MAPPING TIDAL AND SEASONAL MOVEMENTS OF THE MAXIMUM TURBIDITY ZONE IN ESTUARINE WATERS FROM REMOTELY SENSED

jm.froidefond(at)epoc.u-bordeaux1.fr The study concerns the quantification of suspended sediments in turbid estuarine waters from high spatial resolution remotely sensed data (SPOT-HRV, Landsat-ETM+). It is applied to the following areas: the Gironde and the Loire estuaries (France), where suspended sediment concentrations (SPM) in surface waters vary from 10 to 2000 mg l-1. The methodology is essentially empirical. Based on in situ optical measurements, calibration rela-tionships are established between the remote sensing reflectance (Rrs) signal and SPM. These relationships, obtained using reflectance ratios between near-infrared (NIR) and visible (VIS) wavebands, are relatively independent of the sediment grain-size and mineralogy, and of the illu-mination conditions (e.g. the cloud cover). Consequently, they can be applied to satellite images, even if no simultaneous in situ measurements were carried out concurrently with the satellite over-pass. A bio-optical model is used to explain the obtained empirical results. It relates the inherent and apparent optical properties of the considered estuarine waters. Scattering by sediments is mod-elled using the Mie theory. Absorption by sediments is modelled according to the existing knowl-edge in the domain. The modelled optical properties are validated by comparison with in situ measurements. The established calibration relationships allow an accurate estimation of concentrations, once sat-ellite data are corrected for atmospheric effects. Resulting SPM maps permit to observe the sea-sonal movements of the maximum turbidity zone (MTZ) and the resuspension phenomena in the Gironde estuary