Jean-Marie Froidefond

Spectral signature of highly turbid waters: Application with SPOT data to quantify suspended particulate matter concentrations

An experimental method for determining water composition from ‘‘ocean colour’’ satellite data, in visible and near-infrared (NIR) wavelengths, is applied to highly turbid waters. Numerous spectroradiometric measurements are carried out in the Gironde estuary, for suspended particulate matter (SPM) concentrations ranging between 35 and more than 2000 mg l � 1 . Empirical relationships are established between remote-sensing reflectance (Rrs) in SPOT-HRV bands and SPM concentration through these numerous in situ measurements. We observed that remote-sensing reflectance increases with SPM concentration and that the SPOT bands saturate at the highest turbidities. The best correlations are obtained for the NIR band XS3 (790–890 nm) and for the reflectance ratios: Rrs(XS3)/Rrs(XS1) and Rrs(XS3)/Rrs(XS2). The XS1 and XS2 visible bands are only used to determine SPM concentrations in the lower part of the estuary (where the SPM concentrations are lower). As a result, SPM concentrations within the surface waters in the estuary are estimated up to 2000 mg l � 1 with an accuracy better than ±35%. The algorithm is finally applied to a SPOT scene. Satellite data are corrected for atmospheric effects using a radiative transfer code and in situ reflectance measurements; as a result, the horizontal distribution of SPM is retrieved. Moreover, the high spatial resolution HRV-SPOT sensor shows detailed sedimentary flows, especially in the visible XS1 and XS2 spectral bands. D 2002 Elsevier Science Inc. All rights reserved.

Migration of mud banks and changing coastline in French Guiana

Abstract The French Guiana coast is characterized by migrating mud banks which act not only as markers of hydrodynamic and climatic processes but also have an important impact on coastal evolution. The role of mud banks was documented by vertical aerial photographs taken in 1979, 1982 and 1984. These allow the delineation of the intertidal parts of the banks at low tide and at identical tidal ranges. The following results were obtained: (1) Six mud banks separated by interbank eroding areas are present along the French Guiana coast. The intertidal parts of three of these banks are cut by river mouths. (2) Between 1979 and 1984 the intertidal parts of the banks migrated to the west-northwest along the coast at rates ranging from 320 to 1220 m/yr (mean rate = 900 m/yr). (3) In the same period the extent of the intertidal areas colonized by new mangroves (60 km 2 ) was nearly the same as that of the areas bearing older mangroves (58 km 2 ) that were lost by erosion, while the nature of the surface covered by the intertidal parts of the banks remained nearly the same in spite of the migration of these banks. These results suggest the existence of hydrodynamic circulation cells linked to large coastal mud banks along northeastern South America. Additionally, historical data (1955–1972) show long-term variations in the velocity of mud-bank migration.

Optical algorithms at satellite wavelengths for Total Suspended Matter in tropical coastal waters

Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.

SPOT shallow water bathymetry of a moderately turbid tidal inlet based on field measurements

The determination of shallow water depth in high-energy tidal inlets is essential to model and to forecast the navigation channel position or the short-term topographic evolution of beaches. Colour satellite imagery provides, at low cost, complete maps of areas that are difficult to map by traditional hydrographic means due to their size and their rough underwater morphology. In order to calibrate SPOT images to derive bathymetric maps, a simple method applied to shallow waters of a moderately turbid tidal inlet has been carried out. Its development is based on a set of field measurements, including the reflectance of the water and of the bottom sediment, the vertically averaged diffuse attenuation coefficient, and the concentrations of inorganic particles in suspension, of chlorophyll a and pheopigments and of dissolved organic carbon (DOC). From these data, it appears, first, that the water reflectance is directly linked to the depth, and second, that the water reflectance varies slowly with the turbidity in this area for total suspended matter (SM) concentrations lower than 9 mg l−1. The extinction of light with depth has been characterized in the inlet. The relationship obtained is slightly different from that observed in clear waters. The bathymetric models established for the clearest waters do not calculate the depth accurately. In the contrary, reflectance code adapted to Case II water type, calibrated with in situ measurements, allows us to retrieve depth down to 6 m and for total SM concentrations ranging from 0.2 to 9 mg l−1. This relationship has been applied to five SPOT images of the mouth of Arcachon lagoon. The accuracy of the derived map has been assessed by in situ depth measurements. The mean difference between measured depths and computed depths is about 20%. This accuracy is adequate to assess the inlet morphodynamics quantitatively, which is necessary for middle-term to short-term mathematical modeling of the 3D inlet evolution.

Spectral remote sensing reflectances of coastal waters in French Guiana under the Amazon influence

The French Guiana coast, located about 500 km north of the Amazon mouth, is characterized by mangroves and large mud banks migrating northwestwards. These banks are composed of fine clay sediments originating from the Amazon River. In November 1998, field work was carried out on the Mahury River to measure the water colour and the concentration of suspended particles in surface water. An increase of the remote sensing reflectance due to high concentration of mineral particles was observed. The Mahury River survey (November 1998), together with an oceanographic survey onboard the ANTEA vessel (PRECHICO cruise, April 1999), enabled us to identify several classes of remote sensing reflectance spectra characterizing the continental shelf between Cayenne and the Brazil frontier: (1) light blue waters above the shelf break; (2) low salinity dark brown waters above the continental shelf, originating from the Amazon River; (3) green waters above the 20-m isobath; and (4) beige waters close to the coast and in the rivers mouths. The normalized water leaving radiances (nLw) computed from these spectra in the spectral bands of SeaWiFS are examined. The standard OC2 performs poorly in retrieving chlorophyll from the reflectance data because of the relatively high suspended particulate matter concentrations.

Distribution of suspended matter in a coastal upwelling area. Satellite data and in situ measurements

Oceanographic measurements and satellite images recorded during the same period show that the data from the visible wavelengths of channel 1 and 2 of the AVHRR sensor (satellite NOAA-11) can be used to establish the distribution of total suspended particulate matter (TSPM) on the inshore continental shelves. The near-infrared channel (C2 = 725–1100 nm) is used for atmospheric corrections of channel C1 (580–680 nm) when concentrations in TSPM are low. However, when TSPM are high, they influence the channel C2 data. In this case, we use a subtraction between C1 data and a C1 value as constant. The fit function obtained between the TSPM measured by filtration and the corrected satellite reflectances is relatively linear for the low concentrations. This fit function permits to attribute an estimated concentration value to each satellite reflectance. In the Bay of Biscay, the additional use of a multiparameter probe and of AVHRR/NOAA-11 infrared data (channels 4 and 5, thermic infrared) makes it possible to show the presence of a coastal upwelling. The cold water rises to the surface near the coast. It is enriched in suspended matter, both removed by the waves and discharged by the rivers. These waters are then driven westwards, by the tidal currents and the continental northerly winds. The observation of complementary data showed that the coastal upwelling was active during the entire summer period of 1989 and possibly contributed to the cooling of coastal waters.

Analysis of the turbid plume of the Gironde (France) based on SPOT radiometric data

Abstract A SPOT satellite image covering the Gironde estuary turbid plume during a flood (flow > 2000 m 3 /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.

Method for the quantification of suspended sediments from AVHRR NOAA-11 satellite data

Abstract Oceanographic data (Ecofer, Givapa and Gastel cruises) and satellite images evidence that the channels 1 and 2 (visible and near infra-red regions) of AVHRR/NOAA-11 can be used to estimate the total concentration of suspended sediments on the sea surface, with a daily frequency when under cloudless skies. The near infra-red channel is used for atmospheric corrections. The fit function obtained between the concentrations determined by filtration and the corrected satellite data is linear between 0-5mgl -1 and lOmgl-1. The satellite data are also compared to ship data scattering measurements. The results match. This fit function is applied to each satellite value to obtain the estimated concentration. The total weight of the suspended sediments is calculated by multiplying each of the estimated concentrations by the area and by the number of pixels. The relative errors are evaluated to provide an interval of accuracy. This method is applied to the inshore continental shelf off the mouth of the Giro...

Water masses and suspended matter circulation on the French Guiana continental shelf

Abstract This paper discusses the Amazonian solid and liquid fluxes along the coast of French Guiana. The seasonal hydrology and meteorology shows that suspended matter is locally limited to the zone of bank migration in a nepheloid layer. There is more turbidity on the bank as a result of resuspension of significant amounts of particles. The suspended load on the shelf is subject to the circulation of water masses: at the surface the Guiana Current and at the bottom tidal currents (inner shelf) and local counter current (outer shelf). The path of the currents emphasizes that the low amount of suspended matter observed offshore do not influence the build up of the mud prism, which is due to coastal transport. The energy flux and the volume of sediments transported in the banks has been calculated and the results are comparable to those obtained in Guiana and Suriname. However, variable coastal morphology induces differences in energy fluxes. As a result, some areas undergo both erosion and sedimentation.

Variabilité du panache turbide de la Gironde par télédétection. Effets des facteurs climatiques

Abstract The Gironde estuary (France) discharges to the ocean an important amount of suspended particulate matter in the form of turbid plumes. The surface plume is more particularly studied from coastal oceanographic surveys and NOAA/AVHRR satellite data collected during a French programme of coastal oceanography (PNOC-Atlantic). The AVHRR reflectances are atmospherically corrected according to an algorithm based on the clear water concept. The comparison with suspended sediment concentrations are realised by direct and indirect calibrations. The correlations obtained show that at a same concentration the reflectance varies, probably because of varying particle (floc) size and composition. The shape and the surface measured from low resolution (4 km × 4 km) and high resolution (1 km × 1 km) AVHRR data are then compared to the forcings introduced by the fluvial output, the tide, neap and spring tide and the wind variations. The latter have an important effect on the orientation of the distal part of the turbid plume.