Short-term to Decadal-scale Sand Flat Morphodynamics and Sediment Balance of a Megatidal Bay: Insight from Multiple LiDAR Datasets

Levoy, Franck; Anthony, Edward J.; Dronkers, J.; Monfort, Olivier; Montreuil, Anne-Lise

doi:10.2112/si88-006.1

Cited by 6 publications

(10 citation statements)

References 63 publications

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“…This easy-to-implement method could usefully complement airborne lidar surveys [32] which require a heavier implementation. The lidar survey allows quantifying the sedimentary budget but concerns only a small part of the bay close to the Mont.…”

Section: Discussionmentioning

confidence: 99%

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Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

Deroin¹

2023

Preprint

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The Mont-Saint-Michel is known worldwide for its unique combination of the natural site and the Medieval abbey at the top of the rocky islet. But the Mont is also located within an estuarine complex, which is considerably silting up. For two decades, large-scale works were planned to prevent the Mont from being surrounded by the expanding salt meadows. The construction of a new dam over the Couesnon River, the digging of two new channels, and the destruction of the causeway were the main operations carried out between 2007 and 2015. The remote sensing approach is fully suitable for evaluating the real impact of the engineering project in both time and space, particularly the expected large-scale hydrosedimentary effects, for reestablishing the maritime landscape around the Mont. Sentinel-2 satellite data have been used for the period from 2015 to 2023. Aster data were used for the previous period covering 2000 to 2017. Aerial photographs and an ALOS scene have been also used. The remote sensing approach is based on time-series images. It allows identifying local or regional consequences and temporary or permanent effects. The migration of the different channels (especially for the new west and east Couesnon river courses) and the erosion-progradation balance of the vegetation through space and time are the main features to study. Between 2007 and 2023, the erosion of the salt meadows is significant to the south-west of the Mont (− 150 ha) but more limited to the south-east (− 65 ha). The erosion effect is limited to the immediate environment because the vegetation fringe of the uppermost tidal flat along the main dike is slightly increasing (+ 35 ha) to the west and to the east (+ 40 ha). During the same period, the sedimentation considerably increased to the north-east of the Bay, between the Bec d’Andaine, the Grouin du Sud and Tombelaine islet, which seems now facing the same silting-up problem. At this stage, the remote-sensing survey indicates mixed results for the engineering project.

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Section: Discussionmentioning

confidence: 99%

“…Seminal lidar works were carried out for the study of the vegetation of the schorre [31]. More recently, lidar data have been acquired close to the Mont to control the efficiency of the new facilities [32] and to study facies models for tidal point bars lacking a three-dimensional perspective [33].…”

Section: Previous Remote Sensing Studiesmentioning

confidence: 99%

Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

Deroin¹

2023

Preprint

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“…This propagating uncertainty δu DoD is used as the limit of detections min LoD, as shown in Wheaton et al (2009). All elevation changes between − min LoD and + min LoD for each DoD were discarded (Levoy et al, 2019; Montreuil et al, 2014).…”

Section: Methodsmentioning

confidence: 99%

The formation and morphodynamics of complex multi‐hooked spits and the contribution of swash bars

Bas

Levoy

Robin

et al. 2021

Earth Surf Processes Landf

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Numerous studies have been devoted to recurved and hooked spits in micro-and meso-tidal environments, but none have really focused on the peculiar right-angle plan shape exhibited by some spit hooks. The morphodynamics of a complex multihooked spit in a macrotidal inlet in Normandy, France, have been investigated using aerial photographs and 12 topographic LiDAR surveys from 2000 to 2017, complemented by hydrodynamic data. Merville Spit is a 2.5 km-long hooked spit, composed of four 'nested' ridges each with a right-angle hook at its tip, that has developed in the vicinity of the sand-rich outlet of the Orne River. Monitoring of the two last ridges shows that ridge formation starts with the attachment of the tip of a swash bar to the fulcrum point of the previous ridge, and ends following its westward elongation and counter-clockwise rotation when its hook impinges on the Orne outlet.The main factor controlling ridge plan-view shape is swash bar dynamics in the vicinity of the spit. The offshore wave direction has no significant impact on ridge and hook behaviour due to the large number of swash bars on the ebb-tidal delta. Over the study period, between 5 and 10 swash bars massively assured sand supply to the upper foreshore, favouring the growth of existing ridges, but also enabling the building of a new ridge. The bars contribute to imparting hook right angles at the tips of the ridges by creating local wave refraction or by providing a sheltering effect. Strong tidal currents in the Orne channel act as a hydrodynamic barrier that stops hook elongation at the edge of the river's outlet. The similar behaviour of the two studied ridges has been synthesized into a new and original conceptual model of complex multi-hooked spit formation and evolution under the influence of swash bars.

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“…1A). It is a hypertidal environment, with a tidal range reaching up to 15.3 m during the highest spring tides (Larsonneur, 1989; Tessier, 1993; Larsonneur et al ., 1994; Levoy et al ., 2000) and 8.5 m for mean tides (Levoy et al ., 2019). The tidal regime is semi‐diurnal with slight periodical diurnal inequality.…”

Section: Study Area and Geological Settingmentioning

confidence: 99%

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

et al. 2021

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Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

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Short-term to Decadal-scale Sand Flat Morphodynamics and Sediment Balance of a Megatidal Bay: Insight from Multiple LiDAR Datasets

Cited by 6 publications

References 63 publications

Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

Evaluating the Impact of Engineering Works in Tidal Flats Using Time-Series Satellite Images – Case of the Mont-Saint-Michel Bay, France

The formation and morphodynamics of complex multi‐hooked spits and the contribution of swash bars

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

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