沙质河床冲刷过程中床沙级配的模拟

研究了沙质河床冲刷过程中床沙级配的模拟方法。在该方法中采用了新的描述床沙级配变化的动力学方程和床沙级配在交换层内垂向变化与水流强度、河床冲刷强度的关系,反映了冲刷过程中交换层内向床面补充的物理过程。通过室内实验率定了相关参数,建立了床沙级配变化动力学方程的数值求解方法,模拟了床沙冲刷粗化。计算结果与室内实验结果符合较好,表明所建议的方法反映了河床冲刷中床沙级配变化的物理事实,可用于沙质河床冲刷粗化模拟。     

Statistical properties of sediment bed profiles in alluvial channels

The objective of this study is to investigate in detail the statistical properties of series of bed elevations measured on gravel-bed and sand-bed alluvial channels, in order to identify means of quantifying bed roughness effects on streamflow. The semivariogram is used as the basic statistical method for investigating roughness properties of bed profiles obtained from field work and laboratory experiments. For sand bedforms, the semivariograms include exponential and periodic components from which can be obtained reliable measures of bedform spacing and height, as well as information on the degree of regularity of bedform arrangement. Because of the irregular nature of gravel-bed profiles, the approach in this case uses the semivariogram to investigate fractal properties of series of bed elevations to determine scales of bed roughness associated with grain sizes and small-scale bedforms and to estimate the Hausdorff dimension corresponding to each scale. These superimposed scales of roughness may be responsible for the greater flow resistance generally observed in gravel-bed rivers rather than predicted from the theoretical friction equation.     

Uncertainty analysis in bed load transport prediction of gravel bed rivers by ANN and ANFIS

River flow is a complex dynamic system of hydraulic and sediment transport. Bed load transport have a dynamic nature in gravel bed rivers and because of the complexity of the phenomenon include uncertainties in predictions. In the present paper, two methods based on the Artificial neural networks (ANN) and adaptive neuro-fuzzy inference system (ANFIS) are developed by using 360 data points. Totally, 21 different combination of input parameters are used for predicting bed load transport in gravel bed rivers. In order to acquire reliable data subsets of training and testing, subset selection of maximum dissimilarity (SSMD) method, rather than classical trial and error method, is used in finding randomly manipulation of these subsets. Furthermore, uncertainty analysis of ANN and ANFIS models are determined using Monte Carlo simulation. Two uncertainty indices of d factor and 95% prediction uncertainty and uncertainty bounds in comparison with observed values show that these models have relatively large uncertainties in bed load predictions and using of them in practical problems requires considerable effort on training and developing processes. Results indicated that ANFIS and ANN are suitable models for predicting bed load transport; but there are many uncertainties in determination of bed load transport by ANFIS and ANN, especially for high sediment loads. Based on the predictions and confidence intervals, the superiority of ANFIS to those of ANN is proved.     

宜昌砾石层的沉积环境及地貌意义

宜昌砾石层上覆中更新世网纹红土,下伏白垩纪红层,共有22个岩性层组成,总厚超过100m。通过对宜昌地区剖面出露砾石层的沉积相分析表明,善溪窑和云池剖面由下至上出现冲积扇扇中-扇根亚相;李家院剖面与上述两剖面间有沉积间断,由下至上出现冲积扇扇端-扇中亚相。粒度分析也证明了该砾石层中的砂体属冲积扇中的辫状水道沉积,与沉积相分析所得结论一致。研究认为,宜昌砾石层具有典型的冲积扇沉积环境特征,主要为冲积扇扇顶部分。该冲积扇在宜昌东南地区如此大规模的发育,显示其形成时具有强大的水动力条件,通过对其形成的地貌环境意义进行探讨,初步认为,该冲积扇为长江三峡贯通的产物。据前人研究推断,宜昌砾石层的形成时代应该在1.08～0.73MaB.P.,三峡贯通应在1.0MaB.P.之前。     

Influence of gravel segregation on gluing solution solidification in a railway ballast

Influence of gravel segregation on gluing solution distribution in a realistic railway ballasts is investigated numerically. The rheological characteristics of the gluing solution are accounted for by using a coupled elasto-visco-plastic stress model with an asymptotic time increasing viscosity. The realistic railway ballast is accomplished by using the vertex identification technique, in which the regulations of the gravel composition in ballast practice are followed. Specifically, a uniformly distributed one and a segregated one induced by the difference in gravel size and gravel shape are generated. The rheological model of the gluing solution and twofold ballasts are incorporated into the finite element code of ANSYS\(^{\copyright }\) to study the flow characteristics and time-dependent solidification of the gluing solution. Relatively concrete vertical and lateral gravel–gravel adhesion in twofold ballasts can be generated by using denser network arrangement of the gluing solution; the vertical penetration in the uniform ballast is slightly deeper than that in the segregated ballast, while a slightly reverse tendency in the lateral extension near the upper and central parts appears. However, the difference in the solidified gluing solution distributions in both ballasts is essentially insignificant, indicating that the influence of the gravel segregation needs not necessary to be taken into account in gluing practice when percolation is not taken into account.     

Quaternary evolution of a meandering gravel bed river in central Spain

The river Jarama is a medium-sinuosity meandering river with gravel bedload. Throughout the Quaternary it developed a large number of terraces in a repetitive sequence of aggradation-stability-degradation-stability stages, which are studied in the middle reach of the river, applying geomorphological and sedimentological methods, as well as kadiocarbon dating techniques. The Upper Terrace System of earliest Pleistocene to mid Pleistocene age forms isolated fragments vertically disconnected and colluviated, and their sequential distribution of facies implies lateral shifting of channels and a welldeveloped flood plain. The Middle Terrace System of mid-to-late Pleistocene age has differentiated levels in the upstream reaches while downstream they condense giving a thick multi-episodic series, corresponding to the tectonic tilting of the basin. Classical point bar fining-upward sequences of a medium- to high-sinuosity river (with gravels and sands as bedload) are clearly differentiated in them. A major degradation stage follows this group of terraces, with an incision of up to 30 m. Then, during a relatively long period of stability, a forest of riparian species developed, prior to the last aggradational stage of the Lower Terrace System. The aggradation resulted in a fining-upward sequence where lateral accretion is clearly visible in the gravels, due to migration of the medium- to low-sinuosity river. Sedimentation continued and was active up to the 1950s, but since then, intense anthropogenic modifications led to renewed incision of the river.     

洞庭湖古湖滨砾石层的发现及意义

洞庭湖曾经是中国第一大淡水湖,解放初期湖泊面积为4350km^2,如今仅有2691km^2.由于洞庭湖承纳“四水”,吞吐长江,是长江中游最重要的调蓄洪湖泊,其变化直接影响到长江中游的防洪和生态环境.因此,洞庭湖的演化一直为学术界所关注.     

Lateritised gravel bed: A new guide horizon for lateritic gold?

Geological studies made in the area of Mara Rosa in Goias State, Brazil, where small occurrences of gold are known in river placers, have revealed an interesting and extensive “lateritised gravel bed” overlying an inexpressive nodular lateritic crust horizon. The gravel has pebbles of vein quartz, quartzites and gneisses, which are interlocked and compacted by intense lateritisation with brown hydrous Fe-oxide (goethite). The underlying nodular crusts grade from deep brown to mottled, and occasionally have cavities. They possibly represent an interface between the bed rock and the gravel bed. The intensity of lateritisation is attributed to the existence of volcanogenic mafic sequences in the rock. The presence of pebbles of the same nature in the small placers with Au, suggest that Au has been dissolved and precipitated during the lateritisation process. Its abundance is mainly restricted to the lateritic gravel bed, which in turn has contributed Au to the stream placers. Thus, it is considered as a new guide horizon for lateritic Au.     

An evaluation model of artificial neural network to predict stable width in gravel bed rivers

Regime width of alluvial channels is a vital problem in river morphology and channel design. Many equations are available in the literature to predict regime width of alluvial rivers. In general, there are many approaches to estimate regime width; however, none of them is widely accepted at present. This is due to the fact that most hypotheses have many constrains which may lead to simplify governing conditions and also lack of knowledge of some physical processes associated with channel formation and maintenance. Intelligent models are a new approach to describe complex problems one of which is artificial neural networks. In this research, initially, gravel bed rivers database was used in bankfull discharge condition to train various dimensional and non-dimensional neural-network schemes with three and four variables as input data, respectively. Then, the same database was applied to fit regression analysis to estimate regime width; this led to drive dimensional and non-dimensional equations. Finally, dimensional and non-dimensional neural-network models and regression equations were compared together based on 50% error bands with other dataset. Results show that neural network can adequately estimate the regime width in gravel bed rivers and multilayer perceptron network with one hidden layer and eight hidden neurons based on dimensional data set was selected as optimum network to predict regime width. A sensitivity analysis also shows that bankfull discharge has a greater influence on regime width of gravel bed channels than the other independent parameters in dimensional scheme of neural network.     

The response of subaqueous dunes to floods in sand and gravel bed reaches of the Dutch Rhine

Abstract The branches of the River Rhine in the Netherlands, characterized by a sand–gravel bed in the upstream part and a sand bed in the downstream part of the river system, show migrating dunes, especially during floods. In the last 20 years, these dunes have been studied extensively. High-resolution echo-sounding measurements of these dunes, made with single and multibeam equipment, were analysed for three different sections of the Rhine river system during several floods. This analysis was done to quantify the growth, decay and migration rates of the dunes during floods. In addition, the migrating dunes were used to calculate bedload transport rates with dune tracking. The results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods, and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain. The relations are used to show that the growth and migration rate of dunes, and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations.     

Large-scale rhomboid bed forms and sedimentary structures associated with hurricane washover*

Hurricane washover fans from the Texas Gulf Coast exhibit large-scale rhomboid bed forms developed on washover deposits of fine sand with varying shell content. Washover processes inferred from aerial photographs, storm characteristics, and physical settings suggest that these bed forms are the product of (1) storm surge flooding or (2) high wind shear stress. Multiple bed forms, including large-scale rhombs, are responsible for sedimentary structures preserved in washover deposits. Proximal channels exhibit scour and fill sequences capped by mud drapes. Mid-channel fan deposits also have scour bases marked by shell lags which are overlain by horizontal laminations and foreset and backset laminae. Distal fan sediments are relatively shell free and are interbedded with tidal flat deposits characterized by bioturbated, alternating sand and mud laminae. Rhomboidal patterns can form on the free surface of water in response to five processes: (1) wave interference from two externally independent sources, (2) wave interference from refraction of a single set of wave fronts, (3) standing oblique waves caused by bed roughness elements, (4) standing oblique waves formed at channel boundaries and channel transitions, and (5) wind stress. Geologically, standing oblique waves from unidirectional nearly supercritical flow is probably the most important process in rhomboid bed form development.     

Effect of open-framework gravel on suffusion in sandy gravel alluvium

Acta Geotechnica - Some accidents of dams are attributed to suffusion around open-framework gravel (OFG) in sandy gravel alluvium. However, whether OFG can be really an internal seepage exit of...     

卵石地层中CFG桩的设计及其施工技术

某工程项目位于北京市门头沟区,场地内卵石、碎石地层层厚大,施工困难,选用CFG桩进行地基处理。考虑场地地层条件、施工设备、项目工期以及工程造价,按变形控制对CFG桩进行设计,同时根据场地条件及CFG桩特点制定了施工流程。对CFG桩施工中可能出现的问题,提出了相应的质量控制措施,如埋设护筒、选用合适的旋挖钻头等。     

Interrelationships between bed morphology and bed-material transport for a small,gravel-bed channel*

Bed conditions (micro-relief, textural associations and packing structural arrangements) in the gravel-bed channel of Seale's Brook are shown to be closely interrelated; various categories are identified and related to mode of bed material transport and deposition. Entrainment of bed material, commonly treated as a simple function of particle weight and channel hydraulics, is also shown to be strongly affected by varying and variable bed conditions. In particular, the classic concept of competence appears to be of restricted utility in such channels; resistance of bed material to fluid drag and to particle impact is augmented, over large parts of the channel bed, by its interlocking structure, made possible by the wide range in particle calibre, and by the characteristic disc and blade shapes of the slate debris. Particle mobility, as indicated by distance of travel of labelled bed material, is only partly a function of particle weight; indeed, although particle mobility decreases from small pebbles to large cobbles, it also decreases for the finest bed material (very small pebbles). This appears to be explicable, partly in terms of the ease of entrainment (and duration of travel), and, partly in relation to the ease of transport of material over an uneven channel bed surface. Particle mobility is greatest for material in open and infilled structures and smallest for sediment in tight structural arrangements. Local bed slope also exerts an influence on the probability of particle entrainment and on particle mobility. The findings emphasize the need for combining sedimentological and engineering approaches to bed material transport in coarse-bedded channels, and, at the same time, illustrate some of the reasons for the existence of indeterminacy in the modelling of bed-material transporting processes.     

仿和田玉籽料的方法及鉴定

和田玉籽料以其洁白温润博得了越来越多人的喜爱,加之其有限的资源和丰厚的利润,市场上出现的和田玉籽料仿制品也越来越多。介绍了和田玉的分类及外部特征,分析了仿和田玉籽料的两种常用方法——滚料磨圆和染色处理做假皮,从其外形、颜色、硬度、密度和光泽等特征及仪器测试方面,讨论研究了和田玉籽料与其仿制品之间的差异,以达到鉴定和田玉籽料真伪的目的。     

Analysing laser‐scanned digital terrain models of gravel bed surfaces: linking morphology to sediment transport processes and hydraulics

The grain‐scale topography of a sediment surface is a key component of a fluvial system, affecting aspects including sediment transport, flow resistance and ecology. However, its effect is hard to quantify because of the need for grain‐scale elevation data from in situ fluvial gravel surfaces which are difficult to collect. The sediment surface properties are, therefore, commonly estimated as a function of the sediment grain‐size distribution; however, because of additional factors, such as grain packing and shape, there is not necessarily a unique relationship between the two. A new methodology has been developed that uses terrestrial laser scanning to collect grain‐scale topographic data from in situ fluvial gravel surfaces, from which digital terrain models are created. This paper investigates methods of analysing such digital terrain models, and possible sedimentological interpretations that can be drawn from the analysis. Eleven digital terrain models from exposed gravel surfaces in two contrasting rivers (the River Feshie and Bury Green Brook) were analysed by calculating: the distribution of surface elevations, semivariograms, surface inclinations, surface slopes and aspects and grain orientation. The distribution of surface elevations and surface slope and aspect analysis were found to be most informative. In the River Feshie, grain‐size was interpreted as being a dominant control on sediment surface structure and gravel imbrication was identified. In Bury Green Brook, the location of the digital terrain models within the riffle–pool sequence was the dominant control on surface structure and grain orientation. Such digital terrain models therefore provide a new approach to measuring and quantifying the topography of fluvial sediment surfaces.     

Social change and segregation in Copenhagen

This article examines socio-spatial changes resulting from economic and social restructuring in Copenhagen in recent decades. The main features of these changes have involved a sharp decline in manufacturing employment, a rise in the participation of women in the labour market, increasing unemployment among unskilled workers, a growing number of immigrants in obsolete dwellings and social housing estates, a relative decline in the number of families, and a growth in the numbers of single parent households. Do these changes challenge the classic models of segregation, or can they still be considered valid? Empirical evidence is produced concerning the socio-spatial effect of these changes in Copenhagen. The conclusion is that the classical model of social segregation is still valid, although some additional dimensions must be included. Furthermore, the overall socio-spatial structure of Copenhagen remains relatively intact, largely because of the inertia of the urban landscape and its social relations. However, the radical nature of the restructuring processes can also be called into question.     

兰州黄河阶地卵砾石层渗透系数测试研究

采用多孔抽水、钻孔注水和单环注水试验测得兰州黄河Ⅰ级阶地卵砾石层渗透系数分别为34.19m/d、32.68m/d和26.50m/d。颗粒分析成果表明,研究区卵砾石层孔隙体系形式为卵砾类骨架形成架空大孔隙,含量较少的中细砂小颗粒难以完全充填架空大孔隙,仅起到减少大孔隙通道的作用,其渗透系数大于中细砂但小于一般意义上不含或仅含极少量中细砂的卵砾石层。对比分析可知,基于特征粒径、不均匀系数的半经验渗透系数预测值具有很大的离散性和变异性,与实测值有较大差异,作为工程建设设计参数时应通过多种方法验证其准确性。依据水文地质测试成果,结合相关资料工程经验值及地区经验,推荐研究区降水方案设计含水层渗透系数取34.19m/d。