Magnetic properties of tidal flat sediments of the Yangtze Estuary and its relationship with particle size

Since the 1970s, environmental magnetism has found extensive applications in diverse research areas of geoscience[1]. In China, a lot of environmental magnetic studies have been carried out on loess, lake and marine sediments for the purpose of paleoclima…     

Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics

Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand-dominated layers (SDLs) and mud-dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.     

Seasonal,tidal, and geomorphic controls on sediment export to Amazon River tidal floodplains

Mainstem–floodplain material exchange in the tidal freshwater reach of major rivers may lead to significant sequestration of riverine sediment, but this zone remains understudied compared to adjacent fluvial and marine environments. This knowledge gap prompts investigation of floodplain-incising tidal channels found along the banks of tidal rivers and their role in facilitating water and suspended-sediment fluxes between mainstem and floodplain. To evaluate this role, and how it evolves along the tidal river and with time, we measured water level, flow velocity, temperature, and suspended-sediment concentration (SSC) in four tidal channels along the tidal Amazon River, Brazil. Eleven deployments were made during low, rising, high, and falling seasonal Amazon discharge. Generally, channels export high-SSC water from the mainstem to the tidal floodplain on flood tides and transfer low-SSC water back to the mainstem on ebbs. Along the length of the tidal river, the interaction between tidal and seasonal water-level variations and channel–floodplain morphology is a primary control on tidal-channel sediment dynamics. Close to the river mouth, where tides are large, this interaction produces transient flow features and current-induced sediment resuspension, but the importance of these processes decreases with distance upstream. Although the magnitude of the exchange of water and sediment between mainstem and floodplain via tidal channels is a small percentage of the total mainstem discharge in this large tidal-river system, tidal channels are important conduits for material flux between these two environments. This flux is critical to resisting floodplain submergence during times of rising sea level. © 2019 John Wiley & Sons, Ltd.     

Seasonal variation of river and tide energy in the Yangtze estuary,China

In many large estuaries there are significant variations in flow conditions due to the interaction between tide (with spring–neap changes) and river discharge (with wet–dry seasons), which is key to understanding the evolution of the morphology and the resultant equilibrium state. To explore whether there exists an equilibrium state, and what might control such a state in such a dynamic environment, both numerical and analytical methods have been used to investigate the relative importance of tide and river contributions to the work done locally and globally over a wide range of discharge conditions in the Yangtze estuary. In particular, we have quantified the contributions from the tidal flow, the river flow and the tide–river interaction in terms of energy and its dissipation under different river discharge conditions. Model results suggest that there is a state of minimum tidal work for the case representing the wet season, when river and tide are doing uniform work locally and minimum work globally, within the bi‐directional tidal reach for tide and along the whole estuary for river. We also observe that the system is not optimized for other conditions (peak discharge and low flows during the dry season), but the system would tend to do the minimum work possible given the constraints on the system (e.g. imposed forcing conditions and available sediment supply). Results, therefore, are consistent with the use of these two energetic optimization principles, and the proposed method could be applicable to other alluvial estuaries. Copyright © 2015 John Wiley & Sons, Ltd.     

Benthic ecosystem development in an artificial tidal flat constructed from dredged spoil

Dredged spoil (DS) was used as a silt and clay additive in the construction of artificial tidal flats from mountain sand (MS). As the ratio of DS in the sediment media increased, the number of emerging macrobenthos increased. The composition of the macrobenthic community was also affected by the addition of DS, and the changes might be dependent on the ratio of DS to MS. In addition, the macrobenthos in the artificial tidal flats was more abundant than that in the control tidal flat, which was constructed with natural tidal flat sediment. With a silt and clay content of 25%, polychaetes Ceratonereis erythraeensis and Capitella sp. and the gastropod Batillaria cumingii were dominant, whereas no bivalves were present. With less silt and clay (5% and 10%), the bivalves Ruditapes philippinarum and Musculista senhousia were observed in the artificial flats, while their numbers in the control tidal flat were lower.     

The influence of neap–spring tidal variation and wave energy on sediment flux in salt marsh tidal creeks

Sediment flux in marsh tidal creeks is commonly used to gauge sediment supply to marshes. We conducted a field investigation of temporal variability in sediment flux in tidal creeks in the accreting tidal marsh at China Camp State Park adjacent to northern San Francisco Bay. Suspended‐sediment concentration (SSC), velocity and depth were measured near the mouths of two tidal creeks during three 6‐ to 10‐week deployments: two in winter and one in summer. Currents, wave properties and SSC were measured in the adjacent shallows. All deployments spanned the largest spring tides of the season. Results show that tidally averaged suspended‐sediment flux (SSF) in the tidal creeks varied from slightly landward to strongly bayward with increasing tidal energy. SSF was negative (bayward) for tidal cycles with maximum water surface elevation above the marsh plain. Export during the largest spring tides dominated the cumulative SSF for each deployment. During ebb tides following the highest tides, velocities exceeded 1 m s^?1 in the narrow tidal creeks, resulting in negative tidally averaged water flux, and mobilizing sediment from the creek banks or bed. Storm surge also produced negative SSF. Tidally averaged SSF was positive in wavy conditions with moderate tides. Spring tide sediment export at the creek mouth was about twice that at a station 130 m further up the tidal creek. The negative tidally averaged water flux near the creek mouth during spring tides indicates that in the lower marsh some of the water flooding directly across the bay–marsh interface drains through the tidal creeks, and suggests that this interface may be a pathway for sediment supply to the lower marsh as well. Copyright © 2018 John Wiley & Sons, Ltd.     

Morphodynamic characteristics of the dextral diversion of the Yangtze River mouth,China: tidal and the Coriolis Force controls

This paper examines the morphological development of the Yangtze River mouth, which has been diverting southeasterly (dextrally), according to historical (150 years) chart‐based digital evolution model and on‐site measured tidal flow data. We reveal a significantly narrowing of the northern river mouth branch from formerly >30 km wide to presently 10 km wide due to rapid siltation. Net siltation there, however, decreases gradually, which largely contrasts with the fact that the siltation has shifted to the southern river mouth area, as shown by many newly‐emerged estuarine islands, sandy shoals and bifurcated branches. Our data have further demonstrated that the ebb flow that dominates in the study area changes its direction gradually from east to southeast from the inner to outer river mouth area, and its duration is much longer than the flood flow in the inner river mouth area, but nearly equal at the river mouth area. Accordingly, the sediment transport pathway has been diverted from east to southeast. We examine whether the Coriolis Force could explain the dextral diversion of the ebb flow and the altered morphodynamical processes. Although too weak to strengthen the tidal flows, the Coriolis Force can drag the ebb flow southeasterly, and so influence sediment transport paths at the estuarine scale. The Coriolis Force is limited in the inner river mouth, but substantial at and in the outer river mouth area when gradually free of estuarine topographic constraints. The Coriolis Force causes an offset in propagation of in‐out flow directions at the river mouth area to form a slack water setting prone to estuarine siltation. Using the present approach also enables explanation of the morphological development of the Holocene Yangtze delta‐coast that extends to the southeast. Copyright © 2010 John Wiley & Sons, Ltd.     

Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision

A detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred. The measured bore, within the Arcins Channel of the Garonne River (France), was undular in nature and was followed by well‐defined secondary wave motion. Due to the local river geometry a collision between the Arcins channel tidal bore and the bore which formed within the main Garonne River channel was observed about 800 m upstream of the sampling site. This bore collision generated a transient standing wave with a black water mixing zone. Following this collision the bore from the main Garonne River channel propagated ‘backward’ to the downstream end of the Arcins channel. Velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage, including during the 'backward' bore propagation. Large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion, consistent with the murky appearance of the flood tide waters. Copyright © 2015 John Wiley & Sons, Ltd.     

Heavy metal pollution and assessment in the tidal flat sediments of Haizhou Bay,China

The heavy metal inventory and the ecological risk of the tidal flat sediments in Haizhou Bay were investigated. Results show that the average concentrations of heavy metals in the surface sediments exceeded the environment background values of Jiangsu Province coastal soil, suggesting that the surface sediments were mainly polluted by heavy metals (Cd, Cr, Cu, Mn, Pb and Zn). In addition, the profiles of heavy metals fluxes can reflect the socio-economic development of Lianyungang City, and heavy metals inputs were attributed to anthropogenic activities. Cr, Cu, Pb and Zn were mainly present in the non-bioavailable residual form in surface sediments, whereas Cd and Mn were predominantly in the highly mobile acid soluble and reducible fractions. The ecological risk of the polluted sediments stemmed mainly from Cd and Pb. According to the Sediment quality guidelines (SQGs), however, the adverse biological effects caused by the heavy metals occasionally occurred in tidal flat.     

Trends in suspended sediment grain size in the upper Yangtze River and its tributaries,as influenced by human activities

Abstract

Based on data from five hydrometric stations, Pingshan station on the Jinshajiang River, Gaochang station on the Minjiang River, Wulong station on the Wujiang River, Wusheng station on the Jialingjiang River and Yichang station on the Yangtze River, a study has been made of the temporal variation in grain size of suspended sediment load in the upper Yangtze River. The results show that in the past 40 years, the grain size of the suspended sediment load in the main stem and major tributaries of the upper Yangtze River has had a decreasing trend, that can be explained by the effect of reservoir construction and implementation of soil conservation measures. The reservoirs in the upper Yangtze River Basin, all used for water storage for hydro-electric generation and/or irrigation, have trapped coarse sediment from the drainage area above the dam and, thus, the sediment released now is much finer than before the construction of the reservoirs. The downstream channels are all gravel-bedded or even in bedrock, with little fine sediment, and thus, the released flow can hardly get a supply of fine sediment through eroding the bed. Then, after the downstream adjustment, the grain size of suspended sediment is still fine. Large-scale soil conservation measures have significantly reduced sediment yield in some major sediment source areas. The relatively coarse sediment is trapped and, thus, the sediment delivered to the river becomes finer.     

A two-dimensional model for suspended sediment transport in the southern branch of the Rhine–Meuse estuary,The Netherlands

For the southern branch of the Rhine–Meuse estuary, The Netherlands, a two-dimensional horizontal suspended sediment transport model was constructed in order to evaluate the complicated water quality management of the area. The data needed to calibrate the model were collected during a special field survey at high river runoff utilizing a number of techniques: (1) turbidity probes were used to obtain suspended sediment concentration profiles; (2) air-borne remote sensing video recordings were applied in order to obtain information concerning the spatial distribution of the suspended sediment concentration; (3) an acoustic probe (ISAC) was used to measure cohesive bed density profiles and (4) an in situ underwater video camera (VIS) was deployed to collect video recordings of the suspended sediment. These VIS data were finally processed to fall velocity and diameter distributions and were mainly used to improve insight into the relevant transport processes, indicating significant erosion of sand from the upstream Rhine branch. For quantitative calibration of the model, the data from the turbidity profiles were used. Sedimentation and erosion were modelled according to Krone and Partheniades. The model results showed a good overall fit to the measurements, with a mean absolute error of 18 per cent (standard fault = 1 per cent), corresponding to concentrations of about 0·020 (upstream) to 0·005 kg m⁻³ (downstream). The overall correlation between observed and simulated suspended sediment concentrations was 0·85. The remote sensing video recordings were used for a qualitative calibration of the model. The distribution pattern of the suspended sediment on these photos was reproduced quite well by the model. However, a more accurate calibration technique is needed to enable the use of aerial remote sensing as a quantitative calibration method. Copyright © 1999 John Wiley & Sons, Ltd.     

Suspended sediment balance for the mainstem of Changjiang (Yangtze River) in the period 1964–1985

Suspended sediment dynamics during the period 1964–1985 are examined along the mainstem of Changjiang (Yangtze River). The period represents a basin condition prior to major changes in land management policy and dam building on the river's mainstem. The downstream sediment dynamics reflect basin geology and topography and channel morphology. Sediment exchange within the mainstem was calculated by the development of reach sediment balances that reveal complex temporal and spatial patterns. There is relatively little sediment exchange in the upper, bedrock‐controlled reaches, with systematic increases in the downstream alluvial reaches. Degrading, transfer, and aggrading reaches were identified. Relations between input and output in all reaches were significant but no relation was found between sediment exchange and input/output. Comparison between ‘short‐term’ (22 years) and ‘long‐term’ (52 years) records demonstrates the importance of the record length in studying the suspended sediment dynamics in a large fluvial system. The longer record yielded better correlation and different trends than the shorter record. Sediment transfer (output/input ratio) changes downstream: the dominance of the upstream contributing area in sustaining the appearance of net degradation through most of the river system highlights the importance of reach length on characterisation of suspended sediment dynamics in large fluvial systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Decadal link between longitudinal morphological changes in branching channels of Yangtze estuary and movement of the offshore depo-center

In estuaries, the morphology of inland and offshore areas usually evolves synergistically. This study examines the decadal link between longitudinal changes in morphology of branching channels and movement of the offshore depo-center (where sediment deposition rate is maximum) of the Yangtze River estuary, under intense human interference. Integrated data analysis is provided on morphology, runoff discharge, and ebb partition ratio from 1950 to 2017. Channel-volume reductions and change rates between isobaths in branching channels reflect the impact of estuarine engineering projects. Ebb partition ratio and duration of discharge ≥ 60 000 m³ s^-1 act as proxies for the water excavating force in branching channels and runoff intensity. It is found that deposition occurs in the lower/upper sub-reaches (or further downstream/upstream channels) of the inland north/south branching channels, and the offshore depo-center moves southward or southeastward, as runoff intensity grows; the reverse occurs as runoff intensity declines. This is because the horizontal circumfluence in the Yangtze estuary rotates clockwise as ebb partition ratios of the north/south branching channels increase/decrease for increasing runoff, and conversely rotates anticlockwise for decreasing runoff. Land reclamation activities, the Deepwater Channel Project, and the Qingcaosha Reservoir have impacted greatly on longitudinal changes of morphology in the North Branch and the South Passage and on ebb partition ratio variations in the North/South Channel and the North/South Passage. Dam-induced runoff flattening has enhanced deposition in the upper/lower sub-reaches of the north/south branching channels and caused northward movement of the offshore depo-center, except in areas affected by estuarine engineering projects. Dam-induced longitudinal evolution of branching channel morphology and offshore depo-center movement will likely persist in the future, given the ongoing construction of large cascade dams in the upper Yangtze and the completion of major projects in the Yangtze estuary. © 2020 John Wiley & Sons, Ltd.     

Retrieval of suspended sediment concentrations in large turbid rivers using Landsat ETM+: an example from the Yangtze River,China

Suspended sediment concentration (SSC) is a critical parameter in the study of river sediment transport and water quality variation, but traditional measurement methods are costly and time‐consuming. This paper is focused on presenting a methodology that may be useful in estimating SSC which is of key importance in process geomorphology and hydrology. In previous studies, remote sensing has been applied to estimate the SSC of sea waters as well as low turbid inland waters like lakes, reservoirs and short river reaches visible within a single Landsat satellite image coverage. Rivers, especially highly turbid large rivers, have largely been ignored. The dataset used in this paper includes measured SSC and multi‐temporal Landsat ETM+ images covering most part of the Yangtze River. Using an effective easy‐to‐use atmospheric correction method that does not require in situ atmospheric conditions, retrieved water reflectance of Band 4 was found to be a good SSC indicator within the large SSC range 22–2610 mg l^–1. The newly developed regression relation between SSC and water reflectance of Band 4 appears to be able to provide a relatively accurate SSC estimate directly from Landsat ETM+ images for the Yangtze River from the upper, the middle to the lower reaches. With the relation it is possible to estimate or map out SSC dynamics of large rivers which lack SSC data due to constraints of conventional measurements. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of aquifer parameters based on measurements of tidal effects on a coastal aquifer near Beihai,China

Measurements of the tide and groundwater levels in coastal zones are of importance in determining the properties of coastal aquifers. The solution to a one‐dimensional unsteady groundwater flow model in a coastal confined aquifer with sinusoidal fluctuation of the tide shows that the tidal efficiency decreases exponentially with distance and the time lag increases linearly with distance from the coast. The aquifer property described by the ratio of storage coefficient to transmissivity is determined if the damping constant of the tidal efficiency or the slope of the time lag with distance are obtained on the basis of tidal measurements. Hourly observations of the tide and groundwater levels at 10 wells on the northern coast near Beihai, China show that with distance from the coast, tidal efficiency decreases roughly exponentially and the time lag increases roughly linearly. The estimated ratio of storage coefficient to transmissivity of the confined aquifer ranges from 1·169 × 10^?6 d m^?2 to 1·83 × 10^?7 d m^?2. For a given transmissivity of 750 m² d^?1, the storage coefficient of the aquifer is 8·7675 × 10^?4 with the tidal efficiency method and 1·3725 × 10^?4 with the time lag method. The damping constant of the tidal efficiency with distance can be defined as the tidal propagation coefficient. The value of the confined aquifer is determined as 0·0018892 m^?1. Copyright © 2007 John Wiley & Sons, Ltd.     

Comparative study on magnetic minerals of tidal flat deposits from different sediment sources in Jiangsu coast,Eastern China

Understanding the sources of sediments deposited in tidal flats is critical for reconstructing the evolution of coastal regions impacted by large rivers. Environmental magnetism can be an effective tool to track sediment sources and analyze the sediment properties. We evaluate several magnetic parameters from the tidal flat sediments along the Jiangsu coast. The results show that the sediments of Liandao Island have the lowest values of magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM) and saturation isothermal remanence magnetization (SIRM). In addition to these, the values of χ, ARM and SIRM display a generally increasing trend from the north to the south along the Jiangsu coast. The strong relations between χ, ARM and SIRM show that the changes of magnetization of the tidal flat sediments mainly reflect the changes of concentration rather than those in grain sizes of magnetic particles. The main magnetic phase is magnetite, with a small amount of hematite. The strong relationship between χ and the 2-16 μm grain-size fraction suggests that magnetite is enriched in the finer silt fraction. The sediment sources is the main factor that influences the magnetic properties of the tidal flat sediments from the Jiangsu coast. Combined with the previous studies, the results indicate that the tidal flat sediments of Liandao Island were mainly derived from the nearshore rock weathering. The Yellow River is the dominant supplier for the north of Dafeng, while the Yangtze River is the dominant supplier for the south of Rudong. The coast between Dafeng and Rudong is a transition zone influenced by both rivers. This study provides a basis against which possible future variations in sediment composition resulting from catchment changes can be assessed.     

Distribution,potential sources and ecological risks of two persistent organic pollutants in the intertidal sediment at the Shuangtaizi Estuary,Bohai Sea of China

Spatial distribution, source apportionment, and potential ecological risks of sixteen polycyclic aromatic hydrocarbons (PAHs) and seven endocrine disrupting compounds (EDCs) in the intertidal sediment at the Shuangtaizi Estuary, Bohai Sea of China were analyzed. Results showed that the total PAH concentrations ranged from 28.79 ng g^? 1 dw to 281.97 ng g^? 1 dw (mean: 115.92 ng g^? 1 dw) and the total EDC concentrations from 0.52 ng g^? 1 dw to 126.73 ng g^? 1 dw (mean: 37.49 ng g^? 1 dw). The distribution pattern for the PAHs was generally different from that of the EDCs possibly due to their distinct sources and n-octanol-/water partition coefficients (K_OW). Qualitative and quantitative analytical results showed that PAH sources were mainly from a mixture of pyrogenic and petrogenic contributions. The higher levels at the southeast of Geligang indicated that the EDC pollutants may have mainly originated from the plastic industry and other chemical plants located along the Liao River. Ecological risk assessment revealed that PAHs exhibited low ecotoxicological effects, whereas EDCs, especially 4-tert-octylphenol and bisphenol A, had high ecological hazard to the estuarine biota.     

A study of scale effect on specific sediment yield in the Loess Plateau,China

Based on data from 148 hydrometric stations in the Yellow River Basin, an analysis of regional scale relationship, or the relationship between specific sediment yield and drainage basin area, has been undertaken in the study area of the Loess Plateau. For different regions, scale relationship in log-log ordinate can be fitted by two types of lines: straight and parabola, and for each line, a function was fitted using regression analysis. The different scale relationships have been explained in terms of the difference in surface material distribution and landforms. To offset the scale-induced influence, calcu-lation has been done based on the fitted functions, in order to adjust the data of specific sediment yield to a common standard area. Based on the scaled data, a map of specific sediment yield was con-structed using Kriging interpolation. For comparison, a map based on the un-scaled data of specific sediment yield was also constructed using the same method. The two maps show that the basic pattern of specific sediment yield was basically the same. The severely eroded areas (Ys >10000 t km-2a-1) were at the same locations from Hekouzhen to Longmen in the middle Yellow River Basin. However, after the adjustment to a common standard area, the very severely eroded area (Ys >20000 t km-2a-1) became much enlarged because after the adjustment, all the values of Ys in the lower river basin in those regions became much larger than before.     

Abrupt behaviours of streamflow and sediment load variations of the Yangtze River basin,China

Monthly sediment load and streamflow series spanning 1963–2004 from four hydrological stations situation in the main stem of the Yangtze River, China, are analysed using scanning t‐test and the simple two‐phase linear regression scheme. Results indicate significant changes in the sediment load and streamflow from the upper reach to the lower reach of the Yangtze River. Relatively consistent positive coherency relations can be detected between streamflow and sediment load in the upper reach and negative coherency in the middle and lower reaches. Interestingly, negative coherency is found mainly for larger time scales. Changes in sediment load are the result mainly of human influence; specifically, the construction of water reservoirs may be the major cause of negative coherency. Accentuating the human influence from the upper to the lower reach results in inconsistent correlations between sediment load and streamflow. Decreasing sediment load being observed in recent years has the potential to alter the topographical properties of the river channel and the consequent development and recession of the Yangtze Delta. Results of this study are of practical significance for river channel management and evaluation of the influence of human activities on the hydrological regimes of large rivers. Copyright © 2012 John Wiley & Sons, Ltd.