Derivation of sediment resuspension rates from acoustic backscatter time-series in tidal waters

The main objective of the study was to investigate seasonal sediment dynarnics on the Laptev Sea shelf.The Laptev Sea comprises one of the largest Siberian shelf areas and is characterized by seasonal ice coverage and thus, by a strong seasonality in sediment input.The pathways and the final fate of the sediments derived from the Siberian hinterland are central questions for understanding the complex land-shelf-ocean interactions and their seasonal variations.In order to characterize seasonal variations in suspended particulate matter (SPM) dynamics on the eastem Laptev Sea shelf, one-year Acoustic Doppler Current Profiler (ADCP) records and complementary optical backscatter profiles from the ice-free period were analyzed.

The purpose of this work is to investigate via data analysis and numerical modeling the SPM (suspended particulate matter) dynamics of a heavily contaminated partially urban estuary, the Lower Passaic River estuary (LPR), NJ. Accordingly, I investigate the quantity and mechanics of variation of fine and coarse SPM in the LPR via data analysis. Data analysis focuses on the parameters that affect SPM dynamics at six moored stations occupied during the Fall and Spring seasons, from near the estuary mouth to tidal freshwater. A 3D hydrodynamic model (Delft3D-FM) is used to analyze the effects of estuary topography on the dynamic distribution of bed shear stress, τb, and to interpret the observations. Moored data from a station seaward of the LPR are used to for model calibration. This work will address three primary issues. The first is to determine bulk settling velocity (Wsb) values and the factors that affect Wsb along the estuarine salinity gradient. The second is to determine the quantity of fine and coarse SPM throughout the water column distributed in Rouse-like and Modified-Rouse profiles, and to (a): investigate the dynamical importance of advection in influencing SPM profile structure for fine and coarse SPM, and (b) determine how the SPM concentration varies with particle size, river flow, and tidal range. These two issues are analyzed using acoustic Doppler current profiler (ADCP) data. An ADCP provides simultaneous profiles of velocity and acoustic backscatter (ABS); the ABS signal can be converted to SPM concentration using appropriate calibration data. Finally, Delft3D-FM was set up on a grid of a generic, convergent estuary similar to the LPR. This grid was used to investigate how oceanographic factors (e.g., channel curvature and tidal range to depth ratio), natural and man-made roughness elements (e.g., grains, meanders, and bridge pilings), and external forcing by river inflow influence the distribution of bed shear stress in a stratified estuary similar to the LPR. To investigate the behavior of bulk settling velocity Wsb (the first question), friction velocity (u*) estimated from the ADCP velocity profile taking into consideration the effect of density stratification due to salinity intrusion. A log-linear velocity equation used when the water column stratified, and a logarithmic velocity profile used to estimate shear velocities,u* for unstratified conditions. Suspended sediment concentration, SSC, was estimated from ADCP acoustic backscatter (ABS) and calibrated against gravimetric SSC samples. Time series of profiles of flow velocity and SSC, and shear velocities used

Monitoring and modelling of suspended particulate matter (SPM) is an important task especially in coastal environments. SPM concentration is one of the major parameters that regulates the penetration of light into the ocean and hence the primary production. In the past several SPM models have been developed for the North Sea. However, due to waves in shallow water and strong tidal currents in the southern part of the North Sea, this is a challenging task. In general there is a lack of measurements to determine appropriate exchange coefficients. In many satellite borne ocean colour images of the North Sea a plume is visible, which is caused by the scattering at SPM in the upper ocean layer. The intensity and length of the plume depends on the wave and current climate. It is well known that the SPM plume is especially obvious shortly after strong storm events. In this paper a SPM transport model is presented using the synergy of satellite borne ocean colour data and numerical modeling to derive the vertical exchange coefficients due to currents and waves. This results in a model that for the first time is able to reproduce the temporal and spatial evolution of the plume intensity. The SPM model is a quasi-3D model which consists of 3 components: ocean dynamics, SPM vertical exchange and SPM exchange processes with the sea bed. The component for vertical exchange of SPM considers sedimentation and resuspension. The SPM exchange processes with the sea bed take into account erosion, bioturbation by benthos as well as diffusion in the bottom layers.

For offshore constructions the knowledge on sediment transport dynamics is essential and the quantification of suspended particulate matter (SPM) is of major importance. The Laptev Sea shelf is one of the largest Siberian shelf seas and ice-covered for about nine months a year. In order to use indirect measuring devices for the quantification of SPM concentration on the Laptev Sea shelf, optical (turbidity meter) and acoustic (ADCP; Acoustic Doppler Current Profiler) backscatter sensors were compared to assess their potential for the investigation of SPM dynamics in an arctic environment. To estimate SPM concentrations from optical backscatter signals, these were converted using the linear relation between the backscatter signals and SPM concentrations derived from water samples. Applying the theoretical interaction of sound in the water with SPM the acoustic backscatter signals were transformed adapting a previously established approach. SPM concentrations estimated from the backscattered signals of both sensors showed a close similarity to SPM concentrations obtained from filtered water samples. While the ADCP offers distinct advantages over the turbidity meter in that it allows measurement of the complete concentration profile, bottom location, and currents, co-deployment of both sensors are recommended for improved SPM measurements.

Dissolved CO2 and buried organic matter budgets have been studied in shelf seas to identify carbon fractions that are exported to the ocean interior or preserved in the sediment. However, the fate of suspended particulate organic matter remains less understood, particularly because its lability is difficult to identify. Analysis of the different fractions of particulate organic matter in the North Sea could contribute to understanding its fate. The particulate organic carbon (POC) concentration follows coastal-offshore gradients that can be predicted with the suspended particulate matter (SPM) concentration. The POC:SPM ratio indeed features a typical exponential decrease with the SPM concentration. While that ratio is higher offshore where SPM concentrations are minimum, it reaches low asymptotic values at the coast where SPM concentrations are high. Such a relationship is actually found in many different systems (coastal zones, estuaries) and at different latitudes. A semi-empirical model has been proposed to fit the observed data of that relationship in the southern North Sea (German Bight: Schartau et al., 2019; Belgian zone: Fettweis et al., 2022). Based on the model assumptions, it is possible to separate two fractions of POC: the fresh fraction, that is assumed to accumulate during the bloom and to be degraded within the season, and a more refractory POC fraction. More detailed calculations allow this latter fraction to be divided into a slow POC, which includes the refractory detritus, and a mineral POC, that is the POC adsorbed on the surfaces of clay minerals. We assume that suspended mineral particles in the North Sea provide a total surface area saturated with adsorbed organic matter, also considering an underlying dynamic equilibrium between adsorption and desorption of organic matter. We then calculate the SPM budget in the North Sea from satellite remote sensing and vertical concentration profiles obtained from in situ observations. On this basis, we can use the semi-empirical model to establish a budget of the fresh, refractory detrital and mineral fractions of POC on the shelf.

Seasonal and interannual variations of suspended particulate matter in a West-African lagoon (Nokoué lagoon, Benin): Impact of rivers and wind

The influence of waves and seagrass communities on suspended particulates in an estuarine embayment

Large rivers have been previously shown to be vertically heterogeneous in terms of suspended particulate matter (SPM) concentration, as a result of sorting of suspended solids. Therefore, the spatial distribution of suspended sediments within the river section has to be known to assess the riverine sedimentary flux. Numerous studies have focused on the vertical distribution of SPM in a river channel from a theoretical or experimental perspective, but only a few were conducted so far on very large rivers. Moreover, a technique for the prediction of depth‐integrated suspended sediment fluxes in very large rivers based on sediment transport dynamics has not yet been proposed. We sampled river water along depth following several vertical profiles, at four locations on the Amazon River and its main tributaries and at two distinct water stages. Depending on the vertical profile, a one‐ to fivefold increase in SPM concentration is observed from river channel surface to bottom, which has a significant impact on the ‘depth‐averaged’ SPM concentration. For each cross section, a so‐called Rouse profile quantitatively accounts for the trend of SPM concentration increase with depth, and a representative Rouse number can be measured for each cross section. However, the prediction of this Rouse number would require the knowledge of the settling velocity of particles, which is dependent on the state of aggregation affecting particles within the river. We demonstrate that in the Amazon River, particle aggregation significantly influences the Rouse number and renders its determination impossible from grain‐size distribution data obtained in the lab. However, in each cross section, the Rouse profile obtained from the fit of the data can serve as a basis to model, at first order, the SPM concentration at any position in the river cross section. This approach, combined with acoustic Doppler current profiler (ADCP) water velocity transects, allows us to accurately estimate the depth‐integrated instantaneous sediment flux. Copyright © 2010 John Wiley & Sons, Ltd.

Abstract. Sediment transport dynamics were studied during ice-free conditions under different atmospheric circulation regimes on the Laptev Sea shelf (Siberian Arctic). To study the interannual variability of suspended particulate matter (SPM) dynamics and their coupling with the variability in surface river water distribution on the Laptev Sea shelf, detailed oceanographic, optical (turbidity and Ocean Color satellite data), and hydrochemical (nutrients, SPM, stable oxygen isotopes) process studies were carried out continuously during the summers of 2007 and 2008. Thus, for the first time SPM and nutrient variations on the Laptev Sea shelf under different atmospheric forcing and the implications for the turbidity and transparency of the water column can be presented. The data indicate a clear link between different surface distributions of riverine waters and the SPM transport dynamics within the entire water column. The summer of 2007 was dominated by shoreward winds and an eastward transport of riverine surface waters. The surface SPM concentration on the southeastern inner shelf was elevated, which led to decreased transmissivity and increased light absorption. Surface SPM concentrations in the central and northern Laptev Sea were comparatively low. However, the SPM transport and concentration within the bottom nepheloid layer increased considerably on the entire eastern shelf. The summer of 2008 was dominated by offshore winds and northward transport of the river plume. The surface SPM transport was enhanced and extended onto the mid-shelf, whereas the bottom SPM transport and concentration was diminished. This study suggests that the SPM concentration and transport, in both the surface and bottom nepheloid layers, are associated with the distribution of riverine surface waters which are linked to the atmospheric circulation patterns over the Laptev Sea and the adjacent Arctic Ocean during the open water season. A continuing trend toward shoreward winds, weaker stratification and higher SPM concentration throughout the water column might have severe consequences for the ecosystem on the Laptev Sea shelf.

A detailed record of suspended particulate matter (SPM) concentrations in the benthic boundary layer (BBL) 1.5 m above an accumulation bottom and 13.5 m below the surface was obtained from frequent (30 min interval) beam attenuation measurements made with a Sea Tech transmissometer in the main basin of Lake Erken, a moderately deep (mean depth 9 m, maximum depth 21 m) dimictic lake in central Sweden. Concentrations of SPM (g m−3) were not as strongly correlated to the beam attenuation coefficient (c, [m−1]), as were concentrations of the inorganic SPM fraction. Apparently, this was caused by large optically inactive organic particles which significantly affected the measurements of SPM, but had little effect on the attenuation of light. When the water column was thermally stratified, SPM concentrations in the BBL showed a seasonal increase which was related to an increase in the thermocline depth. As the epilimnion deepened, there was also a marked increase in the occurrence of rapid and large changes in SPM concentration. After the loss of stratification, the amount of SPM and the temporal variability in its concentration was reduced. Since surface waves could not influence sediment resuspension at the depth of measurement, these data show the importance of internal waves in promoting sediment resuspension in areas of sediment accumulation. The relatively short period in each summer, when the thermocline reaches a sufficient depth to allow for resuspension over accumulation bottoms, can have important consequences for both the redistribution of lake sediments and the internal loading of phosphorus.

Suspended particulate matter fluxes through the Straits of Dover, English Channel: observations and modelling

The accurate measurement of suspended particulate matter (SPM) concentrations in coastal waters is of crucial importance for ecosystem studies, sediment transport monitoring, and assessment of anthropogenic impacts in the coastal ocean. Ocean color remote sensing is an efficient tool to monitor SPM spatio-temporal variability in coastal waters. However, near-shore satellite images are complex to correct for atmospheric effects due to the proximity of land and to the high level of reflectance caused by high SPM concentrations in the visible and near-infrared spectral regions. The water reflectance signal (ρw) tends to saturate at short visible wavelengths when the SPM concentration increases. Using a comprehensive dataset of high-resolution satellite imagery and in situ SPM and water reflectance data, this study presents (i) an assessment of existing atmospheric correction (AC) algorithms developed for turbid coastal waters; and (ii) a switching method that automatically selects the most sensitive SPM vs. ρw relationship, to avoid saturation effects when computing the SPM concentration. The approach is applied to satellite data acquired by three medium-high spatial resolution sensors (Landsat-8/Operational Land Imager, National Polar-Orbiting Partnership/Visible Infrared Imaging Radiometer Suite and Aqua/Moderate Resolution Imaging Spectrometer) to map the SPM concentration in some of the most turbid areas of the European coastal ocean, namely the Gironde and Loire estuaries as well as Bourgneuf Bay on the French Atlantic coast. For all three sensors, AC methods based on the use of short-wave infrared (SWIR) spectral bands were tested, and the consistency of the retrieved water reflectance was examined along transects from low- to high-turbidity waters. For OLI data, we also compared a SWIR-based AC (ACOLITE) with a method based on multi-temporal analyses of atmospheric constituents (MACCS). For the selected scenes, the ACOLITE-MACCS difference was lower than 7%. Despite some inaccuracies in ρw retrieval, we demonstrate that the SPM concentration can be reliably estimated using OLI, MODIS and VIIRS, regardless of their differences in spatial and spectral resolutions. Match-ups between the OLI-derived SPM concentration and autonomous field measurements from the Loire and Gironde estuaries’ monitoring networks provided satisfactory results. The multi-sensor approach together with the multi-conditional algorithm presented here can be applied to the latest generation of ocean color sensors (namely Sentinel2/MSI and Sentinel3/OLCI) to study SPM dynamics in the coastal ocean at higher spatial and temporal resolutions.

ABSTRACTAim To present a general, process‐based river model for suspended particulate matter (SPM).Location General approach based on processes; data from Europe and Israel.Methods The model has been tested and calibrated using an empirical river model for SPM and validated (blind‐tested) using data from seven European sites. This modelling gives mean monthly SPM concentrations in water for defined river sites. The model is based on processes in the entire upstream river stretch (and not for given river segments) and calculates the transport of SPM from land to water, primary production of SPM (within the upstream river stretch), resuspension, mineralization and retention of SPM in the upstream river stretch (but not bed load of friction materials, such as sand). The catchment area is differentiated into inflow (∼ dry land) areas and outflow area (∼ wetland areas dominated by relatively fast horizontal SPM‐fluxes). The model is simple to apply in practice as all driving variables may be accessed readily from maps. The driving variables are: latitude, altitude, continentality, catchment area and mean annual precipitation.Results Modelled values have been compared to independent empirical data from sites covering a relatively wide domain (catchment areas from 93 to 5250 km2, precipitation from 400 to 660 mm year−1, altitudes from −210 to 150 m a.s.l., latitudes from 47 to 59° N and continentalities from 200 to 1000 km from the ocean). When blind‐tested, the model predicts annual SPM‐fluxes well.Conclusion When modelled values are compared to empirical data, the slope is almost perfect (1.03) and the r2‐value is 0.9996. This is good, given the fact that there are several simplifications in the model structure. It must, however, be stressed that there are only seven validation cases and that this model has not been tested for small catchments.

An estimate of the suspended particulate matter (SPM) transport in the southern North Sea using SeaWiFS images, in situ measurements and numerical model results

Measuring Fluxes in Tidal Estuaries: Sensitivity to Instrumentation and Associated Data Analyses