Particle assemblage characterization in the Rhone River ROFI

Abstract

International audienceAn innovative experiment was carried out in the vicinity of the Rhône River mouth in February 2014. An instrumentalpackage, composed of a CTD, a LISST-100 type B (1.25–250 μm), and a LISST-HOLO (20–2000 μm), wasused to characterize the hydrological parameters and suspended particles properties (concentration, size, composition,shape, and effective density) in the region of freshwater influence (ROFI) of the Rhône River. Besides,a coastal SLOCUMglider, equipped with a CTD and optical backscattering sensors at severalwavelengths,was deployedto detail the spatial description of the hydrological parameters and some particle properties. Large riverdischarge (annual flood ~5000 m3 s−1) and strong wind conditions favored the dispersal of the river plume onthe shelf. Surface suspended particulate matter concentrations decreased rapidly seaward from 20 mg L−1next to the river mouth to 1.5 mgL−1 at the shelf break. A persistent bottomnepheloid layerwas observed acrossthe shelf with concentrations decreasing from8 mg L−1 at the coast to 1 mg L−1 at the shelf break. Observationsshowed that most of suspended particles were mainly flocculated in micro and macro-flocs (30–400 μm) ininner-shelf waters. The particle assemblage in the Rhône River plume and in the bottomnepheloid layer becameprogressively finer seaward and the associated effective density increased from 370 to 1600 kgm−3. Outside theplume, planktonic organisms increasingly contributed to the total volume concentration. Finally,we demonstratedthe ability of gliders, equipped with optical backscattering sensors at severalwavelengths, to describe the finescale distributions of suspended particles, and provide an index of their size distribution

Potential of High Spatial and Temporal Ocean Color Satellite Data to Study the Dynamics of Suspended Particles in a Micro-Tidal River Plume

“…On 17 and 23 February, wind direction varied from the north-west to the south-west around 11:00 UTC. These wind conditions, associated to dilution and flocculation processes, likely explain the observed decrease of the plume extent after 14:00 UTC for these two days [53] (Figure 12). These short time variations (few hours) of plume shape under changing wind directions support a dominant and fast effect of wind stress on the river plume.…”

“…Given the low impact of the intercept value on the SPM estimation in the relatively turbid waters of the Rhône River, and given the large range of solar and sensor zenith angles spanned by the three sensors, the intercepts in Equations (3A)-(3C) were set to 0. The resulting relationships (Equations (3A)-(3C)) show a significant scatter (R 2 of~0.61) that can be explained by (i) the difficulty to measure in the field the SPM concentration within the thin Rhône River plume, i.e., a less than 1 m thick turbid surface layer floating over underlying layers; (ii) the natural variations of SPM composition and size distribution along the river plume where different types of suspended particles (mainly sediments) mix and form aggregates in the presence of organic matter (e.g., [8,53]) and (iii) measurement uncertainties. Nevertheless, these relationships are more robust than those proposed by [54] as they are based on a larger range of SPM concentrations (3-60 g¨m´3 in this study compared to 1-10 g¨m´3 in [54]) and show a slightly better determination coefficient (R 2 of 0.61 for this study compared to 0.56 in [54]).…”

Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season

“…However, particles less than the smallest size class or bigger than the largest size class affect the measurements in the spectrum [42][43][44][45][46][47][48][49][50][51][52]. In this study, we followed the recommendation of former authors to remove the first and last classes for calculating the general slope of the particle size distribution and the mean apparent diameter D 50 [42][43][44][45][46][47][48][49][50][51][52]. Based on data between class #2 and class #31, D 50 was thus calculated as the diameter corresponding to 50% of the cumulative volume concentration of aggregates between 1.48 and 212 µm.…”

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…However, if the river plume hydrodynamics has been studied in details for~30-40 years (see a review in [98]), few studies were dedicated to the suspended matter characteristics (grain size distribution, excess of density, organic content, mixing processes at the interfaces) and their behaviour in the plume or at its boundaries, and in the bottom nepheloid layer (such as [164,165]). …”

“…Gliders enable quick profiling along transects facing mouths, equipped with sensors such as CTD probes, turbidimeter, bio-optical sensors and, soon, in situ laser grain size meters (like the LISST 200-X). Many et al [164] have recently documented sections of suspended matter in the surface plume and in the bottom nepheloid opposite the Rhône River mouth. Stereo restitution, which is adapted to build Digital Elevation Model (DEM) over sandy environments, may fail over mudflats which hold back residual tidal water.…”