Variation Of Suspended Sediment Concentration Research Articles

3-D Simulation of the Suspended Sediment Transport in the Jiao jiang Estuary: Based on Validating by Remote Sensing Retrieval

Lu, C.Z.; Li, H.; Dai, W.Q.; Tao, J.F.; Xu, F.; Cybele, S.; Zhang, X.Y., and Guo, H.T., 2018. 3-D Simulation of the Suspended Sediment Transport in the Jiaojiang Estuary: Based on Validating by Remote Sensing Retrieval. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 116–120. Coconut Creek (Florida), ISSN 0749-0208.Suspended sediment concentration (SSC) in Jiaojiang Estuary is characterized by high concentration and significant diurnal dynamics. The large-scale reclamation will influence the sediment transport in Jiaojiang Estuary. At present, ship-borne data, remote sensing observations, and numerical modeling technique are the main technological methods to acquire the distributions of SSC. However, every method has its own limitations in application. Conventional ship-borne survey requires a lot of manpower and resources to obtain sediment distribution of an estuary although its accuracy is high. The cloudy and rainy environment has been the bottle-neck for monitoring the land and ocean surface. With the influence of land-use by human activities (such as tidal flats reclamation) being strengthened gradually, previous numerical models are not suitable for the new environment. This paper aims to combine numerical modeling technique with remote sensing observations to acquire a higher-resolution surface suspended sediment field in time and space. MIKE3 3D model was used to simulate the suspended sediments and it is composed of hydrodynamic module and mud transport module. The surface-layer sediment distribution was calculated using suspended sediment dynamic mathematical model in MIKE 3. Based on Geostationary Ocean Color Imager (GOCI) and the simultaneous surface layer sediment concentration data, a remote sensing model was developed to retrieve the distribution of SSC in the Jiaojiang Estuary. The degree of matching between the remotely sensed SSC and the numerically simulated SSC is up to 72%, both of which reveal temporal-spatial variation of the SSC. The research provides a reference for real-time monitoring and forecasting the dynamic variation of the SSC.

Dynamically Constrained Interpolation of the Sparsely Observed Suspended Sediment Concentrations in Both Space and Time: A Case Study in the Bohai Sea

AbstractThe observed suspended sediment concentrations (SSCs) obtained from the water sampling are usually sparsely distributed in both space and time, which are traditionally applied just to calibrate other types of observations. In this study a dynamically constrained interpolation methodology (DCIM) is developed to interpolate these sparsely observed SSCs in the Bohai Sea. In this method the suspended sediment transport model is taken as dynamical constraints to interpolate the observations. Meanwhile, the interpolated results are optimized iteratively by adjusting the key model parameters using the adjoint method.The DCIM is first verified using the synthetic observations produced by twin model runs. The modeling results reveal that this method is effective at interpolating the sparsely observed artificial SSCs, even when the observations are heavily contaminated by data noise. Then, the sparsely observed practical SSCs obtained from a large area survey in the Bohai Sea are interpolated using the DCIM. The interpolated results are verified by randomly selected independent observations. The discrepancies between the interpolated SSCs and the observations are significantly decreased. When all the observations are interpolated, the final interpolated SSCs captured a majority (96.88%) of observations with a factor of 2 and the correlation coefficient between the observed and interpolated SSCs is 0.98. Besides, the interpolated results have presented the reasonable dynamical variations of SSCs in the space and time domains. The modeling results indicate that the DCIM is an effective tool for interpolating the sparsely observed SSCs in both space and time.

High-Frequency Monitoring of Suspended Sediment Variations for Water Quality Evaluation at Deep Bay, Pearl River Estuary, China: Influence Factors and Implications for Sampling Strategy

Suspended sediment (SS) is an important water quality indicator of coastal and estuarine ecosystems. Field measurement and satellite remote sensing are the most common approaches for water quality monitoring. However, the efficiency and precision of both methods are typically affected by their sampling strategy (time and interval), especially in highly dynamic coastal and estuarine waters, because only limited measurements are available to analyze the short-term variations or the long-term trends of SS. Dramatic variations of SS were observed, with standard deviation coefficients of 48.9% and 54.1%, at two fixed stations in Deep Bay, China. Therefore, it is crucial to resolve the temporal variations of SS and its main influencing factors, and thus to develop an improved sampling strategy for estuarine ecosystems. Based on two years of continuous high-frequency measurements of SS and concurrent tidal and meteorological data, we demonstrated that the tide is the dominant factor influencing the SS variation among tide, wind (speed and direction), and rainfall in Deep Bay, China. For the monitoring of maximum suspended sediment concentration (SSC), the recommended optimum sampling time coincides with the occurrence of the ebb tides, whereas multiple sampling times are recommended for monitoring of minimum SSC. Although variations of SS are also affected by other factors, the recommended sampling strategy could capture the maximum and minimum SSC variations exactly more than 85% days in a year on average in Deep Bay. This study provides a baseline of SS variation and direct sampling strategy guidance for future SS monitoring and could be extended to other coastal or estuarine waters with similar climatological/tidal exposures.

Spatio-temporal Analysis of suspended sediment Concentration in the Yongjiang Estuary Based on GOCI

The concentration and spatio-temporal variation of suspended sediment concentration in the estuary area are of great significance to the nearshore engineering, port construction and coastal evolution. Based on multi-period GOCI images and corresponding measured suspended sediment concentration (SSC) data, three inversion models (the linear regression model, the power exponent model and the neural network model) were established after rapid atmospheric correction. The results show that the absolute error of the three models is 0.20, 0.16 and 0.10kg/m3 respectively, and the relative errors are 38%, 23% and 18% respectively. The accuracy of the neural network (8-17-17-1) is the best. The SSC distribution diagrams in an ebb and flow cycle are obtained using this ANN model. The results show that with Yongjiang estuary for segmentation, the high concentration area is located in the north and the lower is in the south around Jintang Island deeper water area. When the tide rises, the water flow disturbs a large amount of sediment, and then the sediment concentration increases and high area high concentrations water body moves along the SE-NW. When the tide falls, flow rate decreases and the sediment concentration decreases. However, with the falling tide, the concentration of suspended sediment in the northern sea areas gradually increases, and is higher than 1kg/m3, and gradually moves along the NW-SE until to the estuary.

Remote Sensing Studies of Suspended Sediment Concentration Variation in Barito Delta

The dynamic of suspended sediment concentration in Barito Delta depend on the process in upstream. Agriculture, mining, and illegal logging in Barito River upstream has an effect for suspended sediment concentration in Barito Delta. The objective of research is to estimate the variation of suspended sediment concentration in Barito Delta. The data of research consist of Landsat 7 in year 2011 and measurement result data of suspended sediment concentration both in wet season and dry season in year 2011. Data analysis is regression analysis to estimates the variation of suspended sediment concentration in Barito Delta. The method of research compares three types of spectral transformation for suspended sediment that is Normalized Suspended Material Index (NSMI), Normalized Differences Suspended Sediment Index (NDSSI), and band ratio (green/blue). The result of the transformation is compared with the value of the field measurement. Based on the result of the comparison can be known the suitable type of transformation for the suspended sediment estimation in Barito Delta. The result of research explains that NSMI has the highest value to estimate the variation of suspended sediment concentration in Barito Delta.

Quantitative Assessment on Multiple Timescale Features and Dynamics of Sea Surface Suspended Sediment Concentration Using Remote Sensing Data

AbstractWind‐waves, tidal currents, and some other dynamic factors dominate the suspended sediment concentration (SSC) variations in shallow seas and it is difficult to quantitatively evaluate the effects of individual dynamic factors on SSC modulation. This work used the long‐term Moderate Resolution Imaging Spectroradiometer (MODIS) and the high temporal‐resolution Geostationary Ocean Color Imager (GOCI) remote sensing data to quantify the sea surface SSC variations on multiple timescales (intratidal, spring‐neap, seasonal, and long‐term timescales) in the Bohai Sea, and further quantitatively evaluated the effects of corresponding dynamic factors on the SSC modulation. The results indicated that the monsoon associated wind‐waves and stratification played the most important role in modulating SSC, with seasonal SSC variation of 8.1 mg/L in the Bohai Sea. The intratidal current variations played the secondary important role, causing SSC variation of 5.8 mg/L. The spring‐neap tidal current variations led to SSC variation of 3.1 mg/L in the Bohai Sea. In the long run (2003–2014), the SSC of the Bohai Sea decreased slightly with SSC variation of 2.8 mg/L (decline rate: 0.23 mg/L/year), which may be caused by the weakening wind, decreasing sediment load from the Yellow River or the massive reclamation in recent decades. Probably due to the topography, sea bed sediment grain size and river plume, SSC variations in the southern Bohai Sea were more pronounced than those in the northern Bohai Sea.

Remote Sensing Studies of Suspended Sediment Concentration Variations in a Coastal Bay During the Passages of Atmospheric Cold Fronts

Mississippi River, the largest river in North America, carries a significant amount of sediment into the northern Gulf of Mexico coastal waters. In this region, recurring wind events associated with atmospheric cold fronts between fall and the following spring dominate resuspension and transport of the sediment. In this paper, based on a time series of the moderate-resolution imaging spectroradiometer satellite images with a moderate spatial resolution of 250 m during the passages of three atmospheric cold fronts in March 2013, considering the entire process of cold fronts, the distribution and variation of total suspended sediment (TSS) concentration and underlying hydrodynamic mechanism were analyzed. The TSS concentration distribution and sediment transport in the Atchafalaya Bay within the Mississippi River estuary were found to be positively correlated with the magnitude of local winds and waves, and negatively correlated with bathymetry. During the prefrontal stage, concentrations in the offshore water always decreased as a result of the wind-induced landward sediment transport. In the shallower water, the variations of TSS concentration are dominated by the intensity of wind stress. In the postfrontal period, the variations of TSS were controlled by wind-driven currents, resuspension, and vertical mixing. Although there were some exceptional situations due to the variations of the characteristics of wind and wind-induce wave, in general, the TSS concentration decreased in the entire region.

Spatiotemporal Variation in Suspended Sediment Concentrations and Related Factors of Coastal Waters Based on Multispatial Satellite Data in Gyeonggi Bay, Korea

ABSTRACT Eom, J.; Choi, J.-K.; Won, J.-S.; Ryu, J.-H.; Doxaran, D.; Ruddick, K., and Lee, S., 2017. Spatiotemporal variation in suspended sediment concentrations and related factors of coastal waters based on multispatial satellite data in Gyeonggi Bay, Korea. The variations in suspended sediment concentration (SSC) in turbid coastal waters around Gyeonggi Bay, Korea were analyzed using multiresolution ocean-color satellite imagery. Geostationary Ocean Color Imager (GOCI) and Land Satellite 7 (Landsat-7) Enhanced Thematic Mapper Plus (ETM+) images were atmospherically corrected, and an empirical algorithm was employed to generate maps of SSCs in the study area and investigate daily and annual variabilities in coastal water turbidity. SSC values were highest around 6 hours before high tide and around low tide, and the maximum values had strong positive relations with the tidal range near the sand ridge and channel (R2 values of 0.74 and 0.72, respectively), which implies that the main driver of the diurnal...

Hydrodynamic and Biological Mechanisms for Variations in Near-Bed Suspended Sediment Concentrations in a Spartina alterniflora Marsh—a Case Study of Luoyuan Bay, China

Near-bed suspended sediment concentration (SSC) was measured at the margin and in the interior of a Spartina alterniflora marsh in Luoyuan Bay, a semi-enclosed embayment sheltered from the East China Sea, in January and July 2008. Variations in SSC at the two stations correlated to the tidal cycle: high SSC occurred in the early stage of the flood tide and the later stage of the ebb tide, while SSC was significantly lower at all other times. The SSC increased from the neap tide to the spring tide in both winter and summer. The high SSC in winter was primarily a result of stronger hydrodynamic processes, whereas the high SSC in summer was primarily attributable to biological activity. The particulate organic carbon (POC) in the S. alterniflora marsh of Luoyuan Bay primarily originates from freshwater and marine algae and phytoplankton, while the contribution of marsh detritus is limited. The contribution of POC to near-bed SSC is less than 5% in both winter and summer. During winter, the near-bed SSC was closely related to the bottom shear stress; therefore, variations in SSC were controlled by hydrodynamic processes. However, the correlation between bottom shear stress and near-bed SSC was poor in summer because of the interference of burrowing organisms during the late stage of the ebb tide. The maximum estimated contribution of turbid water flowing out of caves due to the biological activity of burrowing organisms was approximately 30.3 mg L−1 or 20.2 to 50.5% with a mean value of 34.9% of total SSC at the late stage of the ebb tide. Our findings also suggest that the relative importance of hydrodynamic processes and biological activity can vary considerably between seasons.

Surficial sediment erodibility from time-series measurements of suspended sediment concentrations: development and validation

Numerical models of fine sediment transport depend on different approaches to parameterize the erosion properties of surficial sediment strata. These properties, namely the critical shear stress for erosion and the erosion rate coefficient, are crucial for reproducing the short-term and long-term sediment dynamics of the system. Methods to parameterize these properties involve either specialized laboratory measurements on sediment samples or optimization by model calibration. Based on observations of regular patterns in the variation of suspended sediment concentrations (SSC) over the tidal cycle in a small, narrow estuary, an alternate approach, referred to as the entrainment flux method, for quantifying the erosion properties of surficial bed strata is formulated and applied. The results of this method are shown to be analogous to the erosion data used to formulate the standard linear erosion formulation developed by various authors. The erosion properties inferred from the entrainment flux method are also compared to direct measurements of erodibility on sediment samples from the same site using the Gust microcosm apparatus. The favorable comparison of the two approaches suggests that the entrainment flux method can be used to infer and quantify the erodibility of surficial sediment strata in similar small and narrow estuaries. This method has certain advantages, chiefly its ease of implementation and the fact that it uses SSC time series which would typically be expected to be available for the study of or for model application at a given site. Guidelines for selecting the appropriate dataset for the application of the method are also presented.

The variations of sediment transport patterns in the outerChangjiangEstuary andHangzhouBay over the last 30 years

AbstractThe research objective is to investigate the variations of sediment transport in outer Changjiang Estuary and adjacent Hangzhou Bay, induced by the decline of Changjiang River sediment discharge and massive land reclamation in the last three decades. A synchronous hydrographic survey was conducted along two transects (at the bay‐mouth and outer Changjiang Estuary, respectively) during the spring‐neap tides of January and July 2014. The results show that the suspended sediment grain size, current velocity, suspended sediment concentration (SSC), and the water and sediment fluxes varied with the tidal cycles. Quantitative correlations with the tidal range were found for SSC and fluxes. These data have been compared with those at the same hydrographic stations in the summer and winter of the early 1980s. Along the outer Changjiang Estuary transect, the SSCs and sediment fluxes decreased in the winter, but no apparent changes occurred in the summer. The SSCs in the northern Hangzhou Bay decreased in both summer and winter, while the southern bay mouth has evolved from a low SSC region to a high SSC region. The findings clarify that the SSC and sediment flux changes in this area have only an indirect connection to the dramatic riverine sediment decline, because the sediment resuspension by the strong tidal currents provided a major source. At the present stage, the impact of the riverine sediment decline is insignificant for the SSC variation off the Changjiang River mouth. Finally, a sediment flux model is proposed to explain and predict the morphological evolution trends.

Modeling sediment mobilization using a distributed hydrological model coupled with a bank stability model

AbstractIn addition to surface erosion, stream bank erosion and failure contributes significant sediment and sediment‐bound nutrients to receiving waters during high flow events. However, distributed and mechanistic simulation of stream bank sediment contribution to sediment loads in a watershed has not been achieved. Here we present a full coupling of existing distributed watershed and bank stability models and apply the resulting model to the Mad River in central Vermont. We fully coupled the Bank Stability and Toe Erosion Model (BSTEM) with the Distributed Hydrology Soil Vegetation Model (DHSVM) to allow the simulation of stream bank erosion and potential failure in a spatially explicit environment. We demonstrate the model's ability to simulate the impacts of unstable streams on sediment mobilization and transport within a watershed and discuss the model's capability to simulate watershed sediment loading under climate change. The calibrated model simulates total suspended sediment loads and reproduces variability in suspended sediment concentrations at watershed and subbasin outlets. In addition, characteristics such as land use and road‐to‐stream ratio of subbasins are shown to impact the relative proportions of sediment mobilized by overland erosion, erosion of roads, and stream bank erosion and failure in the subbasins and watershed. This coupled model will advance mechanistic simulation of suspended sediment mobilization and transport from watersheds, which will be particularly valuable for investigating the potential impacts of climate and land use changes, as well as extreme events.

Seasonal changes of polycyclic aromatic hydrocarbons in response to hydrology and anthropogenic activities in the Pearl River estuary, China

The behaviours of PAHs (containing 2–6 aromatic rings) in the Pearl River estuary were examined each month in 2011. This study was designed to investigate the abundance of 16 priority PAHs and their response to the seasonal dynamics of anthropogenic activities and hydrological cycles. Monthly mean concentrations of ∑16PAHs in water and suspended particulate matter (SPM) were 88.31ng/L and 252.31ng/L respectively, with higher concentrations in the wet season (April to September). Heavy precipitation in the wet season resulted in relatively increased PAH input via riverine discharges and atmospheric deposition. Seasonal variations in suspended sediment concentration (SSC), temperature and salinity have considerably affected the PAH phase association. Higher SSC in the wet season contributed to higher concentration of the PAHs in SPM, and higher temperature and lower salinity facilitated desorption from SPM. The PAH sources were largely attributed to vehicular emissions, coal combustion and coke ovens.

The impacts of land reclamation on suspended-sediment dynamics in Jiaozhou Bay, Qingdao, China

A three-dimensional, high-resolution tidal model coupled with the UNSW sediment model (UNSW-Sed) based on Finite Volume Coastal Ocean Model (FVCOM) was set up to study the suspended-sediment dynamics and its change in Jiaozhou Bay (JZB) due to land reclamation over the period 1935 to 2008. During the past decades, a large amount of tidal flats were lost due to land reclamation. Other than modulating the tides, the tidal flats are a primary source for sediment resuspensions, leading to turbidity maxima nearshore. The tidal dynamics are dominant in controlling the suspended-sediment dynamics in JZB and have experienced significant changes with the loss of tidal flats due to the land reclamation. The sediment model coupled with the tide model was used to investigate the changes in suspended-sediment dynamics due to the land reclamation from 1935 to 2008, including suspended-sediment concentrations (SSC) and the horizontal suspended-sediment fluxes. This model can predict the general patterns of the spatial and temporal variation of SSC. The model was applied to investigate how the net transport of suspended sediments between JZB and its adjacent sea areas changed with land reclamation: in 1935 the net movement of suspended sediments was from JZB to the adjacent sea (erosion for JZB), primarily caused by horizontal advection associated with a horizontal gradient in the SSC; This seaward transport (erosion for JZB) had gradually declined from 1935 to 2008. If land reclamation on a large scale is continued in future, the net transport between JZB and the adjacent sea would turn landward and JZB would switch from erosion to siltation due to the impact of land reclamation on the horizontal advection of suspended sediments. We also evaluate the primary physical mechanisms including advection of suspended sediments, settling lag and tidal asymmetry, which control the suspended-sediment dynamics with the process of land reclamation.

Controls on soil organic carbon stocks in tidal marshes along an estuarine salinity gradient

Abstract. Tidal marshes are sedimentary environments and are among the most productive ecosystems on Earth. As a consequence they have the potential to reduce atmospheric greenhouse gas concentrations by sequestering organic carbon (OC). In the past decades, most research on soil organic carbon (SOC) storage in marsh environments has focused on salt marshes, leaving carbon dynamics in brackish and freshwater marshes largely understudied and neglecting the diversity among tidal marshes. We therefore conducted an extensive sampling campaign to quantify and characterize SOC stock in marshes along a salinity gradient in the Scheldt estuary (Belgium and the Netherlands). We find that SOC stocks vary significantly along the estuary, from 46 in freshwater marshes to 10 kg OC m−2 in salt marshes. Our data also show that most existing studies underestimate total SOC stocks due to shallow soil sampling, which also influences reported patterns in OC storage along estuaries. In all sampled tidal marsh sediments the SOC concentration is more or less constant from a certain depth downward. However, this concentration decreases with increasing salinity, indicating that the amount of stable SOC decreases from the upper estuary towards the coast. Although the net primary production of macrophytes differs along the estuary, our data suggest that the differences in OC storage are caused mainly by variations in suspended sediment concentration and stable particulate OC (POC) content in the water along the estuary. The fraction of terrestrial suspended sediments and POC that is transported downstream of the maximum turbidity zone is very limited, contributing to smaller amounts of long-term OC sequestration in brackish and salt marsh sediments. In addition, high rates of sediment deposition on freshwater tidal marshes in the maximum turbidity zone promote efficient burial of OC in these marsh sediments.

Measuring and modeling vertical gradients in suspended sediments in the Solimões/Amazon River

AbstractAccurately measuring sediment flux in large rivers remains a challenge due to the spatial and temporal cross‐sectional variability of suspended sediment concentrations in conjunction with sampling procedures that fail to accurately quantify these differences. This study presents a field campaign methodology that can be used to improve the measurement of suspended sediment concentrations in the Amazon River or similarly large rivers. The turbidity signal and Rouse model are together used in this study to define the spatial distribution of suspended sediment concentrations in a river cross‐section, taking into account the different size fractions of the sediment. With this methodology, suspended sediment fluxes corresponding to each sediment class are defined with less uncertainty than with manual samples. This paper presents an application of this methodology during a field campaign at different gauging stations along a 3,000‐km stretch of the Solimões/Amazon River during low water and flood periods. Vertical concentration profiles and Rouse model applications for distinctive sediment sizes are explored to determine concentration gradients throughout a cross‐section of the river. The results show that coupling both turbidity technology and the Rouse model may improve our understanding of the spatial distribution of different sediments fractions sizes in the Solimões/Amazon River. These data are very useful in defining a pertinent monitoring strategy for suspended sediment concentrations in the challenging context of large rivers.

Erosion mechanisms and sediment sources in a peatland forest after ditch cleaning

AbstractDitch cleaning in drained peatland forests increases sediment loads and degrades water quality in headwater streams and lakes. A better understanding of the processes controlling ditch erosion and sediment transport in such systems is a prerequisite for proper peatland management. In order to relate hydrological observations to key erosion processes in headwater peatlands drained for forestry, a two‐year study was conducted in a nested sub‐catchment system (treated with ditch cleaning) and at two reference sites. The treated catchment was instrumented for continuous discharge and turbidity monitoring, erosion pin measurements of changes in ditch bed and banks and time‐integrated sampling of suspended sediment (SS) composition. The results showed that ditch cleaning clearly increased transient suspended sediment concentrations (SSCs) and suspended sediment yields (SSYs), and resulted in temporary storage of loosely deposited organic sediment in the ditch network. After exhaustion of this sediment storage, subaerial processes and erosion from ditch banks became dominant in producing sediment for transport. Recorded SSCs were higher on the rising limbs of event hydrographs throughout the study period, indicating that SS transport was limited by availability of erosion‐prone sediment. A strong positive correlation (R2 = 0.84, p < 0.001) between rainfall intensity (above a threshold of 1 mm h−1) and average SSC obtained on the rising limb of hydrographs for the sub‐catchment showed that soil detachment from ditch banks by raindrop impact can directly increase SSC in runoff. At the main catchment outlet, variation in SSC was best explained (R2 = 0.67, p < 0.05) by the linear combination of initial discharge (−), peak discharge (+) and the lag time from initial to peak discharge (−). Based on these factors, ditch cleaning slightly increased peak discharges and decreased transit times in the study catchment. The implications of the results for water pollution management in peatland forests are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Flood-ebb asymmetry in current velocity and suspended sediment transport in the Changjiang Estuary

Time series measurements were conducted on suspended sediment and current velocity from neap tide to spring tide in the South Branch of the upper Changjiang Estuary in the summer of 2011. Strong flood-ebb asymmetry in the current velocity was observed in the South Branch as a result of high river runoff and tide deformation, in which the magnitude and duration of ebb currents were significantly greater than those of flood currents. The suspended sediment concentration (SSC) and suspended median grain size also exhibited remarkable flood-ebb variation; these variables were considerably larger during the ebb than during the flood and increased from neap to spring tide. Affected by the strong asymmetry in the current velocity and SSC between the flood and ebb, suspended sediment flux during the ebb was notably larger than during the flood, and a seaward tidal net flux was observed in each tidal cycle. The balance of sediment flux illustrates that the seaward sediment transport was dominated by river flow and tidal trapping and the landward sediment transport was dominated by the Stokes drift and the shear effect. Notable resuspension occurred during the spring and moderate tides. The critical velocity for the resuspension of bed sediments was estimated based on the correlation between current velocity with SSC and suspended median grain size. The results show that the critical velocity was approximately 40 cm/s during the flood phases and approximately 80 cm/s during the ebb phases because the surficial flood bed sediments located in the lower reach are much finer than the surficial ebb bed sediments located in the upper reach. The flood-ebb variation in the critical erosion velocity has significant effect on the intratidal variation of SSC and sediment transport process, and it is a common phenomenon in many estuaries of the world due to the complicated spatial distribution of bed sediments.

Remote sensing observations and numerical studies of a super typhoon-induced suspended sediment concentration variation in the East China Sea

By integrating remote sensing observations and a numerical modeling technique, we studied the influences of super Typhoon Saomai on the suspended sediment concentration (SSC) in the Yangtze River estuary and its adjacent coastal areas in the East China Sea. First, three consecutive Moderate Resolution Imaging Spectroradiometer (MODIS) images acquired during the post-typhoon stage were used to estimate the SSC. Then, we implemented a hydrodynamic model, including a sediment transport module, based on Delft3D to simulate the sediment erosion, re-suspension, transport and deposition processes in the study area during the passage of Typhoon Saomai. The model-simulated water level was validated against the in situ station data to show the feasibility of the model. The simulated SSC results agree reasonably well with the satellite observations. Time series of the simulation results showed that the model revealed the whole SSC variations during this extreme weather event and made up for the scarcity of in-situ and satellite observations. SSC significantly increased during the passage of the typhoon and decreased gradually during the post-typhoon stage. Modeled results also reveal that the spring-neap tidal effect significantly controlled the distribution and variation of SSC in the shallower coastal water (<20–30m in depth) and the typhoon-induced re-suspension is evident in most of the study area, especially in the coastal waters near the Yangtze River estuary and Hangzhou Bay during the passage of the typhoon. Finally, based on the simulation results, we discuss the dynamic mechanisms including turbulent energy, bed shear stress and vertical mixing that caused the SSC variation.

Decline in suspended sediment concentration delivered by the Changjiang (Yangtze) River into the East China Sea between 1956 and 2013

The temporal evolution of suspended sediment concentration (SSC) in a river debouching into the ocean provides vital insights into erosion processes in the watershed and dictates the evolution of the inner continental shelf. While the delivery of sediment from rivers to the ocean has received special attention in the recent past, few studies focused on the variability and dynamics of river SSC, especially in the Changjiang (Yangtze) river, China, the longest river in Asia. Here, variations in SSC delivered by the Changjiang River to the East China Sea and possible causes of its variability were detected based on a long-term time series of daily SSC and monthly water discharge measured at the Datong gauging station. The SSC data are further compared to a hydrological analysis of yearly precipitation covering the entire catchment. The results indicate the presence of a decline in SSC in the period 1956–2013, which can be divided into three phases: (i) high SSC (0.69kg/m3) in the wet season and low SSC (0.2kg/m3) in the dry season from 1956 to 1970; (ii) relative high SSC (0.58kg/m3) in the wet season and low SSC (0.15kg/m3) in the dry season from 1971 to 2002; and (iii) low SSC (0.19kg/m3) in the wet season and very low SSC (0.09kg/m3) in the dry season after 2002. These three periods have a mean yearly SSC values of 0.62, 0.42, and 0.18kg/m3, respectively. Compared with 1956–1970, the slope of the rating curve between SSC and water discharge decreased, respectively, by 2% and 30% during the period 1971–2002 and 2002–2013. Soil erosion, dam construction, and banks reinforcement along the Changjiang River are the main causes of SSC variations. Fluctuations in water discharge are also controlling the SSC long-term variations. Specifically, from 1956 to 1970, the effect of soil erosion overrules that of dam impoundment, which is likely responsible for the high SSC; during the period 1970–2002, the influence of dam impoundment increases while that of soil erosion decreases, which together produce a small reduction in SSC. Since 2002, the impact of soil erosion further decreases and large-scale sediment trapping behind the Three Gorges Dam is responsible for the occurrence of extremely low SSC. The results presented herein for the Changjiang River can inform a better management strategy of sediment resources and water quality for both the river and the coast. Our conclusions can be well applied to other rivers discharging in the ocean subject to similar human activities.