Variations in tidal currents and suspended sediment concentration of the upper part of the North Channel of Changjiang Estuary over the past 10 years

The tidal current duration （TCD） and velocity （TCV） and suspended sediment concentration （SSC） were measured in the dry season in December, 2011 and in the flood season in June, 2012 at the upper part of the North Channel of Changjiang Estuary. They were assimilated with the measured data in 2003, 2004, 2006 and 2007, using the tidal range＇s proportion conversion. Variations in TCD and TCV, preferential flow and SSC have been calculated. Influences of typical engineering projects such as Qingcaosha fresh water reservoir, Yangtze River Bridge, and land reclamation on the ebb and flood TCD, TCV and SSC in the North Channel for the last 10 years are discussed. The results show that： （1） currently, in the upper part of North Channel, the ebb tide dominates; after the construction of the typical projects, ebb TCD and TCV tends to be larger and the vertical average ebb and flood SSC decrease during the flood season while SSC increases during the dry season; （2） changes in the vertical average TCV are mainly contributed by seasonal runoff variation during the flood season, which is larger in the flood season than that in the dry season; the controlling parameters of increasing ebb TCD and TCV are those large-scale engineering projects in the North Channel; variation in SSC may result mainly from the reduction of basin annual sediment loads, large-scale nearshore projects and so on.

Intratidal and neap-spring variations of suspended sediment concentrations and sediment transport processes in the North Branch of the Changjiang Estuary

Profles of tidal current and suspended sediment concentration （SSC） were measured in the North Branch of the Changjiang Estuary from neap tide to spring tide in April 2010. The measurement data were analyzed to determine the characteristics of intratidal and neap-spring variations of SSC and suspended sediment transport. Modulated by tidal range and current speed, the tidal mean SSC increased from 0.5 kg/m3 in neap tide to 3.5 kg/ma in spring tide. The intratidal variation of the depth-mean SSC can be summarized into three types： V-shape variation in neap tide, M-shape and mixed M-V shape variation in medium and spring tides. The occurrence of these variation types is controlled by the relative intensity and interaction of resuspension, settling and impact of water exchange from the rise and fall of tide. In neap tide the V-shape variation is mainly due to the dominant effect of the water exchange from the rise and fall of tide. During medium and spring tides, resuspension and settling processes become dominant. The interactions of these processes, together with the sustained high ebb current and shorter duration of low-tide slack, are respon- sible for the M-shape and M-V shape SSC variation. Weakly consolidated mud and high current speed cause significant resuspension and remarkable flood and ebb SSC peaks. Setting occurs at the slack water periods to cause SSC troughs and formation of a thin flufflayer on the bed. Fluxes of water and suspended sediment averaged over the neap-spring cycle are all seawards, but the magnitude and direction of tidal net sediment flux is highly variable.

Temporal and spatial changes of suspended sediment concentration and resuspension in the Yangtze River estuary

A detailed analysis of suspended sediment concentration (SSC) variations over a year period is presented using the data from 8 stations in the Yangtze River estuary and its adjacent waters, together with a discussion of the hydrodynamic regimes of the estuary. Spatially, the SSC from Xuliujing downwards to Hangzhou Bay increases almost constantly, and the suspended sediment in the inner estuary shows higher concentration in summer than in winter, while in the outer estuary it shows higher concentration in winter than in summer, and the magnitude is greater in the outer estuary than in the inner estuary, greater in the Hangzhou Bay than in the Yangtze River estuary. The sediments discharged by the Yangtze River into the sea are resuspended by marine dynamics included tidal currents and wind waves. Temporally, the SSC shows a pronounced neap-spring tidal cycle and seasonal variations. Furthermore, through the analysis of dynamic mechanism, it is concluded that wave and tidal current are two predominant factors of sediment resuspension and control the distribution and changes of SSC, in which tidal currents control neap-spring tidal cycles, and wind waves control seasonal variations. The ratio between river discharge and marine dynamics controls spatial distribution of SSC.

A review on quantitative analysis of suspended sediment concentrations

Information on the concentration of suspended sediments in coastal waters is necessary for the understanding and management of the coastal environment. Traditionally, suspended sediment concentration （SSC） has been measured by time-consuming and costly boat surveys which allow the accurate measurement of SSC for single points in space and time. In order to obtain the instantaneous measurement of SSC, a variety of remote sensing method has been adopted. Remote sensing from airborne and spaceborne sensors has been proven to be a useful adjunct to such surveys as it provides an instantaneous and synoptic view of sediments that would otherwise be unavailable. Dominique Durand and Jerome Bijaoui in 2000 presented a feasible study on optical remote sensing of shallow-water environmental parameters. Yogesh C. Agrawal and H. C. Pottsmith in 2001 tried to use Laser Diffraction Sensors to measure Concentration and Size Distribution of Suspended Sediment. Francisco Pedocchi and Marcelo H. Garcia in 2006 made an evaluation of the LISST-ST instrument for suspended particle size distribution and settling velocity measurements. H.K.Ha and W-Y.Hsu in 2009 tried to measure suspended cohesive sediment concentration using ADV backscatter strength. Shuisen Chen, Ligang Fang in 2009 managed to use remote sensing of turbidity in seawater intrusion reaches of Pearl River Estuary. In this article, the brief review of most of the technologies or methods used to observe the suspended sediment concentration is executed. As the most powerful technology in the remote sensing, acoustic backscatter device is discussed in detail. A comparison between those traditional and modern technologies is made to clarify its future application and development.

Experimental Study on Suspended Sediment Concentration and Its Vertical Distribution under Spilling Breaking Wave Actions in Silty Coast

In this paper, flume experiments are focused on sediment transport inside and outside the surf zone. According to the energy dissipation balance principle of sediment-laden flow and the similarity between energy dissipation of spilling breaking wave and hydraulic jump, formulas are proposed to predict time averaged suspended sediment concentration under both non-breaking and breaking waves. Assuming that the sediment diffusion coefficient, which is related with energy dissipation, is proportional to water depth, formulas are proposed to predict close-to-bed suspended sediment concentration and vertical distribution of suspended sediment under spilling breaking waves, and the prediction shows a good agreement with the measurement.

Remote Sensing Retrieval of Surface Suspended Sediment Concentration in the Yellow River Estuary

Accurate assessment of surface suspended sediment concentration （SSSC） in estuary is essential to address several important issues： erosion, water pollution, human health risks, etc. In this study, an empirical cubic retrieval model was developed for the retrieval of SSSC from Yellow River Estuary. Based on sediments and seawater collected from the Yellow River and southeastern Laizhou Bay, SSSC conditions were reproduced in the laboratory at increasing concentrations within a range common to field observations. Continu- ous spectrum measurements of the various SSSCs ranging from 1 to 5700 mg/1 were carried out using an AvaField-3 spectrometer. The results indicated the good correlation between water SSSC and spectral reflectance （Rrs） was obtained in the spectral range of 726-900 nm. At SSSC greater than 2700 mg/L, the 740-900 nm spectral range was less susceptible to the effects of spectral reflectance saturation and more suitable for retrieval of high sediment concentrations. The best correlations were obtained for the reflectance ratio of 820 nm to 490 nm. Informed by the correlation between Rrs and SSSC, a retrieval model was developed （R2 = 0.992）. The novel cubic model, which used the ratio of a near-infrared （NIR） band （740-900 nm） to a visible band （400-600 nm） as factors, provided robust quantifica- tion of high SSSC water samples. Two high SSSC centers, with an order of 103 mg/1, were found in the inversion results around the abandoned Diaokou River mouth, the present Yellow River mouth to the abandoned Qingshuigou River mouth. There was little sedi- ment exchange between the two high SSSC centers due to the directions of the residual currents and vertical mixing.

Delineating suspended sediment concentration patterns in surface waters of the Changjiang Estuary by remote sensing analysis

Three Landsat TM imageries （taken on 18 May 1987,4 August 1998 and 28 July 2007） were used as the data source to identify the spatial and temporal variations of the suspended sediment concentration （SSC） in surface waters of the Changjiang Estuary.Atmospheric correction was carried out to determine the water-leaving reflectance using the FLAASH module.A regression equation between surveyed SSC and suspended sediment index was chosen to retrieve the SSC from the Landsat TM images.In addition,tidal harmonic analysis was performed to calculate tidal conditions corresponding to the acquisition time of satellite images.The results show that the SSC spatial patterns are similar to the in situ observation results,which show the highest SSC in the region of turbidity maximum zone in the Changjiang Estuary.For the period of 1987 to 2007,the SSC pattern is controlled mainly by tidal dynamic conditions and wind speeds,rather than sediment discharges from the river.

Filling of the Three Gorges Reservoir to the 135-m Level: Instant Effects on the Yangtze Discharge and Suspended Sediment Concentration Entering the Estuary

Via the valuable opportunity of the Three Gorges Reservoir (TGR) 135-m filling in June 2003, the Yangtze discharge and suspended sediment concentration (SSC) entering the estuary during the period from 15 May to 15 July 2003 were analyzed to examine the instant effects of the filling on them. The Yangtze discharge and SSC entering the estuary in the periods before, during and after the filling clearly indicated three phases: 1) the pre-storage phase characterized by natural conditions, in which the SSC increased with increasing water discharge; 2) the storage phase, during which the SSC decreased dramatically with decreasing water discharge; and 3) the post-storage phase, during which both the SSC and water discharge remained at relatively low levels first until the end of June, then the SSC increased gradually with increasing water discharge. It seems that the times for the instant effects of the decreasing discharge downstream from the upper Yangtze on the Yangtze discharge and SSC entering the estuary due to the TGR 135-m filling to take place were about 5 d and 1 d respectively, while both were about 18 d for those of the increasing discharge. This probably reflects the buffering and resultantly hysteresis of the 1800-km stretch from the upper Yangtze to the estuary. The results are helpful for scientific and hydrological investigation of the Yangtze mainstream downstream from the TGR Dam and of the estuarine and adjacent coastal waters.

Estimation of suspended sediment concentrations using Pulse-coherent Acoustic Doppler Profiler (PCADP)

The objective of the study is to investigate the suitability of using Pulse-coherent Acoustic Doppler Profiler (PCADP) to estimate suspended sediment concentration (SSC). The acoustic backscatter intensity was corrected for spreading and absorption loss, then calibrated with OBS and finally converted to SSC. The results show that there is a good correlation between SSC and backscatter intensity with R value of 0.74. The mean relative error is 22.4%. Then the time span of little particle size variation was also analyzed to exclude the influence of size variation. The correlation coefficient increased to 0.81 and the error decreased to 18.9%. Our results suggest that the PCADP can meet the requirement of other professional instruments to estimate SSC with the errors between 20% and 50%, and can satisfy the need of dynamics study of suspended particles.

Experimental Research on Quantitative Inversion Models of Suspended Sediment Concentration Using Remote Sensing Technology

Research on quantitative models of suspended sediment concentration （SSC） using remote sensing technology is very important to understand the scouting and siltation variation in harbors and water channels. Based onlaboratory study of the relationship between different suspended sediment concentrations and reflectance spectra measured synchronously, quantitative inversion models of SSC based on single factor, band ratio and sediment parameter were developed, which provides an effective method to retrieve the SSC from satellite images. Results show that the bl （430-500nm） and b3 （670-735nm） are the optimal wavelengths for the estimation of lower SSC and the b4 （780-835nm） is the optimal wavelength to estimate the higher SSC. Furthermore the band ratio B2/B3 can be used to simulate the variation of lower SSC better and the B4/B1 to estimate the higher SSC accurately. Also the inversion models developed by sediment parameters of higher and lower SSCs can get a relatively higher accuracy than the single factor and band ratio models.

Suspended sediment concentration mapping based on the MODIS satellite imagery in the East China inland, estuarine, and coastal waters

The purpose of this research is to improve the retrieval accuracy for the suspended sediment concentration(SSC) from in situ and satellite remote sensing measurements in turbid East China estuarine and coastal waters. For this aim, three important tasks are formulated and solved: 1) an estimation of remote-sensing reflectance spectra R_(rs)(λ) after atmospheric correction; 2) an estimation of R_(rs)(λ) from the radiometric signals above the air-water surface; and 3) an estimation of SSC from R_(rs)(λ). Six different models for radiometric R_(rs)(λ) determination and 28 models for SSC versus R_(rs)(λ) are analyzed based on the field observations made in the Changjiang River estuary and its adjacent coastal area. The SSC images based on the above-mentioned analysis are generated for the area.

Numerical simulation on the process of saltwater intrusion and its impact on the suspended sediment concentration in the Changjiang(Yangtze)estuary

To study the relationship between sediment transportation and saltwater intrusion in the Changjiang (Yangtze) estuary, a three-dimensional numerical model for temperature, salinity, velocity field, and suspended sediment concentration was established based on the ECOMSED model. Using this model, sediment transportation in the flood season of 2005 was simulated for the Changjiang estuary. A comparison between simulated results and observation data for the tidal level, flow velocity and direction, salinity and suspended sediment concentration indicated that they were consistent in overall. Based on model verification, the simulation of saltwater intrusion and its effect on sediment in the Changjiang estuary was analyzed in detail. The saltwater intrusion in the estuary including the formation, evolution, and disappearance of saltwater wedge and the induced vertical circulation were reproduced, and the crucial impact of the wedge on cohesive and non-cohesive suspended sediment distribution and transportation were successfully simulated. The result shows that near the salinity front, the simulated concentrations of both cohesive and non-cohesive suspended sediment at the surface layer had a strong relationship with the simulated velocity, especially when considering a 1-hour lag. However, in the bottom layer, there was no obvious correlation between them, because the saltwater wedge and its inducing vertical circulation may have resuspended loose sediment on the bed, thus forming a high-concentration area near the bottom even if the velocity near the bottom was very low during the transition phase from flood to ebb.

Laboratory Application of Laser Grain-Size Analyzer in Determining Suspended Sediment Concentration

Suspended sediment concentration(SSC) is an important parameter in marine sedimentology. With the development of technology, many acoustic and optical devices, such as the Laser In-Situ Scattering and Transmissometry, have been designed to measure in situ SSC and grain size distribution. But due to fund or other restrictions, many experiments were only conducted in laboratory, using an indoor laser grain-size analyzer and gravimetric method to measure grain size distribution and concentration, respectively. In this study the laboratory experiment is simplified by omitting the tiring step of gravimetric method. The connections between SSC and other parameters(obscuration, D50 and sorting index) were investigated based on 124 surface sediment samples collected from different offshore areas. A new method is developed for determining SSC in laboratory using a laser grain-size analyzer.

Short-term variations and influencing factors of suspended sediment concentrations at the Heisha Beach,Guangdong,China

Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes.Thus,a field measurement was conducted at the Heisha Beach,located on the west coast of the Zhujiang River(Pearl River)Estuary,to investigate the short-term variation in suspended sediment concentrations(SSCs)and the relationship between the SSC and turbulent kinetic energy,bottom shear stress(BSS),and relative wave height.Based on extreme event analysis results,extreme events have a greater influence on turbulent kinetic energy than SSC.Although a portion of the turbulent kinetic energy dissipates directly into the water column,it plays an important role in suspended sediment motion.Most of the time,the wave-current interaction is strong enough to drive sediment incipience and resuspension.When combined,the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone.The relative wave height also has a strong correlation with SSC,indicating that the combined effect of water depth and wave height significantly impacts SSC variation.Water depth is mainly controlled by the tide on the beaches;thus,the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.

Feasibility study on the binary-parameter retrieval model of ocean suspended sediment concentration based on MODIS data

This paper brought out a new idea on the retrieval of suspended sediment concentration, which uses both the water-leaving radiance from remote sensing data and the grain size of the suspended sediment. A principal component model and a neural network model based on those two parameters were constructed. The analyzing results indicate that testing errors of the models using the two parameters are 0.256 and 0.244, while the errors using only water-leaving radiance are 0,384 and 0.390. The stability of the models with grain size parameter is also better than the one without grain size. This research proved that it is necessary to introduce the grain size parameter into suspended sediment concentration retrieval models in order to improve the retrieval precision of these models.

The synchronicity and difference in the change of suspended sediment concentration in the Yangtze River Estuary

The sediment discharge from the Yangtze River Basin has a stepwise decreasing trend in recent years. The impounding of the Three Gorges Reservoir exacerbated this de- creasing trend and affected the change of the suspended sediment concentration （SSC） in the Yangtze River Estuary through the transmission effect. The SSC data of the Yangtze River Estuary during 1959-2012 showed that： （1） The SSC in the South Branch of the Yang- tze River in the estuary and in the off-shore sea area displayed decreasing trends and de- creased less towards the sea. At the same time, the difference in decreasing magnitude be- tween SSC and sediment discharge became bigger towards the sea. （2） For the North Branch the preferential flow did not change much but the SSC tended to decrease, which was mainly caused by the decrease of SSC in the South Branch and China East Sea. （3） Due to the de- creased runoff and the relatively strengthened tide, the peak area of the SSC in the bar shoal section in 2003-2012 moved inward for about 1/6 longitude unit compared with that in 1984-2002, and the inward-moving distance was in the order of flood season 〉 annual average 〉 dry season. （4） In the inlet of the South Passage, the SSC decreased mainly because the increase caused by resuspension and shore-groove exchange was less than the decrease caused by the sharp SSC decrease in the basin and the sea areas. The reverse was true in the middle section, where the SSC showed an increasing trend. （5） In the inlet of the North Passage, under the combined influence of decreased flow split and sediment split ratios, the decreased SSC in the basin and the sea area and decreased amount of resuspension, the SSC displayed a decreasing trend. In the middle section, because the increased amount caused by sediment going over the dyke was markedly more than the decreased amount caused by external environments, the SSC tended to increase. Holistically, the sharp decrease in sediment discharge caused synchronized SSC decreases in the Yangtze River Estuary. But there were still areas, where the SSC displayed increasing trends, indicatingsynchronicity and difference in the response of SSC to the sharp decrease in sediment discharge from the basin.

The variations of suspended sediment concentration in Yangtze River Estuary

With the Yangtze River Estuary as an example, this paper analyzes the influence of human activity on the sea water content, the sediment content and the regional transport situation. In both flood seasons and dry seasons, as well as in the whole year, the sediment discharge rate and the suspended sediment concentration （SSC） in the estuary area of the Yangtze River show decreasing trends. In the estuary area, the amount of sediment transported into the South and North Branches and the offshore area is also in a decreasing trend over the years. The SSCs at the sections at the entrances of the South Branch, the South Channel, the North Channel and the South Passage in the Yangtze River Estuary show decreasing trends during 1959-2011. The remote-sensing data reveal that for the same runoff and tidal current combination, the SSC in the surface water is decreased, caused by the dramatic decrease in the sediment discharge and the sediment content from the watershed. Although the SSC in the offshore area does not exhibit an apparent decreasing trend, the SSC in the nearshore area drops by 21.42% during the period of 2003-2011 as compared with that during the period of 1985-2002 before the impounding of the Three Gorges Reservoir. The sediment re-suspension in the estuary slows down the reduction of the SSC. The period of2003-2011 is dry years for the Yangtze River, when the flow is relatively low. The peak value of the SSC in the coast area shifts inwards for about 1/60 of longitude. Due to the deepwater channel improve- ment project in the North Passage at the Yangtze River Estuary in 1998, the SSC in the waterway area is decreased. With the de- crease of the divided flow ratio and the sediment splitting ratio in the entrance and the exit of the North Passage, the SSC in the upper and lower reaches of the North Passage during the period of 2008-2011 is decreased by about 14.25% as compared with that during the period of 2000-2007. Primarily due to the jetty at the south side, the SSC in the middle reach shows an increasing trend.

A study of impact of the reclamation and artificial island projects on tidal currents and sediment concentration in radial sand ridge field of the South Yellow Sea

The two-dimensional hydrodynamic model, MIKE21, is applied to simulate the tidal currents and sediment concentration in the radial sand ridges of the South Yellow Sea. Results are in accordance with in situ observations. Then the variations of tidal currents and suspended sediment concentration caused by reclamation and artificial islands projects are simulated. The results show that the impacts are limited around the project areas. After the projects, the fan-shaped, Jianggang centered tidal current pattern would be replaced by a pattern which is formed by two tidal paths. One locates at the Xiyang channel in north-south direction, and the other locates at the Huangshayang channel in east-west direction. The reclamation of Tiaozini separates the waters into south portion and north portion. The changes of sediment concentrations coincide with those of currents. Both the sediment concentrations and tidal currents increase at the northwest of Dongsha and the south of Gaoni, while both decrease at the north and south of Tiaozini, and the east and southwest of Dongsha.

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.

Estimation of suspended sediment flux from acoustic Doppler current profiling along the Jinhae Bay entrance

A Nortek acoustic Doppler current profiler (NDP) was installed on a moving vessel to survey the entrance to the Jinhae Bay on August 22~23, 2001. The current velocity and acoustic backscattering signal were collected along two cross-sections; water samples were also collected during the measurement. The acoustic signals were normalized to compensate for the loss incurred by acoustic beam spreading in the seawater. The in situ calibration shows that a significant relationship is present between suspended sediment concentrations (SSC) and normalized acoustic signals. Two acoustic parameters have been determined to construct an acoustic-concentration model. Using this derived model, the SSC patterns along the surveyed cross-sections were obtained by the conversion of acoustic data. Using the current velocity and SSC data, the flux of suspended sediment was estimated. It indicates that the sediment transport into the bay through the entrance has an order of magnitude of 100 t per tidal cycle.