009

J. Geophys. Res.

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[1] Swash bar development has been well documented from coasts with low to moderate tidal ranges, while studies of the effects of these forms on the morphology and dynamics of the adjacent shore in large tide range environments are rare. The analysis of sequential vertical aerial photographs was combined with field work in order to highlight the effects of swash bar development on the adjacent shoreline in the vicinity of a megatidal inlet (mean spring tidal range of 11 m). Swash bars are observed to form on the ebb tidal delta and to migrate landward before welding onto the coast. A close relationship was noticed between the position of the swash bar and shoreline dynamics. The bar protects the shore against wave attack in a sedimentary system controlled by longshore transport. This protective role is, however, modulated by the large tidal range. As the bar migrates upward toward the high-tide level and the subaerial beach, it develops morphologically into a transverse form that acts as a cross-shore obstacle to longshore sediment transport, thus resulting in shoreline accretion updrift and in strong erosion downdrift. This disturbance may persist for years because of the relatively slow speed of movement of the bar at this stage, an aspect characteristic of large tide range environments. This pattern of behavior differs fundamentally from that documented in the literature where the perturbation of the longshore sediment transport occurs over shorter periods. An original conceptual model of swash bar morphodynamics and repercussions on the adjacent shoreline for this megatidal environment is proposed.Citation: Robin, N., F. Levoy, O. Monfort, and E. Anthony (2009), Short-term to decadal-scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta,

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Section: Onshore Megatidal Swash Bar Migration and Shoreline Evolutiosupporting

confidence: 64%

Section: Comparison Of Megatidal Tidal Inlet Shoreline Behavior With mentioning

confidence: 74%

Section: Study Areamentioning

confidence: 89%

Section: Analysis Of Bar and Shoreline Variationsmentioning

confidence: 99%

See 2 more Smart Citations

Short‐term to decadal‐scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta

Robin¹,

Levoy²,

Monfort³

et al. 2009

J. Geophys. Res.

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“…Their impacts depend on the synchronization of sediment supply and transport events (Houser, 2009), and if shoals are capable of substantially and persistently increasing the beach width (Galiforni‐Silva et al, 2020). However, the welding bar can also disturb the littoral drift and drive foredune erosion on the downdrift end (Davidson‐Arnott & Van Heyningen, 2003; Robin et al, 2009; Robin & Levoy, 2007; Robin, Levoy, et al, 2020). This latter case is not documented in transgressive dune evolution.…”

Section: Introductionmentioning

confidence: 99%

Natural remobilization and historical evolution of a modern coastal transgressive dunefield

Robin

Billy

Lerma

et al. 2023

Earth Surf Processes Landf

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The vast majority of coastal dunes in Europe have been stabilized by increasing vegetation cover since the mid‐20th century. However, some systems may experience a remobilization phase, generally occurring locally and further propagating alongshore, the drivers of which remain poorly documented. This study investigates the evolutionary paths (stabilization/destabilization/remobilization) from 1945 to 2020 of a 2 km‐long modern coastal transgressive dunefield located in southwest France with a holistic approach (GPR profiles, aerial photographs and LiDAR topographic data). Results show a landward migration of the transgressive dune by approximately 233 ± 7.5 m, through two distinct stages of rapid landward migration from 10 to 23 m/yr (Stage I: 1949–1959 and Stage III: 2000–2021) separated by an approximately 40‐year stage of slow to no migration, but with substantial windward slope deflation (Stage II). The onset of Stage II is due to the fixation of vegetation by human action between 1950 and 1959. The onset of Stage III is hypothesized to be driven by long and sustained upper backshore/dune toe erosion beginning in 1968 due to a massive shoal welding that locally disturbed the longshore drift. It induced a destabilization of the dune and erosion of the vegetation cover over some decades. A non‐synchronization is therefore observed between the start of the perturbation (1968), then the migration (2000), in line with the hysteresis concept of Tsoar (2005). This study shows that almost all of the sedimentary volume of the 1945 dune has been remobilized by translation to shape the dune system in its current form. The 2.2 km dunefield has grown by approximately 673 000 ± 190 000 m3 during the 2005–2020 period. Among this volume, there is a new foredune that was built from 2005 between the upper beach and the transgressive dune (volume in 2020 of about 394 000 ± 68 000 m3).

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Sand spit dynamics in a large tidal‐range environment: Insight from multiple LiDAR, UAV and hydrodynamic measurements on multiple spit hook development, breaching, reconstruction, and shoreline changes

Robin

Levoy

Anthony

et al. 2020

Earth Surf Processes Landf

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Sand spits with distal hooks have been well documented from coasts with low to moderate tidal ranges, unlike high tidal‐range environments. Datasets from 15 LiDAR and 3 UAV surveys between 2009 and 2019 on the Agon spit in Normandy (France), a setting with one of the largest tidal ranges in the world (mean spring tidal range: 11 m), combined with in‐situ hydrodynamic records between 2013 and 2017, highlight a three‐stage pattern of spit hook evolution. Stage 1 (2009–2013) commenced with the onshore migration and attachment of a swash bar, followed by persistent spit accretion updrift of the bar and erosion downdrift because of the slow speed of bar migration in this large tidal‐range environment. In stage 2 (2013–2016), three overwash events and a 220 m‐wide breach culminating in the total destruction of the spit during winter 2015–2016 involved the landward mobilization of thousands of cubic metres of sand. These events occurred during short durations (a few hours) when spring high tides coincided with relatively energetic waves, underscoring the importance of storms in rapid spit morphological change. Strong spring tidal currents maintained the breach. Stage 3 (2016–2019) has involved new hook construction through welding of a swash bar and spit longshore extension, highlighting the resilience of the spit over the 10‐year period, and involving a positive sediment balance of 244 000 m3. The three stages bring out, by virtue of the temporal density of LiDAR and UAV data used, a high detail of spit evolution relative to earlier studies in this macrotidal setting. The large tidal range strongly modulates the role of waves and wave‐generated longshore currents, the main process drivers of spit evolution, by favouring long periods of inertia in the course of the spring–neap tidal cycle, but also brief episodes of significant morphological change when storm waves coincide with spring high tides. © 2020 John Wiley & Sons, Ltd.

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Stage and rate of formation of a complex spit in a megatidal environment

Cited by 11 publications

References 0 publications

Short‐term to decadal‐scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta

Short‐term to decadal‐scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta

Natural remobilization and historical evolution of a modern coastal transgressive dunefield

Sand spit dynamics in a large tidal‐range environment: Insight from multiple LiDAR, UAV and hydrodynamic measurements on multiple spit hook development, breaching, reconstruction, and shoreline changes

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