In-water reflectance spectra measured on-board a jet-ski across a complex nearshore zone of case-2 waters during the ECORS experiment

International audienceIn situ reflectance spectra have been measured during the international field experiment ECORS-Truc Vert 2008 (SW France) in the nearshore zone over a complex bathymetry and in moderately turbid waters (SPM 7 m). The jet-ski provides a valuable mean to gather optical data in shallow waters and in environments hard to sample with traditional coastal ships. Main results show reflectance spectra are typical of the presence of mineral particles with light absorption at short wavelengths. The shape and magnitude of the spectra are also very representative of the type of waters and bottom depth : the maximum of reflectance is reached around 500 nm in low turbidity waters (SPM<1 mg/l) while moving to the yellow spectral band (570-580 nm) in moderately turbid shallow waters (SPM=4 mg/l) where rip currents drive sediments outside the surf zone. A very original dataset has been obtained considering the synchronization of optical data, bathymetric surveys and Formosat-2 high resolution satellite image all collected the same day (5 April 2008). In the present study, this dataset is tested to inverse optical models for bathymetry retrieval in order to compute diachronic bathymetric maps as the coastline is changing very fast and need frequent updates of the bathymetry

Extreme winter storm versus Summer storm: morphological impact on a sandy beach

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Pocket beach hydrodynamics : the example of four macrotidal beaches, Brittany, France

International audienceDuring several field experiments, measurements of waves and currents as well as topographic surveys were conducted on four morphologically-contrasted macrotidal beaches along the rocky Iroise coastline in Brittany (France). These datasets provide new insight on the hydrodynamics of pocket beaches, which are rather poorly documented compared to wide and open beaches. The results notably highlight a cross-shore gradient in the magnitude of tidal currents which are relatively strong offshore of the beaches but are insignificant inshore. Despite the macrotidal setting, the hydrodynamics of these beaches are thus totally wave-driven in the intertidal zone. The crucial role of wind forcing is emphasized for both moderately and highly protected beaches, as this mechanism drives mean currents two to three times stronger than those due to more energetic swells when winds blow nearly parallel to the shoreline. Moreover, the mean alongshore current appears to be essentially wind-driven, wind waves being superimposed on shore-normal oceanic swells during storms, and variations in their magnitude being coherent with those of the wind direction. We also found evidence for the contribution of infragravity waves to the formation of an intertidal sand bar (through prolonged duration of swash processes at the same beach level) and to the development of beach cusps (but not to their initiation), for which the observed standing edge wave pattern may have been enhanced by the enclosed pocket-beach setting. The paper highlights some of the specificities of pocket beach with regards to their hydrodynamics and morphodynamics. More detailed observations would be needed, however, prior to a more comprehensive review

Courants induits et dissipation de l'énergie des vagues sur les plages macrotidales de la mer d'Iroise

National audienceSeveral field experiments of both morphological and hydrodynamical measurements occured during springs 2004 and 2005 on four beaches of the Iroise Sea (Finistère). Within these four datasets (sea surface elevation, cross-shore and longshore flow velocities), dissipation of wave energy is investigated through time (accounting with tidal cycle) and space (across the intertidal zone). These results are compared to empirical parametrizations of relative wave height Γs (Raubenheimer et al., 1996 ; Sénéchal et al., 2001), parameter greatly used in hydrodynamic models. Then, the local breaking criteria Γs,b is used to characterize wave-related processes (wave asymetry, breaking-induced undertow) involved in sediment transport in the intertidal zon

Performance and Uncertainty of Satellite-Derived Bathymetry Empirical Approaches in an Energetic Coastal Environment

International audienceObjectives of this study are to evaluate the performance of different satellite-derived bathymetry (SDB) empirical models developed for multispectral satellite mission applications and to propose an uncertainty model based on inferential statistics. The study site is the Arcachon Bay inlet (France). A dataset composed of 450,837 echosounder data points and 89 Sentinel-2 A/B and Landsat-8 images acquired from 2013 to 2020, is generated to test and validate SDB and uncertainty models for various contrasting optical conditions. Results show that water column optical properties are characterized by a high spatio-temporal variability controlled by hydrodynamics and seasonal conditions. The best performance and highest robustness are found for the cluster-based approach using a green band log-linear regression model. A total of 80 satellite images can be exploited to calibrate SDB models, providing average values of root mean square error and maximum bathymetry of 0.53 m and 7.3 m, respectively. The uncertainty model, developed to extrapolate information beyond the calibration dataset, is based on a multi-scene approach. The sensitivity of the model to the optical variability not explained by the calibration dataset is demonstrated but represents a risk of error of less than 5%. Finally, the uncertainty model applied to a diachronic analysis definitively demonstrates the interest in SDB maps for a better understanding of morphodynamic evolutions of large-scale and complex coastal system

Ocean color measurements onboard a jet-ski : consistency for calval exercise of high-resolution satellite imagery?

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Transient swash motions on a gently-sloping beach

International audienceSpectral and wavelet analyses are used to investigate run-up motion on a gently sloping beach with special attention devoted to the coupling between the long-period swash oscillations and the wave grouping outside the surf zone. Runup elevations were measured under random-wave conditions during the laboratory GLOBEX experiments carried out along a 1:80 sloping, fixed beach. A capacitance wire was deployed parallel to the bottom in order to detect the swash edge. The low Iribarren numbers involved in the experiments led to swash motions dominated by surf beat. High correlation levels between the scale-averaged wavelet power and the run-up suggest that low-frequency swash motions are still associated with the modulation of offshore swell waves. In addition, wavelet analysis supports the idea that swash-swash interactions are important in controlling swash oscillations

Observations of waves' impact on currents in a mixed-energy tidal inlet : Arcachon on the southern French Atlantic coast

International audienceCoastal morphodynamic processes around tidal inlets in mixed-energy environments are particularly complex due tosevere tide and wave conditions. Collecting data in this area is generally very challenging .Here we present the first hydrodynamic data collected in the outer inlet of the Arcachon lagoon, a mixed-energy inlet situated on the southern part of the French Atlantic coast. Data consist in vertical current profiles and sea surface elevations collected on the offshore edge of the ebb delta under various tidal and wave conditions. In particular datawere collected during the severe Joachim storm associated to wave heights up to 8 m. Preliminary results indicate that current profiles, intensities and direction are very sensitive to wave conditions. Under energetic conditions, the vertical profile of currents becomeuniform and the longshore drift is enhanced (up to 1.5 m/s). Nevertheless data show that under severe wave conditions(Hm0> 6.0m) and despite high incidence of waves, the longshore drift is weaker (l< 1 m/s) than under ‘usual’ stormconditions associated to Hm 0= 4–5m. More surprisingly, directions of the longshore drift are not always consistent with wave incidenceunder those severe storm conditions and the cross-shore components are very weak while under less energetic conditions weobserve intense offshore currents(upto 0.8 m/s)

New outcomes from spatial remote sensing during the ECORS experiment : towards validation of ocean color products and large-scale bathymetry mapping in a coastal zone

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