Impact of the spatial resolution of satellite remote sensing sensors in the quantification of total suspended sediment concentration: A case study in turbid waters of Northern Western Australia

The impact of anthropogenic activities on coastal waters is a cause of concern because such activities add to the total suspended sediment (TSS) budget of the coastal waters, which have negative impacts on the coastal ecosystem. Satellite remote sensing provides a powerful tool in monitoring TSS concentration at high spatiotemporal resolution, but coastal managers should be mindful that the satellite-derived TSS concentrations are dependent on the satellite sensor's radiometric properties, atmospheric correction approaches, the spatial resolution and the limitations of specific TSS algorithms. In this study, we investigated the impact of different spatial resolutions of satellite sensor on the quantification of TSS concentration in coastal waters of northern Western Australia. We quantified the TSS product derived from MODerate resolution Imaging Spectroradiometer (MODIS)-Aqua, Landsat-8 Operational Land Image (OLI), and WorldView-2 (WV2) at native spatial resolutions of 250 m, 30 m and 2 m respectively and coarser spatial resolution (resampled up to 5 km) to quantify the impact of spatial resolution on the derived TSS product in different turbidity conditions. The results from the study show that in the waters of high turbidity and high spatial variability, the high spatial resolution WV2 sensor reported TSS concentration as high as 160 mg L-1 while the low spatial resolution MODIS-Aqua reported a maximum TSS concentration of 23.6 mg L-1. Degrading the spatial resolution of each satellite sensor for highly spatially variable turbid waters led to variability in the TSS concentrations of 114.46%, 304.68% and 38.2% for WV2, Landsat-8 OLI and MODIS-Aqua respectively. The implications of this work are particularly relevant in the situation of compliance monitoring where operations may be required to restrict TSS concentrations to a pre-defined limit

The spectral reflectance and transparency of river plume waters

The goal of this paper is to present in situ data for the reflectance and transparency of river plumes and to interpret the observed variability in terms of hydrodynamic processes. The data was obtained for river plumes in the Black Sea and in Arctic Ocean regions. Spectral reflectance was measured in the 400-700 nm band by a ship/helicopter-mounted spectroradiometer and was supplemented by shipboard measurements of vertical profiles of the light attenuation coefficient. Remotely-sensed data shows the transition from freshwater to seawater with both sharp boundary fronts and wide mixing zones. Internal fronts and quasi-periodic spatial variability of reflectance were also observed within the river plume. The observations are related to the relevant hydrodynamic processes (fronts, advection and settling of particulate matter, internal waves). A radiance model was developed to match the vertical profile of optical properties to the observed variation of remotely-sensed reflectance

Location of the Rhine Plume Front By Airborne Remote-sensing

The aim of this study was to determine the feasibility of using airborne remote sensing to locate the Rhine plume front. Interest in fronts arises from the desire to predict the fate of pollutants and biological nutrients discharged from rivers into the open sea. Observations were made during flights over the Dutch coastal waters using a vertically-mounted video camera and a side-looking airborne radar (SLAR) designed for oil slick detection. Comparison of radar images with visual observations of the sea colour discontinuity and foam line establish that fronts can indeed be detected by SLAR because of high radar backscatter along the convergence line, where the fresh water jet impinges on saltier water. This provides a sound basis for future investigations using Synthetic Aperture Radar as mounted on ERS-1. An estimation of errors is given, identifying priorities for improvement of the technique. The accuracy achieved is considered sufficient for the validation of hydrodynamic models

Optical remote sensing of chlorophyll a in case 2 waters by use of an adaptive two-band algorithm with optimal error properties

Two-band algorithms that use the ratio of reflectances at 672 and 704 nm have already proved successful for chlorophyll a retrieval in a range of coastal and inland waters. An analysis of the effect of reflectance measurement errors on such algorithms is made. It provides important indications of the range of validity of these algorithms and motivates the development of an entirely new type of adaptive two-band algorithm for hyperspectral data, whereby the higher wavelength is chosen for each input spectrum individually. When one selects the wavelength at which reflectance is equal to the reflectance at the red chlorophyll a absorption peak, chlorophyll a retrieval becomes entirely insensitive to spectrally flat reflectance errors, which are typical of imperfect atmospheric correction, and is totally uncoupled from the retrieval or an estimation of backscatter. This new algorithm has been tested for Dutch inland and Belgian coastal waters. (C) 2001 Optical Society of America

Presentation of a family of turbulence closure models for stratified shallow water flows and preliminary application to the Rhine outflow region

Three turbulence closure schemes, designed for stratified shallow water flows, are presented. They are based upon k-epsilon theory and use respectively two, one or zero transport equations for turbulent variables. The models are first tested on the evolution of a wind-driven turbulent layer in a stratified fluid. The results are at least qualitatively in agreement with observational and experimental data. A discussion is given about the existence of self-similar solutions. The models are compared next with the observational data of the Rhine outflow area. The periodic variation in the density structure, forced by wind and tides and which is clearly visible in the data, is predicted by the model. A physical interpretation of the model results is given in the absence of wind forcing. The effects of estuarine circulation, tidal straining and mixing on the development or breakdown of stratification are well represented by the model calculations

Haline Stratification in the Rhine-meuse Fresh-water Plume - a 3-dimensional Model Sensitivity Analysis

Results are presented of a three-dimensional model study of the tidally-averaged salinity field in the Rhine-Meuse plume. In conditions of low mixing (no wind, neap tide) freshwater emerges from the river as a jet, turns right under the influence of Coriolis force and attaches to the coast as a buoyancy current. Surface residual currents are essentially geostrophic. Within the plume the surface layer is strongly stratified and overlies a bottom mixed layer. Results are strongly sensitive to the parameterization of vertical mixing: models ranging from constant diffusion coefficients through simple algebraic Richardson number dependent formulations to turbulence closure with evolution equations for turbulent kinetic energy and length scale were tested. Turbulence closure with a single evolution equation for turbulent kinetic energy and an algebraic length scale formulation was found to provide a suitable balance between physical realism (assessed by theoretical considerations and the practical ability to represent a well-mixed bottom boundary layer and stratified surface layer) and computational efficiency. Simpler models, which Vary diffusion coefficients as function of Richardson number, may produce similar results, though require more careful calibration. Constant diffusion coefficients are clearly inadequate for the application considered. Even the preferred ''k'' model requires some calibration as a ''background'' or ''ambient'' mixing coefficient had to be introduced to avoid unrealistically strong stratification. The tidally-averaged salinity field was found to be qualitatively similar at neap and spring tide, though cross-shore penetration of the outflow jet was reduced, bottom-mixed layer thickness increased and overall stratification reduced at springs. In contrast, the salinity field was found to be strongly dependent on wind strength and direction, both through wind-induced surface mixing and advection by wind-driven surface currents

A Model Study of the Rhine Discharge Front and Downwelling Circulation

The Rhine is the largest river discharge of the North Sea, and has considerable ecological importance as a source of nutrients and pollutants. While much of the Dutch coastal zone is intensively monitored, it is difficult to perform measurements within the first few km of discharge, a region of very high horizontal and vertical gradients, which influences strongly the fate of freshwater and riverborne material. A fine resolution 3D hydrodynamic model, MU-ROFI, has been developed to simulate the flow in this initial discharge zone. The model is described briefly and a first set of model results is presented and discussed. The process of front formation and associated downwelling on the upstream (with respect to the tide) boundary of the freshwater plume, switching from the South to the North boundary as the tide turns, is illustrated. Qualitative comparison with CTD and airborne remotely sensed measurement indicates that such a behaviour is realistic, at least in conditions of low wind and Neap tide

On the parameters of absorbing layers for shallow water models

Absorbing/sponge layers used as boundary conditions for ocean/marine models are examined in the context of the shallow water equations with the aim to minimize the reflection of outgoing waves at the boundary of the computationaldomain. The ptimization of the absorption coefficient is not an issue in continuous models, for the reflection coefficient of outgoing waves can then be made as small as we please by increasing the absorption coefficient. The optimization of the parameters of absorbing layers is therefore a purely discrete problem. A balance must be found between the efficient damping of outgoing waves and the limited spatial resolution with which the resulting spatial gradients must be described. Using a one-dimensional model as a test case, the performances of various spatial distributions of the absorption coefficient are compared. Two shifted hyperbolic distributions of the absorption coefficient are derived from theoretical considerations for a pure propagative and a pure advective problems. These distribution show good performances. Their free parameter has a well-defined interpretation and can therefore be determined on a physical basis. The properties of the two shifted hyperbolas are illustrated using the classical two-dimensional problems of the collapse of a Gaussianshaped mound of water and of its advection by a mean current. The good behavior of the resulting boundary scheme remains when a full non-linear dynamics is taken into account