Comparisons between acoustic measurements and predictions of the bedload transport of marine gravels

Continuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (q_b) predicted by five bedload transport equations using measurements of the near-bed turbulent current flow. When mean flow data are used in these equations, total q_b estimates are similar to those measured. However, when instantaneous flow data are used, total q_b is over-estimated by approximately one order-of-magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of total q_b over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport.     

In situ acoustic measurements of marine gravel threshold and transport

Measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles. The turbulence in the bottom boundary layer was measured using two electromagnetic current meters, and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material. Visual estimates of bedload were also obtained with an underwater TV camera. The acoustic technique, unlike a conventional bedload sampler, has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected. This has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions. The results of the study show that of the turbulent bursting events which contribute towards the Reynolds stress, only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material. The measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process.     

On Bagnold's sediment transport equation in tidal marine environments and the practical definition of bedload

Bagnold's sediment transport equation has proved to be important in studying tidal marine environments. This paper discusses three problems concerning Bagnold's transport equation and its practical application:

• 1 Bagnold's suspended-load transport equation and the total-load transport equation with are incorrect from the viewpoint of energy conservation. In these equations the energy loss due to bedload transport has been counted twice. The correct form should be for suspended-load transport and for total-load transport with
• 2 The commonly used Bagnold's transport coefficient K varies as a non-linear function of the dimensionless excess shear stress, which can be represented best by the power law , where the coefficient A and exponent B depend on sediment grain size D. The empirical values of A and B for fine to medium grained sands are determined using Guy et al.'s (1966) flume-experiment data.
• 3 The sediment transport rates predicted from this equation are compared with bedform migration measurements in the flume and the field. This comparison shows that the sediment transport rates measured from bedform migrations are higher than the predicted bedload transport rates, but comparable to the calculated total-load (bedload plus intermittent suspended-load) transport rates. This indicates that bedform migration involves both bedload and intermittent suspended-load transport. As a logical conclusion, bedform migration data should be compared with Bagnold's total-load transport equation rather than with his bedload transport equation. In this respect the term ‘bed material’ might be more appropriate than the term ‘bedload’ for estimating sediment transport rate from bedform migration data.

The sediment transport rates predicted from this modified Bagnold transport equation are in good agreement with field measurements of bedform migration rates in four individual tidal marine environments, which cover a wide range of sediment grain size, flow velocity and bedform conditions (ranging from small ripples, megaripples to sandwaves).     

Hydraulic controls of grain-size distributions of bedload gravels in Oak Creek, Oregon, USA

Grain-size distributions of gravels transported as bedload in Oak Creek, Oregon, show systematic variations with changing flow discharges. At low discharges the gravel distributions are nearly symmetrical and Gaussian. As discharges increase, the distributions become more skewed and follow the ideal Rosin distribution. The patterns of variations are established by goodness-of-fit comparisons between the measured and theoretical distributions, and by Q-mode factor analysis. Two end members are obtained in the factor analysis, having (respectively) almost perfect Gaussian and Rosin distributions, and the percentages of the two end members within individual samples vary systematically with discharge. Transformation from Gaussian to Rosin distribution with increasing discharge may be explained by processes of selective entrainment of grains from a bed of mixed sizes. Samples of bed material in Oak Creek follow the Rosin distribution. At high discharges, the transported bedload approaches the grain sizes of that bed-material source and mimics its Rosin distribution. Random-selection processes must be more important to grain entrainment at lower discharges, so that the resulting Gaussian distributions of transported bedload reflect similar distributions of bed stresses exerted by the stream flow. The results from Oak Creek demonstrate that the competence of the flow is reflected in the entire distribution of transported gravel sizes. A sequence of layers of fluvial gravels, modern or ancient, might show systematic variations between coarse Rosin and finer-grained Gaussian distributions, and these could be used to infer frequencies of various discharges and to establish a relationship to the source sediment. With further study, analyses of changing bedload grain-size distributions and their transport rates will lead to a better understanding of downstream variations in grain sizes of bed sediments and how their distributions reflect the progressive development of textural maturity.     

推移质输沙率的非线性研究

目的是从非线性科学角度探讨泥沙运动的规律。通过分析非均匀沙起动的影响因素得到了尖点突变模型的状态变量和控制变量,建立了能描述推移质运动的尖点突变模型。在尖点突变标准方程的基础上,应用尖点突变理论的坐标变换和拓扑变换,导出了输沙强度和水流参数的函数关系式。将输沙强度作为状态变量,水流参数和床沙密实系数作为控制变量。用水槽实验资料和其它推移质输沙率公式进行了对比验证。验证结果表明,计算值与其它推移质输沙率公式和水槽实验结果基本相符,误差一般在-90%～80%之间。说明建立的推移质输沙率公式是合理的,能够反映泥沙的起动和输移规律。     

A method for the measurement of bedload sediment transport and passive faunal transport on intertidal sandflats

A simple and inexpensive sampler to measure bedload sediment transport in shallow subtidal or intertidal areas is described. The cylindrical sub-sediment trap with an aspect ratio of 20 (height: diameter) is an improvement over conventional bedload samplers which are difficult to use in shallow areas or fail to collect the biological material associated with bedload. Traps deployed on a low-energy intertidal sandflat for six months provided daily estimates of bedload transport (quartz grains: 0.001–40 kg m^?1 d^?1), passive infaunal transport (e.g., the bivalveMya arenaria, max: 800 ind m^?1 d^?1), and organic detrital flux (e.g., macrophyte fragments, max: 400 g dry wt m^?1 d^?1). Bedload rates estimated with traps were compared to predictions from a numerical bedload model to evaluate the trap’s collection and retention efficiency. A significant linear regression between observed (trap) and predicted (model) rates (r²=0.65, p<0.001, n=97) indicated that the traps were useful for the measurement of high- and low-frequency variability in bedload transport. Potential applications of the traps in benthic oceanography include recruitment and recolonization studies.     

Simple estimates for the magnitude of bedload transport under a turbid surge

Characteristic length and timescales for a turbid surge are used to estimate bedload transport by the surge, deriving estimates for the conditions under which deposited material will be mobilized as bedload, and of the relative importance of bedload in determining the overall deposit geometry. A critique is provided of the common modelling assumptions which underlie these estimates and of how their consistency can be checked. For large turbidity currents, such as those which emplaced the Marnoso Arenacea turbidites in northern Italy, model predictions of overall geometry are not easy to reconcile with the field data: some possible reasons for this are discussed. The estimates obtained from these characteristic scales are consistent with the widespread presence in turbidites of sedimentary structures which indicate bedload transport; some of these structures from the Marnoso Arenacea Formation are reviewed briefly. However, the estimates suggest that bedload transport is not a major factor responsible for the geometry of the large turbidites in this formation, which exhibit a broad thickness maximum in their proximal region that contrasts with the downstream-thinning geometry of smaller beds. This effect suggests that the explanation for this theoretically unexpected geometry should be sought in other physical mechanisms.     

Validation and implications of an energy‐based bedload transport equation

A recently developed bedload equation (Abrahams & Gao, 2006) has the form i_b = ωG^3˙4, where i_b is the immersed bedload transport rate, ω is the stream power per unit area, G = 1?θ_c/θ, θ is the dimensionless shear stress and θ_c is the associated threshold value for the incipient motion of bed grains. This equation has a parsimonious form and provides good predictions of transport rate in both the saltation and sheetflow regimes (i.e. flows with low and high θ values, respectively). In this study, the equation was validated using data independent of those used for developing it. The data represent bedload of identical sizes transported in various steady, uniform, fully rough and turbulent flows over plane, mobile beds. The equation predicted i_b quite well over five orders of magnitude. This equation was further compared with six classic bedload equations and showed the best performance. Its theoretical significance was subsequently examined in two ways. First, based on collision theory, the parameter G was related to the ratio of grain‐to‐grain collisions to the total collisions including both grain‐to‐grain and grain‐to‐bed collisions, P_g by P_g = G², suggesting that G characterizes the dynamic processes of bedload transport from the perspective of granular flow, which partly accounts for the good performance of the equation. Moreover, examining the ability of two common equations to predict bedload in gravel‐bed rivers revealed that G can also be used to simplify equations for predicting transport capacities in such rivers. Second, a simple dimensionless form of the equation was created by introducing B = i_b/ω. The theoretical nature of the term B was subsequently revealed by comparing this equation with both the Bagnold model and two commonly used parameters representing dimensionless bedload transport rates.     

Spatio-temporal variability of bedload transport rate: analysis and 2D modelling approach

Field measurements to calibrate numerical bedload formulae are largely missing. Measurements using a Large Helley-Smith sampler were performed over a period of five years in the large Alpine Drau River, Austria. Our results reveal the high spatio-temporal variability of bedload transport rates. Commonly used bedload predictors poorly describe measured transport rates. Temporal and localised cross-sectional variation in bedload transport rates are observed in short time frames. To obtain significant mean values, the measurement period has to be extended to cover the existing bedload transport periodicity. The discrepancy between bedload transport measurements and simulation is partially explained by local hydraulic variations. The results can be improved, particularly for verticals where most of the bedload occurs, by relating measured transport rates to local hydraulic parameters. The incorporation of local cross-sectional parameters demonstrates the utility of 2D bedload models and their greater predictive power over similar 1D models.     

The motion of sediment-water mixtures during intense bedload transport: computer simulations

A new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment-water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment-water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle-fluid interaction is expressed by a velocity-squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast-moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental observations.     

Comparison between numerical and analytical solution of solute transport models

The fate and movement of dissolved substances in soils and groundwater has generated considerable concern for the quality of the subsurface environment. Many analytical solutions for the partial differential equations that describe solute and pollutant movement exist. Numerical solutions are more general, and often more difficult to verify. In order to determine the model error, the examination of the ability of numerical methods compared to analytical methods is strongly recommended. The objective of the study is to make a comparison between numerical and analytical solution models for solute transport equation. In this study, the numerical solution calculated with the WAVE-model is compared with the analytical solution calculated with CXTFTT-model. The study scenarios considered variables such as compartment depth, applied flux at the top and soil dispersivity under steady-state conditions. The simulations depend on 27 solute infiltration scenarios. The solute concentrations were calculated with the WAVE-model and the CXTFIT-model for each scenario. The WAVE-model error was evaluated with three methods: absolute average maximum error, relative average maximum error and relative average area error. The study implied that the WAVE-model error increased with the increase of the compartment depth, decreasing soil dispersivity, and decrease in flux. The study leads to the recommendation to use compartment depth as thin as possible to minimise the WAVE-model error. Furthermore, it is more useful to use several numerical solution models, such as SWMS-2D model, to evaluate and examine the WAVE-model.     

Direct numerical simulation of bedload transport using a local, dynamic boundary condition

ABSTRACT Temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages. The primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation (pocket friction angle) of a bed grain to the grains that it rests upon, variability of the near‐bed turbulent velocity field and the local modification of this velocity field by upstream, protruding grains. A model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain‐size sediment bed. Experimental data from the velocity field downstream and below the tops of upstream, protruding grains are presented. From these data, an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model. The temporal variability of near‐bed turbulence is provided by a measured near‐bed time series of velocity over a gravel bed. The distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles. Calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies.     

Size–density sorting of sand-size spheres during deposition from bedload transport and implications concerning hydraulic equivalence

It is well known that sediment sorting according to size, shape and density occurs, but the exact mechanisms involved are poorly understood. To assess the effects of size and density, sand-size spheres of two densities were transported and deposited under controlled flume conditions. Observations on the motion of discrete particles show that grains smaller than bed-roughness grains move continuously and have the same transport velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than bed roughness grains, movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities. Analyses of bulk sediment deposited from plane-bed transport, show that the size and proportion of heavies decreases and that of lights increases with distance transported. For ripple bed transport, however, the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable. These results suggest that size-density sorting in plane-bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple-bed transport is related to deposition on, and recycling through, the bed forms. Application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful.     

Tertiary fluvial gravels and evolution of the Western Canadian Prairie Landscape

The early Tertiary of Western Canada and northern United States was marked by a change from compressional to extensional tectonics. The result was regional uplift and magmatic events. The uplift resulted in a major unconformity and deposition of extensive regional sheets of gravel and sand, of which only isolated remnants remain. These units are the Eocene to Miocene Cypress Hills Formation, the Miocene Wood Mountain Formation, Miocene Flaxville Formation and preglacial Souris River gravels, All four stratigraphic units consist of gravel and sand with lesser amounts of clay. The formations were largely deposited as laterally continuous sheets of braided river gravels, with some occurrences of meandering-river sedimentation. The sediment was deposited several hundred kilometres downstream of their source areas. Paleocurrent data for the Cypress Hills and Wood Mountain formations indicate that regional paleoslope dipped towards the north–northeast. The modern prairie landscape of western Canada began to evolve with the deposition of these gravels during the Eocene with creation of a basin-wide unconformity followed by deposition of an extensive braidplain system that was subsequently uplifted, incised, and molded into its present form.     

海洋沉积物的铁和锌同位素测定

介绍海洋沉积物Fe和Zn同位素化学前处理及测定方法,报道南海西部夏季上升流区两个沉积物柱样的Fe和Zn同位素组成。样品采用HF+HNO3+HClO4常压消解,经脱盐后,转化为氯化物形式并经离子交换柱分离纯化后,用多接收器等离子体质谱法测定Fe和Zn同位素比值。该前处理方法可以快捷地实现海洋沉积物的消解、有机质的去除和海盐脱离;结合相关测试流程,可获得较高的δ56Fe(0.10‰,2SD)和δ66Zn分析精度(0.11‰,2SD)。两个沉积物柱样的δ56Fe值(相对于IRMM-014)和δ66Zn值(相对于JMC3-0749C)随深度变化不明显,两柱之间也无明显差异。总体上,南海西部上升流区1～2 ka以来的沉积物δ56Fe值(0.04‰～0.20‰)和δ66Zn值(0.12‰～0.30‰)与已报道的黄土和气溶胶、火成岩以及大部分海洋沉积物接近,明显高于静海相海洋沉积物的δ56Fe值。     

Geochemical interaction between the freshwater and marine hydrospheres

The results of more than 40 years long authors’ investigations in the field of the freshwater (river input) and marine (ocean waters) hydrospheres are summarized. The latest estimations of the global average concentrations of many chemical elements in river water and suspended matter and in ocean water and suspended matter are presented. It is shown that particulate suspended forms of many elements are predominant in river waters (“rivers are the kingdom of suspended forms of elements”), while their dissolved forms prevail in ocean waters (“ocean is the kingdom of dissolved forms of elements”). Sedimentary and biogeochemical processes of the river material transformation in the river-sea mixing zone (the so-called “marginal filter of the ocean”) were studied thoroughly. It was shown that radical quantitative and qualitative changes of dissolved and particulate suspended substances take place in this zone, resulting in the governed transformation of dissolved forms into suspended particulate forms and their following deposition on the bottom. The first data on the losses of 35 chemical elements in the river-sea mixing zone are presented. These data prove that the concentrations of dissolved elements in river and ocean waters are in regular and close relationship with their losses in the river-sea mixing zone and with the types of element distribution in ocean water column (conservative, biogenic, and lithogenic). This indicates the existence of a geochemical system in the entire (freshwater and marine) hydrosphere, which calls for deep and detailed investigations.