冲积河流泥沙输移幂律函数指数变化规律

冲积河流泥沙输移幂律函数关系与不平衡输沙理论是对河道不平衡输沙同一物理现象的不同描述,两者既有区别也有联系。比较研究发现:对于恒定均匀流不平衡输沙过程,当输沙位于近平衡态时两者含沙量导函数表达式具有一阶近似等价性,当输沙远离平衡态时前者含沙量导函数中隐含考虑有泥沙恢复饱和系数的变化。基于两者等价性,推导建立了幂律函数指数计算表达式,表明指数随泥沙沉速、单宽流量和沿程距离而变化,且随着输移距离的增大呈指数衰减。基于前者含沙量导函数表达式结构特点,分析建立了相应泥沙恢复饱和系数变化的计算表达式。综合以上成果,改进提出了一种变幂指数的泥沙输移幂律函数计算模型。对库里·阿雷克沉沙池沿程断面输沙指数及含沙量计算结果表明,不同距离过水断面输沙指数的变化规律是合理的,含沙量计算值与实测值变化趋势基本符合。     

Beach fetch distance and aeolian sediment transport

An experiment was conducted to examine the influence of fetch distance on aeolian sediment transport on a natural sand beach at Benone Strand, County Londonderry, Northern Ireland. The site consisted of a wide dissipative beach, approximately 150 m wide at low tide and 80 m wide during high tide. Surface moisture levels (and hence dry fetch distance) were dictated by both local groundwater, from a stream outlet across the beach, as well as local tidal levels. An abundant dry sediment supply was available during the experiment. High-resolution (1 Hz) measurements were made of wind speed and direction along with sediment flux. Wind velocity ranged from 2·1 to 8·1 m s^–1 during the study. Second-order polynomial sand transport equations were derived from the wind speed and trap results with r ² values of better than 0·93 for all data. When the data were sorted into velocity bins of 1 m s^–1, there was no discernible relationship between fetch distance and sand transport, with a measured fetch distance range of 10–58 m available during the experiment. Results show that fetch distance is unimportant when an adequate sand supply is available. However, it is suggested that fetch may restrict the development of steady-state transport under sediment-limited conditions. Sediment availability is thus identified as a key variable in aeolian transport studies on natural beaches.     

床面附近泥沙交换率在悬移质输沙计算中的应用

总结并分析了关于悬移质运动方程的源/汇项或底部边界条件的已有的处理方法.认为在悬移质输沙计算中,这些方法因引入了平衡输沙概念、忽略了推移质颗粒对悬移质运动的影响或因缺乏床面附近泥沙质量交换率的理论表达式,均存在着某些不足.运用床面附近泥沙质量交换率理沦,对一般的悬移质输沙方程的底部边界条件进行了新的探讨,并提供了关于一维和垂向二维悬移质输沙的计算实例.     

Modeling the influence of settling velocity on cohesive sediment transport in Tanshui River estuary

Settling velocities of suspended cohesive sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often considered as the principal cause. Turbulence and the suspended sediment concentration, as well as other factors such as salinity, dissolved organic substances, flocculation ability, and the rate of floc growth affect setting velocities. A laterally–averaged finite difference model for hydrodynamics and cohesive sediment transport is developed and applied in the Tanshui River estuary, Taiwan. The model has been calibrated and verified with water surface elevation, longitudinal velocity, salinity, and cohesive sediment measured. The overall performance of the model is in qualitative agreement with the available data. The model is used to investigate the influence of settling velocity on cohesive sediment transport dynamics. The simulation indicates that the turbidity maximum zone is near Kuan–Du. When settling velocities increase the surface cohesive sediment concentration at Kuan–Du station trends to decrease and bottom cohesive sediment concentration increases. Both surface and bottom cohesive sediment concentrations decrease at Taipei Bridge and Pa–Ling Bridge. This implies that suspended sediment advected seaward and deposited. There is consequently a net seaward flux of suspended sediment near surface, and a net landward flux near the bed.     

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.     

Sediment trend models fail to reproduce small-scale sediment transport patterns on an intertidal beach

A rigorous test is presented of the application of sediment trend models to an intertidal beach environment characterized by bar morphology. Sediment samples were collected during low tide from a regular grid and their sediment fall velocity distributions, obtained using a settling tube, were analysed using moment analysis. The net sediment transport direction determined from beach surveys, hydrodynamic measurements, wave ripple observations and sediment transport modelling was compared with predictions by sediment trend models based on the spatial distribution of sediment parameters. It was found that the sediment transport pathways and patterns of sedimentation predicted using sediment trend models were at odds with field observations, and varied significantly depending on whether surface or sub‐surface sediment samples were used. The sediment trend models are thought to fail because, in energetic and morphologically variable beach environments, spatial patterns in sediment characteristics are mainly attributed to the presence of different hydrodynamic regions and associated morphology, rather than sediment pathways. The use of sediment trend models cannot replace the collection of morphological, hydrodynamic and sediment transport data in the field to define relationships between flows, forms and sedimentation patterns on a dynamic intertidal beach.     

海洋环境沉积物输运研究进展

海洋环境中沉积物的输运涉及复杂的过程和机制。20世纪后半叶发展起来的悬沙输运数学模型已经成为海洋沉积动力学的一个有力的研究工具。悬沙输运数学模型的有效运行需要正确的数值解法和模型中所含参数的确定,包括悬沙沉降速度、扩散系数、底床糙度和切应力,以及底边界上的沉降-再悬浮通量。由于复杂的水动力条件、屏蔽效应以及海底生物扰动等因素的作用,海洋环境推移质输运的经验、半经验公式具有一定的局限性。因此,充分考虑以上各种因素是正确预测海洋环境中推移质输运的关键。海洋环境沉积物输运理论的进一步发展需要着重进行各种过程和机制的研究,而这项工作依赖于高精度、高分辨率现场观测仪器的发展和更先进的颗粒态物质运动理论的建立。     

Thresholds of aeolian sand transport: establishing suitable values

This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.     

西部典型矿区风积沙含水层突水溃沙的起动条件与运移特征

突水溃沙是西部矿井地质灾害的主要类型之一,为研究突水溃沙灾害的临界判据与水沙运移特征,以榆横矿区含水层风积沙为例,采用自主研发的水沙两相高速渗流试验设备,对4种不同粒径范围的风积沙进行了沙粒起动试验和溃沙试验,对混合风积沙进行了水沙两相高速渗流试验。得到了影响突水溃沙的2个临界速度条件：第一临界速度即沙粒起动速度,为0.38～1.26 mm/s;第二临界速度即溃沙临界速度,为2.48～3.54 mm/s。试验结果表明：体积膨胀是沙粒群起动的必要条件,突水溃沙灾害是起动的风积沙由量变逐步积累到质变的物理过程,分为3个阶段：（1）水携沙流动,（2）水沙混合流动,（3）沙携水流动。沙粒起动是水动能转变为沙动能的过程,溃沙时水沙混合流体运动表现为颗粒群浓度波传播的特征,沙粒间通过碰撞的形式传递能量。溃沙速率随初始水力梯度的增大呈指数增长,单位时间内溃沙量随水力梯度的增大而呈线性增加。     

莱州湾悬沙输运机制研究

基于2012年实测的潮流、含沙量及表层沉积物数据及资料等,分析了潮流、余流、潮流底应力及底质类型对含沙量变化的影响,并运用物质通量分析方法,探讨了莱州湾悬浮泥沙的输运机制.研究结果表明:研究海域受半日潮控制呈往复流特征,涨、落潮期间近底含沙量与流速及潮流底应力显著相关,存在明显的再悬浮现象,含沙量呈现潮周期变化特征;底质类型与含沙量大小密切相关,细颗粒物质更容易发生悬浮;平流输运与潮泵效应是莱州湾海域的悬沙输运的主要动力因素.     

钙质砂压缩波速与物理性质参数关系研究

开展钙质砂纵波波速与物理性质参数关系的试验研究,对珊瑚礁地基无损检测及工程物探具有重要的理论指导意义和工程应用价值。控制试样的物性参数状态,利用制样装置进行波速测量,揭示钙质砂纵波波速与物性参数的相关关系。试验结果表明：含水率是影响纵波波速的主要因素,与纵波波速呈二次函数关系。CT扫描结果表明,试样粒径越大,颗粒内部孔隙越丰富,这导致试样孔隙比越大,波速故而越小,与粒径、孔隙比呈负相关关系。在密实度相同、不均匀系数不同时,级配良好的砂样纵波波速差异不大。经过侧限压缩后,含水率一定时,粉土纵波波速与孔隙比呈二次关系,且与密度呈良好的线性关系。利用粉土易压缩且与纵波波速紧密变化的特点,可积极探索声波在粉土地基密实度检测的工程应用。     

Aeolian system sediment state: theory and Mojave Desert Kelso dune field example

The sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply, sediment availability and the transport capacity of the wind. The sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system. Numerous factors impact the susceptibility of grains on a surface to transport, but these are cumulatively manifested by the actual transport rate, which serves as a proxy for sediment availability. Transport capacity is the potential sediment transport rate of the wind. Because the three aspects of sediment state can be given as a volumetric rate, they are directly comparable. Plotted simultaneously against time, the generated curves define nine possible classes of sediment state. Sediment supply that is stored occurs because it is transport or availability limited, or generated at a rate greater than the potential or actual transport rates respectively. Contemporaneous or lagged influx to an aeolian system may be limited by sediment availability, but cannot exceed the transport capacity of the wind. For the Kelso dune field in the Mojave Desert of California, a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system. The sediment supply was generated during the latest Pleistocene and earliest Holocene during humid periods of enhanced discharge by the Mojave River to form the Lake Mojave fan delta or terminal fan, and has been calculated over time from the sedimentation rate and the frequency of floods. Estimation of transport capacity over time was based upon modern wind data, with an allowance for greater winds during the Pleistocene based upon climatic models. Sediment availability was approximated by calculation of a modern dune mobility index, with variation over time based upon climatic inferences. While quantifying the Kelso or any natural system is subject to numerous uncertainties, the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at Kelso, as well as illuminating questions for future research.     

Distribution of suspended sediment concentration in wide sediment‐laden streams: A novel power‐law theory

The diffusion equation of suspended sediment concentration in a wide sediment‐laden stream flow is dependent on the vertical gradient of streamwise velocity and the sediment diffusivity. This study aims at investigating the influence of the streamwise velocity laws on the suspended sediment concentration distributions, resulting from the solution of the diffusion equation. Firstly, the sediment concentration distributions are obtained numerically from the solution of the diffusion equation using different velocity laws and compared with the experimental data. It is found that the power‐law approximation produces good computational results for the concentration distributions. The accuracy of using a power‐law velocity model is comparable with the results obtained from other classical velocity laws, namely log‐law, log wake‐law and stratified log‐law. Secondly, a novel analytical solution is proposed for the determination of sediment concentration distribution, where a power‐law, wall‐concentration profile is coupled with a concentration wake function. The power‐law model (for velocity and concentration) is calibrated using the experimental data, and then a generalized wake function is obtained by choosing a suitable law. The developed power‐law model involving the wake function adjusted by an exponent predicts the sediment concentration distributions quite satisfactorily. Finally, a new explicit formula for the suspended‐load transport rate is derived from the proposed theory, where numerical computation of integrals, as needed in the Einstein theory, is avoided.     

The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data

The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and, at many sites, only suspended load data are available. Often the bedload fraction is estimated with ‘rule of thumb’ methods such as Maddock’s Table, which are inadequately field‐tested. Here, the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short‐term and long‐term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year‐to‐year averages. Using published simultaneous short‐term field measurements of bedload and suspended load transport rates, Maddock’s Table is reviewed and updated. Long‐term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel‐bed rivers, whereby in large catchments the bedload fraction is insignificant at ca 1%. For sand‐bed rivers, the bedload fraction may be substantial (30% to 50%) even for large catchments. However, available data are scarce and of varying quality. Long‐term partitioning varies widely among catchments and the available data are currently not sufficient to discriminate control parameters effectively.     

Sediment diffusion during overbank flows

Distinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post-settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion. If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post-settlement period, the model may be used to estimate palaeohydraulic characteristics of post-settlement floods. Calculations indicate that 212 post-settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel.     

Air density effects on aeolian sand movement: Implications for sediment transport and sand control in regions with extreme altitudes or temperatures

Aeolian sand transport results from interactions between the ground surface and airflow. Previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity, but the influence of air density, which strongly constrains airflow characteristics and the resulting sand flow, has not been widely considered. In the present study, entrainment, saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from ?154 m below sea‐level (Aiding Lake) to 5076 m above sea‐level (Tanggula Mountain pass on the Qinghai–Tibetan plateau). At each site, a portable wind tunnel and high‐speed camera system were set up, and the friction wind velocity, threshold friction velocity and sand flow structure were observed systematically. For a given volumetric airflow, lower air density increases the wind velocity. Low air density also creates a high threshold friction velocity. The Bagnold wind erosion threshold model remains valid, but the value of empirical parameter A decreased with decreasing air density and ranged from 0·10 to 0·07, the smallest values reported in the literature. For a given wind velocity, increased altitude reduced total sand transport and creeping, but the saltation rate and saltation height increased. The present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude (for example, alpine deserts and low‐lying lake basins) or on other planets, including Mars. These results also provide theoretical support for improved sand‐control engineering measures. The data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind‐blown sand.     

Complex variations in sediment transport at three large river bifurcations during discharge waves in the river Rhine

River bifurcations strongly control the distribution of water and sediment over a river system. A good understanding of this distribution process is crucial for river management. In this paper, an extensive data set from three large bifurcations in the Dutch Rhine is presented, containing data on bed‐load transport, suspended bed sediment transport, dune development and hydrodynamics. The data show complex variations in sediment transport during discharge waves. The objective of this paper is to examine and explain these measured variations in sediment transport. It is found that bend sorting upstream of the bifurcations leads to supply limitation, particularly in the downstream branch that originates in the outer bend of the main channel. Tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches. Lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport. All bifurcations show evidence of sediment waves, which probably are intrinsic bifurcation phenomena. The complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river. Differences among the studied river bifurcations are mainly due to differences in sediment mobility (Shields value). Because the variations in sediment transport are complex and poorly correlated with the flow discharge, prediction of the sediment distribution with existing relationships for one‐dimensional models is problematic.     

直剪剪切速率对钙质砂强度及变形特征的影响

为深入研究剪切速率对钙质砂强度和变形特征的影响,对钙质干砂进行不同剪切速率条件下的直剪试验。研究结果表明,随剪切速率从0.1 mm/min增至2.4 mm/min时钙质砂抗剪强度先减小后增大,其内摩擦角亦呈现出先减小后增大趋势,临界剪切速率为1.6 mm/min;低法向应力条件下钙质砂试样随剪切速率的增加更易于呈现剪胀现象,高法向应力条件下剪切速率从0.1 mm/min增长至1.6 mm/min时试样整体剪缩量逐渐减小;当剪切速率继续从1.6 mm/min增长至2.4 mm/min时试样最大剪缩量逐渐增加;不同法向应力水平条件下钙质砂加载速率效应的细观机制不同,较低应力水平条件下钙质砂加载速率效应主要由试样内部颗粒错动、换位、重新排列引起,在较高应力水平条件下钙质砂加载速率效应主要由颗粒破碎引起。     

淤泥质河口水沙运动数学模型相关问题

为研究淤泥质河口的水沙运动规律,建立了用于模拟淤泥质河口水沙运动的二维数学模型。该模型采用基于无结构三角网格下的有限体积法对方程组进行离散,结合Roe-MUSCL方法及时间方向的预测-校正格式,使模型在时空方向具有二阶计算精度。模型中分别采用不同方法计算粘性和非粘性泥沙的输移源项,并引入粘性泥沙的起动流速和冲刷率计算公式。采用已有的概化水槽试验数据对模型进行了初步验证。然后模拟了1995年10月小潮及大潮期间海河口的潮流运动与泥沙输移过程,计算得到的潮位、潮流速及含沙量过程与实测过程符合较好,结果表明模型能够用来模拟淤泥质河口粘性和非粘性泥沙的不平衡输移过程。同时还比较了泥沙输移源项的不同处理方式对计算结果的影响,计算表明在淤泥质河口水沙运动数学模型中必须同时考虑粘性和非粘性泥沙的输移。     

Combined tide and wave influence on sedimentation patterns in the Upper Jurassic Swift Formation, south-eastern Alberta

In south‐eastern Alberta, the Oxfordian Swift Formation comprises two unconformity‐bounded sequences. Sequence 1 consists of the shale member. It unconformably overlies Bathonian calcareous shale of the Rierdon Formation and consists dominantly of shale with uncommon hum‐ mocky cross‐stratified siltstone. The shale member comprises up to four parasequences that show a subtle upward‐fining stacking pattern, suggesting overall deepening in a storm‐dominated fully marine basin. Deposition of the shale member was terminated by a fall in relative sea‐level, which in turn initiated sequence 2. During lowstand, a network of north‐east‐ to south‐west‐trending meandering channels incised the top of sequence 1. Despite the preserved morphology of these channels, observations in core suggest that lowstand deposits are absent. These strata were most likely thoroughly reworked by ravinement processes associated with the ensuing transgression and are recorded only by a thin chert pebble lag. Also associated with transgression were significant changes in the physical and ecological conditions in the local Swift basin. These changes were related to modifications in basin configuration and the development of a low‐energy strait, which probably formed as a result of uplift of the Sweetgrass Arch. Within this newly configured basin, brackish‐water conditions prevailed, and sedimentation was dominated by suspension deposition of mud. Bed‐load sediment, composed mostly of very fine and fine sand, made up only a small part of the total sediment flux into the area and was transported principally by low‐energy combined flows with variable‐speed microtidal currents and a low‐energy oscillatory component related to storm waves. Sand was generally deposited in the form of small, discontinuous sand ridges that developed locally throughout the study area – it is these features that form the principal hydrocarbon reservoir bodies in the Swift Formation in the study area. After initiation, ridges built upwards and migrated laterally in response to transport and preferential local deposition of bed‐load sediment. Although common near the unconformity along the top of sequence 1, sand ridges also occur at several higher stratigraphic levels in sequence 2. These latter ridges, however, are not associated with regionally correlatable discontinuities but, instead, were initiated by local bed irregularities, possibly related to breaking internal waves, and were supplied with new sediment transported into the study area. The origin of these sand ridges is therefore not related to changes in relative sea‐level but, instead, to intrabasinal processes in a low‐energy strait.