Low Suspended Sediment Concentration Research Articles

Evolution of suspended sediment concentration profiles in the partially and periodically stratified Changjiang Estuary during dry season

A better understanding of how the vertical profile of suspended sediment concentration (SSC) is influenced by multiple factors is required to better understand the hydrodynamics and sediment dynamics of estuaries. Based on in situ measurements obtained during a spring-neap cycle in the estuarine turbidity maximum (ETM) of the Changjiang Estuary in January 2015, we investigate the intratidal evolution and controlling mechanisms of SSC profiles. The results show that during the dry season the ETM was characterized by periodic salinity stratification, low SSC, suspended sediments with reduced grain size, and low sediment availability for resuspension. Three types of SSC profiles were identified, i.e., profiles with two-layer, linear and quasi-uniform structures. The shapes of the SSC profiles varied with time and generally deviated from the equilibrium profile due to the highly variable hydrodynamic and sediment conditions. During ebbs of spring and intermediate tides, the SSC profiles generally evolved from a two-layer to a quasi-uniform structure via a linear structure. Analysis shows that the linear and quasi-uniform profiles occurred under the combination of vertically well-mixed salinity, negligible resuspension, low settling velocity associated with very fine grains, and shallow water depth. Such an evolution pattern can be well reproduced using a one-dimensional numerical model, with the diffusion coefficient being parabolically distributed in the water column and the four factors mentioned above. The findings reported here imply a high variability of SSC profiles in estuarine environments.

Alternating patterns of cross-shelf suspended sediment transport in the northern East China Sea in winter

Suspended sediment transport across the continental shelf is a key process for the materials exchange between coastal seas and the deep ocean. The cross-shelf transport through the Yangtze Shoal in the northern East China Sea (ECS) exhibits a unique pattern, which is characterized by alternations between high and low values in suspended sediment concentration (SSC). This study is conducted to reveal the underlying mechanisms and their effects on sediment fluxes. Surface SSC retrieved from GOCI data are combined with in situ observations of concentration and particle sizes of suspended sediment to study the temporal and spatial variations of the SSC in the Yangtze Shoal. Additionally, a numerical model for examining ocean circulation based on the FVCOM is established to display the hydrodynamic feature in the study area. The alternating SSC pattern is closely related to the topography of three ridges and two troughs. High SSC with fine particle size can be found over the ridge, while low SSC is detected at the surface layer of water in the trough. Even during the flood and ebb tides, this alternating pattern of high and low SSCs does not change, except for the expansion and contraction of coverage areas of high SSC. The high SSC near the tidal ridge should have resulted from the combined effect of resuspension and upwelling since the stronger bottom shear stress and a surface divergence that may induce an upwelling can be found in this area. Combined with the simulated current, the sediment transport flux between ridges and troughs is estimated. The sediment flux in the ridge is substantially increased than that in the trough, especially during the flood maximum and ebb maximum time. The contribution of suspended sediments from the Yangtze Shoal to the Southwestern Cheju Island Mud (SCIM) is relatively small during the normal weather in winter. However, cross-shelf transport can be remarkably strengthened by strong northerly winds in winter. This work puts forward a detailed cross-shelf transportation process induced by topography.

Coastal acoustic tomography system for monitoring transect suspended sediment discharge of Yangtze river

Suspended sediment discharge (SSD) in the Yangtze River (YR) plays a crucial role in transporting nutrients and sediment to the YR estuaries and the East China Sea. In order to gather information on suspended sediment concentration (SSC) and SSD across an entire river cross-section, we investigated a novel method supported by Coastal Acoustic Tomography (CAT). Consequently, two CAT systems were employed to estimate the cross-section averaged SSC and SSD at the Datong (DT) station of the YR over approximately 2.5 years. Low (0.02 kg/m3), high (0.44 kg/m3), and mean (0.10 kg/m3) SSC values were successfully estimated. The SSD was primarily driven by water discharge, exhibiting low values (204 kg/s) during the dry season and a maximum of 36,299 kg/s during flood events, with the mean value of 3,903 kg/s throughout the observation period. The water sample and CAT results demonstrated high consistency, with correlation coefficients (R2) of 0.92 for SSC between the water sample and CAT results, and 0.96 for SSD between the DT result and CAT results. This study established the ability to monitor transect SSC and SSD in the YR using the CAT sound attenuation approach, offering the advantage of continuous real-time monitoring of transect river flow, SSC, and SSD, even during extreme flood events. For the success of this method, factors such as particle size, sound frequency selection, and the measurable distance between instruments must be meticulously considered. This work highlights the potential of CAT for monitoring riverine transect SSC and SSD.

Winter and summer sedimentary dynamic process observations in the sea area off Qinhuangdao in the Bohai Sea, China

Sedimentary processes in marginal seas play an important role in the biology, physics, and geochemistry as well as ecology of coastal environments and contain abundant information about the material transfer from land to ocean and the regional circulation. Due to the huge sediment discharge of the Yellow River, the Bohai Sea, China is one of the areas with the highest suspended sediment concentration (SSC) in the world. Interestingly, the SSC at the west of the Bohai Sea is low all year round. Thus, it is of great significance to examine the sedimentary dynamic process in this area for better understanding the circulation structure, material exchange and regional environment of the Bohai Sea. Using seabed base observation platform measurements obtained in February 2017 and August 2019, this study examines the winter and summer hydrography and suspended sediment concentration in the sea area off Qinhuangdao located to the west of the Bohai Sea. In summer, the relatively weak residual currents flowed northeastward and showed little correlation with the wind field, especially in the middle layer of the water column. In winter, the residual currents were strengthened, flowing to the northeast during strong wind periods, and predominantly to the southwest during intermittent periods. Moreover, driven by the pressure gradient force associated with the wind-induced sea surface height variations, the winter current was closely related to the wind speed, with correlation coefficients ranging from 0.4 to 0.6 and a time lag of 10 h. The summer SSC was lower and mainly controlled by the tidal current, whereas in winter, owing to the enhanced Reynolds stress and turbulent kinetic energy, strong wind bursts triggered significant sediment resuspension and led to a higher SSC. For the suspended sediment flux (SSF), the advection terms contributed more than 80% in the winter and summer, while the vertical circulation terms contributed 13% in winter, and approximately half that much in summer. Generally, the suspended sediment is transported back and forth, with a little net northward and northeastward motions in winter and summer, respectively. This may explain the low SSC sustaining in coastal Qinhuangdao all year round. These results provide a reference for sedimentary studies conducted in other coastal waters, especially in monsoon-dominating shelf seas.

Changes in the spatial-temporal of the amount of sediment suspended in the Mekong Delta in Vietnam

Sediment depletion in the Vietnamese Mekong Delta (VMD) has brought threats to riverbed and riverbank dynamics. The main objective of this research is to make a relationship between turbidity and suspended sediment concentration (SSCs) and to assess temporal-spatial variations of SSCs in the VMD. In this regard, the SSCs were measured and monitored along the Tien and Hau Rivers under the Japan-ASEAN Science, Technology, and Innovation Platform (JASTIP) project and Mekong River Commission (MRC). Particularly, we installed three monitoring stations in main rivers and conducted two field surveys to collect data for assessing SSC values in the VMD. The results show that the SSCs in the delta are changeable seasonally with high values in the high-flow season and low values in the low-flow season. The Tien and Hau Rivers can be divided into two reaches. The upper reach has high SSC, decreasing seaward, whereas the lower reach has lower SSC, increasing seaward.

The role of earthquakes and storms in the fluvial export of terrestrial organic carbon along the eastern margin of the Tibetan plateau: A biomarker perspective

Driven by earthquakes and intense rainfall, steep tectonically active mountains are hotspots of terrestrial organic carbon mobilization from soils, rocks, and vegetation by landslides into rivers. Subsequent delivery and fluvial mobilization of organic carbon from different sources can impact atmospheric CO2 concentrations across a range of timescales. Extreme landslide triggering events can provide insight on processes and rates of carbon export. Here we used suspended sediment collected from 2005 to 2012 at the upper Min Jiang, a main tributary of the Yangtze River on the eastern margin of the Tibetan Plateau, to compare the erosion of terrestrial organic carbon before and after the 2008 Wenchuan earthquake and a storm-derived debris flow event in 2005. To constrain the source of riverine particulate organic carbon (POC), we quantified lignin phenols and n-alkanoic acids in the suspended sediments, catchment soils and landslide deposits. We found that riverine POC had higher inputs of less-degraded, discrete organic matter at high suspended sediment loads, while the source of POC seemed stochastic at low suspended sediment concentrations. The debris flow in 2005 mobilized a large amount of POC, resulting in an export of lignin within a single day equivalent to a normal year. In comparison, the 2008 Wenchuan earthquake increased the flux of POC and particulate lignin, albeit with limited impact on POC sources in comparison to seasonal variations. Our results highlight the important role of episodic events in the fluvial export of terrestrial carbon.

Role of abiotic drivers on crab burrow distribution in a saltmarsh wetland

Crab burrows play an important role in saltmarsh wetlands and are a useful indicator of wetland condition. The spatiotemporal distribution of crab burrows varies considerably in tidal wetlands. However, the reasons for these variations are poorly understood, in part, due to the limited availability of comprehensive field data. Based on a two-year continuous observation at a tidal wetland in the northern Jiangsu Coast, China, this study explored the relationship between crab burrow density and environmental variables, including median grain size, water content, organic matter content, soil salinity, and elevation. Our results show that the distribution of crab burrows was unimodal across the shore in winter and spring (Nov-Apr) when air temperature was relatively low, while bimodal in summer and autumn (May-Oct) when temperature was relatively high. The density of crab burrows was larger at areas with higher water content, higher organic matter content, and lower soil salinity, while it was lower with stronger hydrodynamics and lower suspended sediment concentration. Crab burrows were more abundant in vegetated areas than in un-vegetated areas. A backward stepwise model selection was performed based on R-square and Akaike information criterion (AIC) to distinguish the main driving factors that determine crab burrow distribution. Results suggested that the principal driving factors were organic matter content and soil salinity in all the seasons, with the addition of water content in warm seasons. Overall, this study provides a comprehensive field dataset for a more in-depth understanding of crab burrow distribution and a scientific basis for sustainable management of tidal wetlands.

Evaluating deep-sea communities' susceptibility to mining plumes using shallow-water data

Increased suspended sediment concentrations (SSC) are a major stressor across aquatic habitats. Here, the literature was synthesized to show that animal responses to increases in relative SSC (test concentration/natural background concentration) were similar in type and negative across different shallow-water (marine, estuarine, freshwater) habitats. Further, animal sensitivities are similar across habitats based on relative SSC and occur starting at low relative SSC increases in all habitats despite differences in natural background SSC. Based upon these similarities in relative SSC sensitivities, deep-sea sensitivity values for acute exposure to increased SSC, where empirical data are almost non-existent, were estimated. Because of the low natural SSC in deep sea environments, very small increases in absolute SSC could result in acute effects. How the methods and results can be used to inform regulatory thresholds are discussed. Because of the large variability in shallow water datasets and differences between deep-sea and shallow-water habitats, deep-sea specific data are needed to verify the estimates and improve their precision. Following the precautionary principle and the results presented here, it is recommended that the threshold for acute plume impacts is set very close to natural background levels.

Variations in the Suspended Sediment Concentration in Mountain-Type Rivers Flowing Into the Sea in the Past 60 years—Taking Nanliu River in Beibu Gulf as an Example

Suspended sediment in the water body of rivers flowing into the sea is of great significance to the accumulation process in river basins and change pattern of landforms. In particular, small and medium rivers entering the sea in mountainous areas exhibit the characteristics of both mountains and streams, and the suspended sediment concentration (SSC) greatly contributes to the formation and evolution of deltas. However, scholars rarely give attention to the factors influencing changes in the SSC in small- and medium-sized mountainous rivers, and few studies have examined SSC changes in small- and medium-sized mountainous rivers. Here, based on daily SSC and flow data obtained at the Bobai Station and Changle Station, the percentile method and regression analysis method are employed to analyse the changes in SSC from the Nanliu River to the Beibu Gulf and possible influencing factors. The main research results indicate that 1) the SSC in river water bodies from 1965 to 2020 generally reveals a downwards trend, with significant annual variations. Specifically, the overall trend can be divided into three stages: 1) the SSC is the highest from 1965 to 1971, and the mean yearly SSC reaches 0.25 kg/m3; 2) the SSC is relatively high from 1972 to 2006, and the mean yearly SSC reaches 0.16 kg/m3; and 3) the SSC is the lowest from 2007 to 2020, and the mean yearly SSC reaches 0.11 kg/m3 2) High values of the SSC are mainly concentrated in the flood season, and low SSC values mostly occur in the dry season. The monthly average SSC in the flood season from April to September and the dry season from October to March exhibits the characteristics of a decrease in the flood season and an increase in the dry season. Moreover, the peak SSC value in the watershed obviously occurs out of sync with the peak flow value. Generally, the former precedes the latter. 3) The curve of the flow rate-sediment ratio is an irregular clockwise rhombus. Moreover, the impact of tropical cyclones, land cover changes, regional GDP per capita, and engineering construction are crucial reasons for the observed variations in the SSC in the Nanliu River Basin. In this paper, the obtained research results provide an important guiding significance for the planning and management of water and sediment resources in the Nanliu River and offer a reference for hydrological planning of other river basins discharging into the sea.

Coastal erosion trend analysis using a combination of remote sensing and hydrodynamic models: Case study of Ca Mau Cape, Mekong Delta

BackgroundCoastal erosion and the loss of protective forest areas have recently become serious problems in the Ca Mau Cape (CMC) area of the Mekong Delta. Specifically, in 2016–2020, the rate of erosion in some subsections of the east coast reached 25–37 m/year and the forest area decreased significantly. ObjectivesTo analyse erosion and accretion trends in the period 2016–2020 and to determine the mechanism underlying erosion in the CMC using a hydrodynamic model focused on tides, currents, waves, and sediment transport. MethodsSatellite images (Google Earth), remote sensing images (Landsat 8 OLI/TIRS C1 Level-1), and a combination of hydrodynamic modules were used to evaluate the combined effects of tides, currents, waves, and sediment transport on erosion. ResultsRemote sensing image analyses revealed that the east coast has a very high rate of erosion, with the highest erosion rates of 35 m/year in both 2016–2018 and 2019–2020, while the west coast showed an accretion trend in 2016–2018, followed by strong erosion in 2019–2020, with a rate of 10 m/year. Hydrodynamic models showed that hydrodynamic factors, such as tides, currents, waves, and a low suspended sediment concentration (SSC), directly contribute to the erosion trend in the area. The differences in water level between the high tide and low tide on the east coast, reaching 2 m (dry season) and 0.8 m (wet season), were consistently two times higher than those on the west coast, i.e. 0.9 m (dry season) and 0.4 m (wet season). The flow rates on the east coast (i.e. 0.4–0.6 m/s, on average) were significantly higher those on the west coast (0.2 m/s, on average). Wave heights on the east coast, reaching 1.5–2 m in the dry season and 0.6–1 m in the wet season, were consistently higher than those on the west coast, reaching 1–1.2 m (dry season) and 0.4–0.6 m (wet season). In addition, the SSC was lowest under strong erosion, with average concentrations of 0.04–0.06 kg/m3 in the wet season and 0.02–0.04 kg/m3 in the dry season. The SSC was highest in the sections showing accretion trends, with concentrations of >0.3 kg/m3 in the wet season and 0.08–0.1 kg/m3 in the dry season. ConclusionEvaluating shoreline changes using remote sensing technology and validating these results using hydrodynamic models provides a better understanding of the mechanism underlying coastal erosion and its development and provides a basis for the development of a coastal protection strategy.

Potential predictability of suspended sediment concentration in the data constrained regions of the Mahanadi River basin, Eastern India

ABSTRACT The study proposes an efficient method to evaluate the suspended sediment concentration (SSC) relative to the traditional sediment rating curves (SRC) for gauged stations and subsequently to predict the SSC in ungauged stations of a major river basin. Multiple environmental control parameters were collected from 16 stations along the Mahanadi River basin (MRB) during the monsoon season. The hysteresis behaviour of SSC is assessed and therefore considered for modelling SSC using linear mixed-effects modelling (LMM). A basin-scale rating model is proposed using principal component analysis and stepwise multiple linear regression for estimating the unmeasured SSC. The findings show that the MRB acts differently in terms of hysteresis, with distinct dilution and flushing regimes in SRC. LMM outscored SRC by nearly doubling the mean covariance and notably reducing the percent bias between observed and predicted data across stations. However, unlike LMM, SRC could not correctly estimate low and high SSCs of ≤ 0.05 g/l and ≥ 1.5 g/l, respectively. The error metrics of the proposed rating model are within acceptable limits for all ungauged stations. Nevertheless, its efficiency varies due to smaller catchment areas, non-linearity in sediment transport with respect to catchment area, and other sampling issues. As a result, compared to other known models applied on the MRB, this model has the lowest error and seems to be the best in predicting monthly SSC.

Net suspended sediment transport modulated by multiple flood-ebb asymmetries in the progressive tidal wave dominated and partially stratified Changjiang Estuary

In many estuaries, current velocity, suspended sediment concentration (SSC), and water depth have notable flood-ebb asymmetries in magnitude or duration. Such asymmetries can make marked impact on estuarine sediment transport and require in-depth analysis of the underlying mechanism. Based on field measurement over a neap-spring cycle in the Changjiang Estuary in January 2015, net suspended sediment transport modulated by multiple asymmetries were analyzed. The results showed that there were notable flood-ebb asymmetries in the current velocity, current duration, SSC, and water depth. The former two were ebb-dominated, while the latter two were flood-dominated. The net suspended sediment transport was landward and exhibited remarkable tidal fluctuation in magnitude. Ebb-dominated velocity and duration asymmetries favored seaward transport, but flood-dominated SSC and water depth asymmetries promoted landward transport. The relative contribution of these asymmetries to the net sediment transport was different and closely related to their asymmetry intensities. The magnitude of net sediment transport was highly modulated by the combined effects of these asymmetries. Further analyses revealed that the ebb-dominated asymmetries in velocity and duration were controlled by river flow, tidal wave deformation, and compensation flow of Stokes drift. The flood-dominated water depth asymmetry was related to the progressive tidal wave. The flood-dominated SSC asymmetry was mainly attributed to the lower SSCs in early ebbs, notable sediment supply limitation in late ebbs, wind wave height asymmetry, and gravitational circulation. Additionally, it is worth noting that the progressive tidal wave can yield strong current asymmetry, notable water depth asymmetry, and obvious wind wave height asymmetry during strong wind periods.

Natural vs. trawling-induced water turbidity and suspended sediment transport variability within the Palamós Canyon (NW Mediterranean)

Increases of water turbidity and suspended sediment transport in submarine canyons have been associated with high-energy events such as storms, river floods and dense shelf water cascading (DSWC), and occasionally with bottom trawling along canyon flanks and rims. To assess the variations on the water column turbidity and sediment transport in the Palamós Canyon linked to both natural and trawling-induced processes, an autonomous hydrographic profiler, as well as a near-bottom current meter and a turbidimeter were deployed in the canyon axis (929 m depth) from February to June 2017, covering a trawling closure (February) and trawling activities (March-June). Periods of enhanced water turbidity during the trawling closure were mostly associated with storms and DSWC events, transporting turbid dense waters into the canyon. In absence of such events, the water column displayed low suspended sediment concentrations (~ 0.3 mg L− 1) until the trawling season began, when particulate matter detachments, ranging between > 1 mg L− 1 and 3.8 mg L− 1, were observed at the water depths where the trawling grounds are found. During the trawling closure, high near-bottom suspended sediment fluxes (35–44 g m− 2 s− 1) were sporadically registered at ~ 920 m depth associated with a major storm and DSWC event. Smaller but more frequent increases of near-bottom suspended sediment fluxes (0.1–1.4 g m− 2 s− 1) were recorded during trawling activities. Despite these smaller trawling-induced suspended sediment fluxes, 30 days of continuous bottom trawling activity transported a total amount of 40 kg m− 2, of similar magnitude to that generated by a major DSWC event (50 kg m− 2). Since bottom trawling in Palamós Canyon is practiced on a daily basis throughout the year, a much larger contribution of anthropogenically derived water turbidity and suspended sediment transport can be expected.

Spatiotemporal Dynamics of Suspended Sediments in the Negro River, Amazon Basin, from In Situ and Sentinel-2 Remote Sensing Data

Monitoring suspended sediments through remote sensing data in black-water rivers is a challenge. Herein, remote sensing reflectance (Rrs) from in situ measurements and Sentinel-2 Multi-Spectral Instrument (MSI) images were used to estimate the suspended sediment concentration (SSC) in the largest black-water river of the Amazon basin. The Negro River exhibits extremely low Rrs values (<0.005 sr−1 at visible and near-infrared bands) due to the elevated absorption of coloured dissolved organic matter (aCDOM at 440 nm > 7 m−1) caused by the high amount of dissolved organic carbon (DOC > 7 mg L−1) and low SSC (<5 mg L−1). Interannual variability of Rrs is primarily controlled by the input of suspended sediments from the Branco River, which is a clear water river that governs the changes in the apparent optical properties of the Negro River, even at distances that are greater than 90 km from its mouth. Better results were obtained using the Sentinel-2 MSI Red band (Band 4 at 665 nm) in order to estimate the SSC, with an R2 value greater than 0.85 and an error less than 20% in the adjusted models. The magnitudes of water reflectance in the Sentinel-2 MSI Red band were consistent with in situ Rrs measurements, indicating the large spatial variability of the lower SSC values (0 to 15 mg L−1) in a complex anabranching reach of the Negro River. The in situ and satellite data analysed in this study indicates sedimentation processes in the lower Negro River near the Amazon River. The results suggest that the radiometric characteristics of sensors, like sentinel-2 MSI, are suitable for monitoring the suspended sediment concentration in large tropical black-water rivers.

Regime shifts in the Changjiang (Yangtze River) Estuary: The role of concentrated benthic suspensions

Channel deepening often triggers positive feedback between tidal deformation, sediment import and drag reduction, which leads to the regime shift in estuaries from low-turbid to hyper-turbid state. In this study, a transition in profiles of suspended sediment concentration (SSC) is hypothesised by including a positive feedback loop of vertical mixing and settling. Such a hypothesis is validated by the historical observations in the North Passage of Changjiang (Yangtze River) Estuary, with decreasing SSC in mid-lower layers and increasing SSC near the bed after the deepening. A mobile pool of concentrated benthic suspensions (CBS) develops in the North Passage, with a tidally averaged length of ~20 km and a mean thickness of ~4 m. The width of the CBS pool is limited (<1 km) as the CBS is concentrated in the Deepwater Navigational Channel. The movements of the CBS pool, combined with tidal asymmetry (e.g., slack-water asymmetry and lateral flow asymmetry), results in sediment trapping in the middle reaches and on the south flank of the channel. Observations by a bottom tripod system show the response of friction/drag coefficient to sediment concentration: (1) nearly linear decrease within low SSC (<10 kg/m3); (2) constant and minimum coefficient (with drag reduction up to 60–80%) in the presence of CBS (10–80 kg/m3). An empirical relationship was derived, which can be used to predict the friction coefficient and the magnitude of drag reduction for sediment transport studies, particularly for modelling regime shifts in estuaries.

Internal erosion of soil pipes: Sediment rating curves for soil pipes

AbstractThe collapse of soil pipes due to internal erosion can result in fully mature gullies. Few studies have measured the rates of sediment detachment and transport through soil pipes in situ. The objectives of this work were to determine suspended sediment concentration (SSC) in soil pipes as a function of pipeflow rate to develop sediment rating curves (SRC) and measure the bedload transport as a function of cumulative flow per storm event. H‐flumes were installed in seven discontinuous gullies formed by pipe collapse and instrumented for pipe discharge measurements and suspended sediment sampling. The typical response to pipeflow was an initial flush of high concentration of suspended sediment followed by a decrease as pipeflow increased (rising limb of hydrograph). Pipeflows were often so dynamic that it was difficult to consistently capture the initial flush of sediment, resulting in weak to non‐existent SRCs. The falling limb of the hydrograph tended to have a relatively low SSC. Thus, soil pipe SRCs tended to be better represented by hysteretic SRCs, although relationships between SSC and flow rate were poorly represented by SRCs. A power law equation given by SSC = aQb was adopted to represent the SRC relationships. Fitting this equation to data showed a correlation between the offset, a, and the slope, b, with the slope decreasing as the offset increases. Both SRC parameters (a and b) were correlated to the contributing area of the individual pipe. Bedload appeared to be an important contributor to sediment transport, with bedload – expressed as an average event sediment concentration (mg l−1) – decreasing as the volume of the event discharge (m3) increased. A significant portion (11–31%) of the bedload material was gravel and aggregates (&gt;2 mm diameter material). While this work was the first to determine SRCs for soil pipes, refined sampling and measurement techniques are needed. © 2020 John Wiley &amp; Sons, Ltd.

Spatio-temporal variability of suspended sediment concentrations in a shallow and turbid lagoon

The Patos Lagoon, located in the southernmost part of Brazil, is a shallow and turbid lagoon of enormous ecological and economical importance. The seasonal, annual, and interannual variability of suspended sediment concentrations (SSC) in the lagoon was investigated in this study based on five years (2002–2006) of hydrodynamics + SSC simulations using the TELEMAC – 3D hydrodynamic model. The model was first calibrated and validated by comparing results with in situ current velocities, salinity, water levels, and SSC measurements, indicating a reasonable/good performance for the majority of the time. Modelling results were then combined with river discharge, wind, precipitation rates, and climate indices (El-Niño Southern Oscillation - ENSO) data to identify their influence on SSC throughout the lagoon. The relation between daily time series of SSC and river discharge was high (r ≈ 0.80), suggesting a strong correlation between these time series in the Patos Lagoon. SSC seasonal variability was also observed, with high SSC from winter to spring, and moderate to low SSC during summer and fall, indicating a relation with the river discharge pattern. On the interannual variability scale, the ENSO influence promoted high SSC values throughout the lagoon, mainly during El Niño years and near the tributaries mouth, while the opposite pattern was observed during La Niña years. Thus, the Patos Lagoon proved to be a dynamic system where SSC are mainly controlled by river discharges at longer timescales and modulated by the wind action at shorter timescales.

Sediment resuspension in winter in an exceptional low suspended sediment concentration area off Qinhuangdao in the Bohai Sea

Seawater with low suspended sediment concentration (SSC) and high salinity dominates the sea area off Qinhuangdao all year round, especially in winter, which is an exceptional case since most of the coastal waters in the Bohai Sea are turbid. Being one of the most famous coastal tourist resorts in the area, it is crucial to keep the seawater around Qinhuangdao clear and unpolluted. Three bottom-mooring systems were deployed in the low SSC region from February to March 2017 for up to 22 days to reveal the processes and mechanisms of local sediment resuspension. The observation results showed that during the strongest wind of the whole deployment period, the sediment resuspension process included a turbulent resuspension event that occurred in the high turbulent kinetic energy period, and a continuous resuspension event when the bed shear stress (BSS) was larger than critical. The turbulent resuspension event was characterised by high-turbidity resuspension of sediments near the bed in the form of pulsation, with a depth of influence not more than the thickness of the bottom boundary layer. It was found to be triggered by strong turbulence driven by storm winds. In contrast the continuous resuspension could be attributed to high wave-dominated BSS. During storm winds, high concentrations of suspended sediment in the upper and middle layers appeared to lag the wind speed by about 8–10 h. In contrast, the turbulent resuspension event occurred at the same time as the strong winds. In-situ measurements from all three moorings indicate that it is the local weak sediment resuspension, even under the influence of storm winds in winter, that causes the formation of the low SSC area off Qinhuangdao.

Research on distribution of suspended sediment concentration in long time series based on mid-resolution imaging spectrometer data and quantile trend analysis

In order to explore the long-term variation trend of suspended sediment concentration (SSC) in the Pearl River Estuary, a multi-voyage in-situ measurements and Moderate-Resolution Imagine Spectrometry (MODIS) Level-1 products were used to establish a quantitative inversion model of the SSC in the Pearl River Estuary. In situ data were gathered with multiple campaigns, while the MODIS sensor was passing over the study area. Then, the Mann–Kendall​ algorithm and the Sen’s slope estimation method were used to analyze the trend of various quantiles SSC. The accuracy of the SSC inversion model was verified by using multiple sets of measured data in which the correlation coefficient was 0.90 and the root mean square error was 4.76. The results showed that: (1) The SSC in the Pearl River Estuary shown a zonal distribution in the northeast–southwest direction, which was gradually reduced from the nearshore to the sea, with an average concentration of more than 90 mg L−1. (2) The SSC in the PRE displayed a decreasing trend, with an averaged SSC reduction rate of about 1.02 mg L−1 yr−1 during 2004–2016. Decreasing rate in high SSC was generally larger than those was in low SSC, with a marked spatial difference. (3) The seasonal variation of dam-induced reduction of the sediment load into the PRE contributed to most of the low-percentile SSC decreases. The water-depth change in the PRE induced by human activities dominated the high-percentile SSC trends. The decrease of the wind speed over the PRE also affected the high-percentile SSC trend.

Relationships Between Chlorophyll‐a and Suspended Sediment Concentration in a High‐Nutrient Load Estuary: An Observational and Idealized Modeling Approach

AbstractThe Guadalquivir estuary is a high‐nutrient load environment, yet it has a reduced primary production because the high suspended sediment concentration (SSC) causes light attenuation. High‐resolution observations revealed relationships between the SSC and Chlorophyll‐a (Chla) concentration from seasonal to intratidal time scales. Local maxima of time‐averaged Chla levels occurred at locations with relatively low SSC. In the upper (lower) part of the estuary, larger Chla concentrations were observed during the wet (dry) season. In contrast to longer time scales, SSC and Chla exhibited in‐phase oscillations during the spring‐neap cycle. Both SSC and Chla were found to peak at maximum ebb and flood. There was no positive correlation at certain apogean neap tides, and instead, Chla exhibited daily variations. An idealized model was developed, which mimicked and helped to explain SSC‐Chla relationships at different time scales. The model accounts for the vertical advection of Chla and SSC, tidal resuspension, radiation‐mediated growth, and effects of the change in stratification stemming from tides and SSC. The specific growth rate of the Chla biomass is inhibited due to high SSC throughout the year. The in‐phase relationship at spring‐neap and tidal scales seems to be due to resuspension of algal microorganisms attached to sediments. Daily variability of Chla during apogean neap tides emerges because tidal shear stress is low and the strong decline in the suspended particulate matter reduces the light attenuation. The light‐mediated growth of part of the biomass, still remaining in the upper layers of the water column, is then driven by the day‐night cycle.