共查询到20条相似文献，搜索用时 31 毫秒
1.
Ronggao Qin Yanqing Wu Zengguang Xu Derrick Xie Chi Zhang 《Hydrogeology Journal》2013,21(6):1235-1246
The combined influence of dip angle and adsorption heterogeneity on solute transport mechanisms in heterogeneous media can be understood by performing simulations of steady-state flow and transient transport in a heterogeneous aquifer with dipping anisotropy. Reactive and non-reactive contaminant transport in various types of heterogeneous aquifer is studied by simulations. The hydraulic conductivity (K) of the heterogeneous aquifer is generated by HYDRO_GEN with a Gaussian correlation spectrum. By considering the heterogeneity of the adsorption distribution coefficient (K d), a perfect negative correlation between lnK and lnK d is obtained by using the spherical grains model. The generated K and K d are used as input to groundwater flow and transport models to investigate the effects of dipping sedimentary heterogeneity on contaminant plume evolution. Simulation results showed that the magnitude of the dip angle strongly controls the plume evolution in the studied anisotropic and heterogeneous aquifer. The retarded average pore-water velocity (v/R) of the adsorption model significantly controls the horizontal spreading of the plume. The bottom plume is intensively retarded in the zones between the dipping lenses of lower hydraulic conductivity and the no-flow bottom boundary. The implications of these findings are very important for the management of contaminated heterogeneous aquifers. 相似文献
2.
Modern geostatistical techniques allow the generation of high-resolution heterogeneous models of hydraulic conductivity containing millions to billions of cells. Selective upscaling is a numerical approach for the change of scale of fine-scale hydraulic conductivity models into coarser scale models that are suitable for numerical simulations of groundwater flow and mass transport. Selective upscaling uses an elastic gridding technique to selectively determine the geometry of the coarse grid by an iterative procedure. The geometry of the coarse grid is built so that the variances of flow velocities within the coarse blocks are minimum. Selective upscaling is able to handle complex geological formations and flow patterns, and provides full hydraulic conductivity tensor for each block. Selective upscaling is applied to a cross-bedded formation in which the fine-scale hydraulic conductivities are full tensors with principal directions not parallel to the statistical anisotropy of their spatial distribution. Mass transport results from three coarse-scale models constructed by different upscaling techniques are compared to the fine-scale results for different flow conditions. Selective upscaling provides coarse grids in which mass transport simulation is in good agreement with the fine-scale simulations, and consistently superior to simulations on traditional regular (equal-sized) grids or elastic grids built without accounting for flow velocities. 相似文献
3.
Stefan Bachu 《Tectonophysics》1991,190(2-4):299-314
Heterogeneity is present in geological sedimentary structures at all scales from pore to basin, and its distribution has an impact on transport processes such as heat and fluid flow. The rock masses at any scale need to be characterized by their effective properties at that scale, based on the individual characteristics of the heterogeneous porous medium. The effective thermal and hydraulic conductivity of sediments characterized by a stochastic distribution of heterogeneity is studied using an inverse approach based on numerical experiments. The simulations, covering a large range of conductivity contrasts, are carried out for actual core-scale cases of shale clasts in a sand matrix, and on a diagrammatic cross-section through a clastic sedimentary group at the basin scale.
The effective conductivity depends primarily on the heterogeneity fraction and on the conductivity contrast between heterogeneities and the embedding matrix, a dependency which can be described by a generalized weighted mean model. This model is better suited to estimate the effective conductivity at any scale than other models like the self-consistent, or any of the arithmetic, geometric or harmonic averages. The effective conductivity has an anisotropic character although the individual components are homogeneous and isotropic. The variation in effective conductivity is significant even for small conductivity contrasts, like in heat flow processes, and exhibits an asymptotic behaviour for large conductivity contrasts characteristic of fluid flow processes. The effective conductivity has a second-order dependence on such heterogeneity characteristics as shape, aspect ratio, orientation, and distribution. Depending on these characteristics, the bounds of effective conductivity values can be narrowed further from the extreme bounds expressed by the arithmetic and harmonic averages. 相似文献
4.
Urban pluvial flash floods have become a matter of widespread concern, as they severely impact people’s lives in urban areas. Hydrological and hydraulic models have been widely used for urban flood management and urban planning. Traditionally, to reduce the complexity of urban flood modelling and simulations, simplification or generalization methods have been used; for example, some models focus on the simulation of overland water flow, and some models focus on the simulation of the water flow in sewer systems. However, the water flow of urban floods includes both overland flow and sewer system flow. The overland flow processes are impacted by many different geographical features in what is an extremely spatially heterogeneous environment. Therefore, this article is based on two widely used models (SWMM and ANUGA) that are coupled to develop a bi-directional method of simulating water flow processes in urban areas. The open source overland flow model uses the unstructured triangular as the spatial discretization scheme. The unstructured triangular-based hydraulic model can be better used to capture the spatial heterogeneity of the urban surfaces. So, the unstructured triangular-based model is an essential condition for heterogeneous feature-based urban flood simulation. The experiments indicate that the proposed coupled model in this article can accurately depict surface waterlogged areas and that the heterogeneous feature-based urban flood model can be used to determine different types of urban flow processes.
相似文献5.
3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples 总被引：1，自引：1，他引：0
Cesare Comina Renato Maria Cosentini Gabriele Della Vecchia Sebastiano Foti Guido Musso 《Acta Geotechnica》2011,6(4):195-203
Monitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the
case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible
preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper
describes a 3D laboratory application of electrical resistivity tomography (ERT) used to monitor solute transport processes.
Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer
cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate
concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes
were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer
the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barriers. 相似文献
6.
Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer 总被引：1，自引：0，他引：1
Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters are usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study. 相似文献
7.
含水层非均质性的刻画是模拟地下水中污染物运移的关键。以渗透系数为研究对象,构建了综合集合卡尔曼滤波方法、有效电阻率模型与地下水运移模型的同化框架,通过融合地球物理观测数据与污染物浓度观测数据来推估渗透系数的空间分布。基于理想算例,验证了该同化框架刻画含水层非均质渗透系数场的有效性,并针对不同初始参数信息与观测类型对比了耦合与非耦合水文地球物理方法的适用性。研究结果表明:基于集合卡尔曼滤波方法同化多种类型的观测数据,可有效地推估非均质参数空间分布。当初始信息较准确时,耦合方法的参数推估精度更高;初始信息存在偏差时,非耦合方法有更好的同化效果。由于非耦合方法计算成本较低且对初始信息缺失时适用性更强,在实际应用中可先基于非耦合方法初步估计参数,再利用耦合方法进一步提高参数推估精度。融合多种类型观测数据可有效提高参数推估效果。 相似文献
8.
Yu-Shu Wu Guoping Lu Keni Zhang Lehua Pan Gudmundur S. Bodvarsson 《Hydrogeology Journal》2007,15(3):553-572
Characterizing percolation patterns in unsaturated fractured rock has posed a greater challenge to modeling investigations than comparable saturated zone studies due to the heterogeneous nature of unsaturated media and the great number of variables impacting unsaturated flow. An integrated modeling methodology has been developed for quantitatively characterizing percolation patterns in the unsaturated zone of Yucca Mountain, Nevada (USA), a proposed underground repository site for storing high-level radioactive waste. The approach integrates moisture, pneumatic, thermal, and isotopic geochemical field data into a comprehensive three-dimensional numerical model for analyses. It takes into account the coupled processes of fluid and heat flow and chemical isotopic transport in Yucca Mountain’s highly heterogeneous, unsaturated fractured tuffs. Modeling results are examined against different types of field-measured data and then used to evaluate different hydrogeological conceptualizations through analyzing flow patterns in the unsaturated zone. In particular, this model provides clearer understanding of percolation patterns and flow behavior through the unsaturated zone, both crucial issues in assessing repository performance. The integrated approach for quantifying Yucca Mountain’s flow system is demonstrated to provide a practical modeling tool for characterizing flow and transport processes in complex subsurface systems. 相似文献
9.
Hybrid analytical and finite element numerical modeling of mass and heat transport in fractured rocks with matrix diffusion 总被引：1，自引：0，他引：1
Christopher I. McDermott Robert Walsh Ralph Mettier Georg Kosakowski Olaf Kolditz 《Computational Geosciences》2009,13(3):349-361
Quantification of mass and heat transport in fractured porous rocks is important to areas such as contaminant transport, storage
and release in fractured rock aquifers, the migration and sorption of radioactive nuclides from waste depositories, and the
characterization of engineered heat exchangers in the context of enhanced geothermal systems. The large difference between
flow and transport characteristics in fractures and in the surrounding matrix rock means models of such systems are forced
to make a number of simplifications. Analytical approaches assume a homogeneous system, numerical approaches address the scale
at which a process is operating, but may lose individual important processes due to averaging considerations. Numerical stability
criteria limit the contrasts possible in defining material properties. Here, a hybrid analytical–numerical method for transport
modeling in fractured media is presented. This method combines a numerical model for flow and transport in a heterogeneous
fracture and an analytical solution for matrix diffusion. By linking the two types of model, the advantages of both methods
can be combined. The methodology as well as the mathematical background are developed, verified for simple geometries, and
applied to fractures representing experimental field conditions in the Grimsel rock laboratory. 相似文献
10.
《Chemical Geology》2007,236(3-4):247-265
A knowledge of the processes involved in streamflow generation are critical to an understanding of solute transport and weathering rates in upland catchments. The determination of specific flow pathways and the formulation of process-based models have proved difficult in such terrains, largely due to the heterogeneous nature of catchments and the necessary limitations of bulked parameter models. Natural geochemical tracers have proved invaluable in developing conceptual models of catchment functioning and for constraining weathering processes and geochemical cycling. Strontium isotopes have been used as a natural tracer to calculate weathering rates for Sr and Ca, and to constrain the dominant flow pathways in two upland forested sub-catchments (Afon Hafren and Afon Hore) of the River Severn at Plynlimon in Central Wales. The dominant source of Sr in the catchments is considered to be from the weathering of silicate minerals. Weathering rates for Sr and Ca in the Afon Hafren, calculated using Sr isotopes, were similar to those derived from mass balance studies. The rates for the Afon Hore were similar for Ca, but significantly different for Sr. The reason for the difference is unclear, but may be due to additional sources (calcite) in the catchment. Strontium isotope ratios for different input sources and compartments within the catchment were characterised and helped to identify potential flow pathways. The data suggest an important role for groundwater inputs in contrast to previous models which indicated a dominant role for soil waters. 相似文献
11.
地下水反应运移模型具有参数个数众多,观测数据类型多样的特点。为了探究不同类型观测数据在反应运移模拟数据同化中的数据价值,构建了三氯乙烯降解反应运移模型的理想算例,基于水头和浓度两种类型观测数据,采用集合卡尔曼滤波方法推估渗透系数和贮水系数的非均质空间分布,讨论了影响同化结果的因素。结果表明:与仅同化水头数据的结果相比,联合同化水头和浓度观测数据推估渗透系数场和贮水系数场时具有更高的精度,在观测数据拟合和模型预测方面也有更好的表现。与目前溶质运移模型、非饱和流模型等地下水模型中的研究结果相似,数据同化结果受样本数量,观测井的数量和位置的影响,合理优化布置监测井和选择样本数量可有效改善数据同化效果并提高计算效率。 相似文献
12.
Comprehensive field measurements of flow and sedimentary processes have been made with the aid of stable scaffolding bridges spaced along the length of a bend of the River South Esk, Scotland. At river stages between about two-thirds full and bankfull, channel width, mean depth and mean flow velocity at a cross-section vary little in the streamwise direction. Flow resistance reaches a maximum at these stages, and the bed topography is stable and in equilibrium with flow and bedload transport. Stable flow geometry is thus related in some way to energy conservation, and to maximization of flow resistance. Detailed observations over a large range of river stages of mean velocity distributions, secondary circulation, water surface configuration, bed shear stress and resistance to flow, bed configurations and bed load transport rates agree with much (but not all) of the comparable published experimental studies and selected theoretical work. Generalized physical models of flow and sediment transport in natural curved channels (Engelund, 1974; Bridge, 1977) are demonstrated to be sound in basis and can simulate the bend studied very well. Although there is a pressing need for further development of these models, the results lend confidence to their use in simulating ancient river sedimentation. Sediment deposited on point bars is the result mainly of bedload transport over a range of near-bankfull stages. The areal distribution of grain-size characteristics and bed configurations at these stages give rise, with lateral deposition, to vertical facies sequences that vary substantially in the streamwise direction. 相似文献
13.
14.
Complex variations in sediment transport at three large river bifurcations during discharge waves in the river Rhine 总被引：1，自引：0，他引：1
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. 相似文献
15.
16.
Understanding flow and transport in low-permeability media is very important in the context of nuclear waste disposal, oil and gas reservoirs and long term evolution of groundwater systems. In low-permeability media, transport by diffusion is often the most important mass transport process. This study investigates the effect of the heterogeneity of diffusion parameters on mass transport in low-permeability media. A geostatistical approach for integrating heterogeneity of diffusion parameters in groundwater flow and transport models is proposed and applied to the Toarcian argillites in France which are studied in the framework of feasibility of storing radioactive waste in deep clayey massifs. Stochastic fields of the diffusion parameters of the Toarcian argillites (France) are generated based on 64 measured values of diffusion coefficient and diffusion accessible porosity and used as input for a 3D local-scale groundwater flow and transport model. The chloride concentrations computed by these heterogeneous models are compared to the measured chloride concentrations and to concentrations calculated with a model in which the Toarcian argillites are subdivided into several homogeneous zones. The heterogeneous simulations result in a slightly better correspondence between measured and calculated values and have the additional advantage that the measured diffusion coefficient values in the Toarcian are perfectly honored in the model. This study shows that small-scale variability of diffusion parameters has a significant effect on solute concentrations and omitting this heterogeneity may be a problem in transport calculations in low-permeability media, depending on the specific setting and objectives of the study. 相似文献
17.
Canul-Macario Cesar Salles Paulo Hernández-Espriú Antonio Pacheco-Castro Roger 《Hydrogeology Journal》2021,29(7):2347-2362
Numerical modelling is increasingly used as a tool for improving management strategies in aquifers and to support the design of comprehensive projects considering natural and anthropogenic processes. Overall, numerical simulation in karstic aquifers poses a major scientific challenge due to the non-Darcian groundwater flow dynamics. In specific cases, the equivalent porous medium approach has shown acceptable results, particularly in poorly karstified aquifers with regional/subregional scales such as this case. The Yucatan coastal karstic aquifer (Mexico) has been defined as a complex regional heterogeneous system, partially confined, thus allowing the discussion of multiple conceptual models. In this research, a two-dimensional numerical model of flow and transport was implemented using SEAWAT for the NW Yucatan aquifer. Four likely conceptual models were audited, calibrated and verified using hydrogeological field data, to select the best one, considering their fit and complexity. The numerical model accuracy was evaluated using the root-mean-square error, Nash Sutcliffe efficiency and the Pearson coefficient. The Akaike information criterion and Bayesian information criterion were included for evaluating the complexity of the numerical models. In addition, the signal of tide propagation into the aquifer was assessed as a proxy to improve the numerical calibration process. Results show that the most complex numerical model has a better calibration than the simpler models, but the model accuracy is worse when compared to less complex numerical models in the verification exercise. This research offers enhancement in the knowledge of numerical modelling in heterogeneous coastal aquifers within a conceptual-model uncertainty setting.
相似文献18.
Downstream lightening and upward heavying: Experiments with sediments differing in density 
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下载免费PDF全文 Sorting and selective transport of particles by material density is important for understanding a wide range of processes, including the formation of mineral placers, deposition of mine tailings and routing of tracers and contaminants. This article describes an experimental study of the transport of mixtures of particles of differing density in a sediment‐feed flume. During the runs, a downstream prograding wedge‐shaped deposit was formed. Results show two sorting processes: (i) longitudinal sorting characterized by preferential deposition of heavy particles in the upstream part of the deposit – downstream lightening; and (ii) vertical sorting with less dense particles preferentially deposited in the lowermost portion of the migrating front – upward heavying. Downstream lightening is the analogue of the well‐known downstream fining observed in the more studied case of mixtures of heterogeneous size with the same density. In both cases, the lighter particles are carried further downstream than the heavier particles. Upward heavying is unexpected when compared with deposits of heterogeneous size and same‐density particles, where the heaviest (i.e. coarsest) particles are deposited in the lowermost part of the front. The physical mechanism underlying this upward heavying might be related to the physics of gravity‐driven granular flows; the front of the deposit acts like a dense granular flow down an inclined plane. In this case, the denser particles settle away from the free surface and at the top of the heap, while the lighter particles flow to the bottom. As the front of the deposit advances, this progressively gives rise to an upward heavying pattern. The application of classical surface‐based fractional bedload transport models suggests that equal mobility is not approached in the case of mixtures of particles with uniform size and different densities. This study hypothesizes that other mechanics related to the physics of the segregation processes in these systems contribute to these results. 相似文献
19.
The grain-size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments. The existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain-size fractions in the transport layer under given hydraulic conditions. The efficiency of these models was tested with the observed data. Subsequently, the patterns of observed grain-size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions (pdf), such as, log-normal, log-hyperbolic and log-skew-Laplace. Tests indicated that a log-skew-Laplace distribution fitted best for 49%, a log-hyperbolic for 31%, and a log-normal for 20% out of forty-five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness. The results of this study would be useful to specify the grain-size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments. 相似文献
20.
为探讨含水层非均质性不同刻画方法对地下水流和溶质运移预测的影响,基于非均质含水层砂箱实验,分别用传统等效均质模型、克立金插值和水力层析刻画含水层渗透系数场,并探讨了先验信息对水力层析结果的影响.将不同方法估算的渗透系数场用以预测地下水流和溶质运移过程,以此判断不同方法估算结果的优劣,分析含水层非均质性对地下水流和溶质运移的影响.结果表明:与克立金插值法相比,水力层析法可以更好地刻画含水层非均质性,较准确地预测地下水流和溶质运移过程;钻孔岩心渗透系数样本值作为先验信息可以提高水力层析法估算结果的精度;传统等效均质模型无法准确预测地下水流和溶质运移过程.含水层非均质性的增强将导致溶质污染羽分布形态和运移路径的空间变异性增强,并且优势通道直接决定溶质的分布及运移路径. 相似文献