In-water reflectance spectra measured on-board a jet-ski across a complex nearshore zone of case-2 waters during the ECORS experiment
Résumé
In 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 <10 mg/l). Measurements of the atmospheric downwelling irradiance (Ed) and in-water upwelling radiance (Lu) were realized on board a jet-ski with TRIOS RAMSES sensors together with water samples to assess the optically significant seawater constituents (SPM, CDOM, Chl-a). Marine reflectance spectra were collected under low-energy swell conditions (Hs=0.5 m) in water depths of 3-25 m and after a long period of very energetic swells (Hs > 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.
Domaines
Océanographie
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