Patrice Castaing

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.

The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon

Some basic features concerning the subtidal circulation of Patos Lagoon were studied through time series analysis of wind, freshwater discharge, and water level records, as well as by means of experiments carried out with a 3D numerical model. The results indicate that during low to moderate river discharge the wind is the main forcing mechanism in time scales associated with meteorological fronts. The two types of wind action, local and non-local effects, are distinguished and their relative importance is evaluated. Salt water enters the system due to a combination of both remote and local wind effects that favors the development of a pressure gradient towards the lagoon during southwesterly winds. This situation is reversed when northeasterly winds dominate. In the inner parts of the lagoon, local wind plays the major role by inducing set up/set down oscillations. An upwind return flow is then developed under these conditions. During high flood periods, normally observed in late winter, the circulation is driven by freshwater discharge.

Mechanisms controlling seaward escape of suspended sediment from the Gironde: A macrotidal estuary in France

Abstract The main parameters controlling suspended-sediment transport in and out of the Gironde estuary are river flow and tides. These combine to control water mixing, density circulation and the trapping of suspended sediment in the turbidity maximum. Seaward escape of sediment is related to the position of the maximum in the estuary, which is a function of river flow; and the amount of sediment in suspension as controlled by tide range. Seaward escape of sediment appears to follow two cycles: (1) a 14-day neap—spring tidal cycle, and (2) a seasonal river flow cycle. Maximum escape occurs during spring tides and high river flow. During these periods, the turbidity maximum is located downstream in the estuary and the increased seaward surface density flow supplies to the shelf large quantities of suspended sediment. At this time, net seaward flux is more than 100 times that measured during conditions of low river flow. Thermal imagery and aerial photography in the inlet, repeated during a spring tide cycle, show an initial seaward drift of the surface estuarine water and suspensions, followed by a northward transport and mixing with water from the adjacent inlet. In succeeding tides, the turbid plume is dispersed, fragmented and transported seaward and to the north in the coastal drift system. Most of the suspended sediment expelled from the estuary is deposited on the shelf between 30 and 70 m depth, where wave resuspension is low and where suspended-sediment concentrations are high enough to supply and maintain a permanent mud deposit.

A synthesis on seasonal dynamics of highly-concentrated structures in the Gironde estuary

Two turbidity peaks can be identified in the Gironde estuary waters. The first one, in a steady position, is possibly related to a mud-trapping zone in the main channel. The second one is the actual turbidity maximum, capable of shifting seasonally along the estuary. Its length and position can be readily predicted as functions of the river flow. Fluid mud presents a similar seasonal pattern, but its occurrence is sensitive to the channel cross-section, especially to its width, which modifies local hydrodynamics and water-bottom exchanges. These observations allow accurate prediction of mud deposition zones according to the season.

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.

The particulate 7Be/210Pbxs and 234Th/210Pbxs activity ratios as tracers for tidal-to-seasonal particle dynamics in the Gironde estuary (France): Implications for the budget of particle-associated contaminants

The short-lived natural radionuclides (7)Be (T(1/2)=53 days), (234)Th(xs) (T(1/2)=24.1 days) and (210)Pb(xs) (T(1/2)=22.3 years), i.e. (234)Th and (210)Pb in excesses of that supported within particles by the decay of their parent isotopes, were analysed in suspended particulate matter (SPM) to study the particle dynamics in the Gironde fluvial estuarine system (France), strongly impacted by heavy metal pollution. From surveys of this land-ocean interface in 2006 and 2007, we established a times series of these radioisotopes and of their activity ratios ((7)Be/(210)Pb(xs) and (234)Th/(210)Pb(xs) ARs) in particles sampled under different hydrological conditions. The particulate (7)Be/(210)Pb(xs) AR varies along the fluvial estuarine system mainly due to variations in (7)Be activities, controlled by riverine, oceanic and atmospheric inputs and by resuspension of old (7)Be-deficient sediments. These processes vary with river discharge, tidal cycle and season. Therefore, seasonal particle transport processes can be described using variations of the SPM (7)Be/(210)Pb(xs) ARs. During high river discharge, the SPM (7)Be/(210)Pb(x) ARs decrease from river to the ocean. The turbidity maximum zone (TMZ) is dispersed and the particles, and the associated contaminants, are rapidly transported from river to coastal waters, without significant retention within the TMZ. During low river discharge, the TMZ intrudes into the fluvial estuary, and the lowest (7)Be/(210)Pb(x) ARs are observed there due to resuspension of (7)Be-deficient sediments. Away from the TMZ, from the middle to lower estuary, SPM (7)Be/(210)Pb(x) ARs increase, indicating that the particles have been recently tagged with (7)Be. We explain this trend as being caused by marine input of dissolved radionuclides, as traced by SPM (234)Th/(210)Pb(xs) ARs, followed by scavenging in the estuary. This result indicates that particle transport models based on (7)Be and trace-metal budgets must consider oceanic dissolved inputs as an additional source of (7)Be and, possibly, of contaminants to estuaries.

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.

Hydrographical Characteristics of the Estuarine Area of Patos Lagoon (30°S, Brazil)

The Patos Lagoon (Fig. 5.1a) situated in Southern Brazil, between 30 and 32°S is a very important water resource system for the Rio Grande do Sul State. It serves as nursery ground for important commercial fish and shrimp species sustaining a fishery production of 182 kg ha-1 yr-1 in average (Castello 1985). Major man activities are navigation and recreation. It also receives untreated domestic, agricultural and industrial sewage disposals produced by population living along the margins of the lagoon. The construction of two 4 km long jetties fixed the entrance and allowed navigation along the entrance channel. About 14 million m3 of sediments where trans-ported to the coastal zone in the first two years after the construction of these structures (Motta 1969). In recent years the channel has been constantly dredged to keep depth around 14 m.

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...