ANALYSIS OF THE TURBID PLUME OF THE GIRONDE (FRANCE) BASED ON SPOT RADIOMETRIC DATA

A SPOT satellite image covering the Gironde estuary turbid plume during a flood (flow > 2000 m3/s) is analyzed. Radiometric data from the three SPOT channels are compared. The findings show that the XS1 (500-590 nm) and SX2 (610-680 nm) channels are complementary for the study of the turbid plume structure; the XS1 channel shows variation ranges higher than XS2 for low radiometric values (low turbidity) and, conversely, the XS2 channel shows range variations higher than XS1 for high radiometric values. The histogram and isocontours of a combination of the two channels (XS1 + XS2) shows the presence of four suspended sediment surface layers discharged over a 48-h period. Two areas of clearer water are located on each side of the turbid plume. They correspond to the lateral penetration of ocean water associated with tidal currents. The XS2 channel variations are compared to the suspended matter concentration measurements. Flux evaluations based on regression analysis are stated. Uncertainty concerning the calibration of the satellite data and the thickness variations of the turbid plume cause a great deal of inaccuracy for the mass transported over a 48-h period.