Xavier Bertin

Numerical modeling of the impact of the Ancão Inlet relocation (Ria Formosa, Portugal)

This work describes the application of hydrodynamic (ELCIRC) and transport (VELA and VELApart) models to the Ria Formosa lagoon (Portugal) to study the impact of the relocation of the Ancao Inlet. Located in the south of Portugal, this lagoon is a mesotidal barrier island system that communicates with the sea through 6 inlets. The Old Ancao Inlet was artificially closed and the New Ancao Inlet was relocated into a westward position. This work investigates the hydrodynamic patterns and the potential pathways of tracers in Ria Formosa in two distinct configurations: before and after the Ancao Inlet relocation. The hydrodynamic model was successfully calibrated and validated against elevation, velocity and inlet discharges data, accurately reproducing the tidal propagation. The inlet relocation increases the magnitude of tidal currents, residual velocities and the tidal prism across the bar, suggesting a better stability. The tracers transport simulations suggest enhanced water exchanges through the Ancao Inlet and smaller residence times in the western part of Ria Formosa with the present configuration. Overall, it is concluded that the Ancao Inlet relocation had a positive contribution towards increasing the water renewal of the western part of the lagoon, thus decreasing its vulnerability to pollution.

Morphological evolution and time-varying bedrock control of main channel at a mixed energy tidal inlet: Maumusson Inlet, France

Abstract Maumusson Inlet, located on the French Atlantic coast, connects the Atlantic Ocean with the Marennes–Oleron tidal bay. The tidal range (2–6 m) and wave climate (mean height 1.5 m) place this tidal inlet in the mixed energy, tide dominant, category of Hayes [(1979) Barrier island morphology. In: Leatherman, S.P. (Ed.), Barrier Island, Academic Press, New York, pp. 1–28]. An innovative method, combining high quality bathymetric data (nine accurate Digital Elevation Models since 1824) with a very high seismic resolution, demonstrates a major tidal inlet evolution from 1824 onwards and its dramatic acceleration since 1970. The chronology of those morphological changes suggests strong coupling between the location of the tidal channel and the behaviour of the adjacent shorelines. The recent shoaling and migration of the inlet channel can be attributed to a decrease in tidal prism due to the filling in sediment of Marennes–Oleron Bay. Seismic data give evidence that the inlet was located on a major incision of the bedrock. It can be inferred that the bedrock exerts control of channel location, this control varying in time as a function of channel depth. A conceptual model is proposed, including the inlet, its adjacent shorelines, the tidal bay and the time-varying bedrock control of main channel location. Such a model could be considered valid in similar cases along other coastlines, i.e. coastlines with a fine unconsolidated sediment sheet.

Monitoring the Topography of a Dynamic Tidal Inlet Using UAV Imagery

Unmanned Aerial Vehicles (UAVs) are being increasingly used to monitor topographic changes in coastal areas. Compared to Light Detection And Ranging (LiDAR) data or Terrestrial Laser Scanning data, this solution is low-cost and easy to use, while allowing the production of a Digital Surface Model (DSM) with a similar accuracy. Three campaigns were carried out within a three-month period at a lagoon-inlet system (Bonne-Anse Bay, La Palmyre, France), with a flying wing (eBee) combined with a digital camera. Ground Control Points (GCPs), surveyed by the Global Navigation Satellite System (GNSS) and post-processed by differential correction, allowed georeferencing DSMs. Using a photogrammetry process (Structure From Motion algorithm), DSMs and orthomosaics were produced. The DSM accuracy was assessed against the ellipsoidal height of a GNSS profile and Independent Control Points (ICPs) and the root mean square discrepancies were about 10 and 17 cm, respectively. Compared to traditional topographic surveys, this solution allows the accurate representation of bedforms with a wavelength of the order of 1 m and a height of 0.1 m. Finally, changes identified between both main campaigns revealed erosion/accretion areas and the progradation of a sandspit. These results open new perspectives to validate detailed morphological predictions or to parameterize bottom friction in coastal numerical models.

The sources of deep ocean infragravity waves observed in the North Atlantic Ocean

Infragravity waves are long-period (25–250 s) ocean surface gravity waves generated in coastal zones through wave-wave interactions or oscillation of the breaking point. Most of the infragravity wave energy is trapped or dissipated near coastlines, but a small percentage escapes into the open oceans. The source of deep ocean infragravity waves is debated, specifically whether they come mostly from regions with strong source waves or from sites with particular morphologies/orientations. We correlate measurements of infragravity waves in the deep North Atlantic Ocean with infragravity wave generation parameters throughout the Atlantic Ocean to find the dominant sources of deep ocean infragravity wave energy in the North Atlantic Ocean. The deep ocean infragravity wave data are from a 5 year deployment of absolute pressure gauges west of the Azores islands (37°N, 35°W) and shorter data sets from seafloor tsunami gauges (DART buoys). Two main sources are identified: one off of the west coast of southern Europe and northern Africa (25°N–40°N) in northern hemisphere winter and the other off the west coast of equatorial Africa (the Gulf of Guinea) in southern hemisphere winter. These regions have relatively weak source waves and weak infragravity wave propagation paths to the main measurement site, indicating that that the site morphology/orientation dominates the creation of deep ocean infragravity waves. Both regions have also been identified as potential sources of global seismological noise, suggesting that the same mechanisms may be behind the generation of deep ocean infragravity waves and global seismological noise in the frequency band from 0.001 to 0.04 Hz.

Seasonal to Decadal Variability of Longshore Sand Transport at the Northwest Coast of Portugal

AbstractLongshore sediment transport (LST) is a major driver of coastal evolution. However, despite the recognition that it presents an unsteady behavior at the seasonal scale, the variability at longer term scales (interannual and decadal) is still far from being properly acknowledged. The present work contributes to the understanding of the seasonal to decadal variability of the LST, benefiting from recent developments in wave hindcast modeling. This work was developed for the northwest coast of Portugal, which is fully exposed to the highly energetic wave regime generated in the Northeast Atlantic Ocean that induces unusually large LST rates. Hindcast offshore waves, between 1953 and 2010, were used as offshore forcing to deduce LST estimates. The mean annual LST, between 1953 and 2010, shows an irregular and noncyclic pattern. Computed mean annual LST, for this coastal stretch, is around 1 million cubic meters directed to the south, with yearly averages ranging from 108,000 to 2.24 million m3 year−1 a...

RELEVANCE OF INFRAGRAVITY WAVES IN A WAVE-DOMINATED INLET

This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave-dominated inlet located on the western coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low-frequency oscillations (i.e., 25–300 s) in water levels and current velocities. While these fluctuations were present over the ebb-tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to 2 h after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m s−1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions.

Limitation of High Water Levels in Bays and Estuaries During Storm Flood Events

This study, which involves process-based numerical models validated with reference to tide gauge measurements, tends to specify in what extent the flooding behind the coastline may limit the water levels reached during exceptional events.

Participatory simulation to foster social learning on coastal flooding prevention

Abstract Due to the increase in coastal flooding risk associated with sea-level rise and increasing population along the coasts, there is a strong need to develop efficient and long-term management strategies. In partnership with the local administration of Oleron Island in France, a participatory simulation model was developed to foster social learning about coastal risk prevention measures with local authorities and managers. This simulation integrates a coastal flooding model and a spatially explicit agent-based model that simulates the development of the area and the management of prevention measures. The participatory set-up includes an immersive environment for participants to remember the coastal flooding simulation displayed and a role game mechanism that simulates the coordination issues between the different decision bodies involved in coastal risk management. A first application proved that participants learn about the water expansion dynamics during flood events and the effects of building, raising and restoring dikes.

Effect of inlet morphology and wave action on pollutant pathways and sediment dynamics in a coastal stream

Hydrodynamics and water renewal of intermittent coa stal streams are highly variable, at various time scales, due to the very active morphod ynamic behavior of their inlets. Due to this strong dynamics, the pathways of water-born e materials – and the consequences of contaminated discharges – can depend strongly on the morphology and environmental conditions. Predicting the fate of contaminants in these systems requires coupled numerical models accounting for the major physical and water quality processes. We aim at improving the understanding of the impact of inlet morphology and wave action on the pollutant and sediment pathways of th ese small coastal systems, based on a suite of calibrated and validated coupled models. T wo analyses, based on particle simulations, are presented to assess sediment dynam ics and pollutant pathways for several conditions. Results show that waves have a major effect on the fate of waterborne materials in the estuary. Wave-induced curren ts sweep away materials coming out of the estuary, while wave-induced setup has a prof ound effect on tidal propagation, water levels and velocities in the estuary, promoti ng he upstream transport of pollutants. 1 National Laboratory for Civil Engineering, Av. do B rasil, 101, 1700-066 Lisbon, Portugal; {aoliveira, afortunato, mguerreiro, xbertin, nbrune au, mfrodrigues, pfreire, lsimoes, gdodet, amendes}@lnec.pt. 2 Faculdade de Ciências da Universidade de Lisboa, Ca mpo Grande, Ed. C6, 1749-016 Lisboa, Portugal; {rtaborda, candrade, amasilva, cm antunes}@fc.ul.pt. 3 CIMA, Universidade do Algarve, Campus de Gambelas, Ed. 7, 8005-139 Faro, Portugal; cloureiro@ualg.pt.