A coupled surface/subsurface flow model accounting for air entrapment and air pressure counterflow

This work introduces the soil air system into integrated hydrology by simulating the flow processes and interactions of surface runoff, soil moisture and air in the shallow subsurface. The numerical model is formulated as a coupled system of partial differential equations for hydrostatic (diffusive wave) shallow flow and two-phase flow in a porous medium. The simultaneous mass transfer between the soil, overland, and atmosphere compartments is achieved by upgrading a fully established leakance concept for overland-soil liquid exchange to an air exchange flux between soil and atmosphere. In a new algorithm, leakances operate as a valve for gas pressure in a liquid-covered porous medium facilitating the simulation of air out-break events through the land surface. General criteria are stated to guarantee stability in a sequential iterative coupling algorithm and, in addition, for leakances to control the mass exchange between compartments. A benchmark test, which is based on a classic experimental data set on infiltration excess (Horton) overland flow, identified a feedback mechanism between surface runoff and soil air pressures. Our study suggests that air compression in soils amplifies surface runoff during high precipitation at specific sites, particularly in near-stream areas.     

Permafrost degradation and subsurface-flow changes caused by surface warming trends

Change dynamics of permafrost thaw, and associated changes in subsurface flow and seepage into surface water, are analysed for different warming trends in soil temperature at the ground surface with a three-phase two-component flow system coupled to heat transport. Changes in annual, seasonal and extreme flows are analysed for three warming-temperature trends, representing simplified climate-change scenarios. The results support previous studies of reduced temporal variability of groundwater flow across all investigated trends. Decreased intra-annual flow variability may thus serve as an early indicator of permafrost degradation before longer-term changes in mean flows are notable. This is advantageous since hydrological data are considerably easier to obtain, may be available in longer time series, and generally reflect larger-scale conditions than direct permafrost observations. The results further show that permafrost degradation first leads to increasing water discharge, which then decreases as the permafrost degradation progresses further to total thaw. The most pronounced changes occur for minimum annual flows. The configuration considered represents subsurface discharge from a generic heterogeneous soil-type domain.     

Discrete fracture model for coupled flow and geomechanics

We present a fully implicit formulation of coupled flow and geomechanics for fractured three-dimensional subsurface formations. The Reservoir Characterization Model (RCM) consists of a computational grid, in which the fractures are represented explicitly. The Discrete Fracture Model (DFM) has been widely used to model the flow and transport in natural geological porous formations. Here, we extend the DFM approach to model deformation. The flow equations are discretized using a finite-volume method, and the poroelasticity equations are discretized using a Galerkin finite-element approximation. The two discretizations—flow and mechanics—share the same three-dimensional unstructured grid. The mechanical behavior of the fractures is modeled as a contact problem between two computational planes. The set of fully coupled nonlinear equations is solved implicitly. The implementation is validated for two problems with analytical solutions. The methodology is then applied to a shale-gas production scenario where a synthetic reservoir with 100 natural fractures is produced using a hydraulically fractured horizontal well.     

Stefan方程在土壤冻融过程模拟中的应用

多年冻土与大气间的相互作用主要是通过活动层中的水热动态变化过程而实现。气候变化背景下的多年冻土活动层冻融过程模拟、多年冻土厚度制图和变化预测是研究冻土区生态环境、水文、工程以及碳循环的基础。根据国内外研究进展,总结了不同修正形式的Stefan方程在多年冻土活动层冻融过程和活动层厚度模拟中的应用进展,对将Stefan方程应用到分层堆积土壤中的不同算法进行了简要介绍,并指出了其在应用过程中存在的问题。Stefan方程首次将地表（或者大气）温度的变化与冰层（或者土层）的冻结融化过程以简单公式的形式联系起来,极大地简化了土壤冻结融化过程的分析计算。由于其输入参数少、形式简单、模拟效果可靠,成为常用模拟土壤冻融过程的方法之一,将其耦合到气候模型、陆面模型和水文模型中的研究也越来越多。Stefan方程最初在研究北极地区湖冰形成过程时提出,在应用到冻土学中后,不同学者在考虑土壤含水量、不同下垫面地气温差、地形和降水等因素后对方程进行了改进,并有多种算法试图将这一方程应用到非均质土壤中,取得了较好的模拟效果。但是,Stefan方程在国内的应用更多地用于简单模拟均质土壤多年冻土活动层厚度的空间分布状况,其应用到非均质土壤中的研究却较少。因此,未来需更深入研究Stefan方程模拟分层土壤的冻融过程,为准确掌握多年冻土对气候变化的响应研究提供最基本的方法。     

Quantifying the effects of subsurface heterogeneity on hillslope runoff using a stochastic approach

The role of heterogeneity and uncertainty in hydraulic conductivity on hillslope runoff production was evaluated using the fully integrated hydrologic model ParFlow. Simulations were generated using idealized high-resolution hillslopes configured both with a deep water table and a water table equal to the outlet to isolate surface and subsurface flow, respectively. Heterogeneous, correlated random fields were used to create spatial variability in the hydraulic conductivity. Ensembles, generated by multiple realizations of hydraulic conductivity, were used to evaluate how this uncertainty propagates to runoff. Ensemble averages were used to determine the effective runoff for a given hillslope as a function of rainfall rate and degree of subsurface heterogeneity. Cases where the water table is initialized at the outlet show runoff behavior with little sensitivity to variance in hydraulic conductivity. A technique is presented that explicitly interrogates individual realizations at every simulation timestep to partition overland and subsurface flow contributions. This hydrograph separation technique shows that the degree of heterogeneity can play a role in determining proportions of surface and subsurface flow, even when effective hillslope outflow is seen. This method is also used to evaluate current hydrograph separation techniques and demonstrates that recursive filters can accurately proportion overland and base-flow for certain cases.     

青藏高原热喀斯特湖演化及其对多年冻土的热影响模型计算研究北大核心CSCD

青藏高原热喀斯特湖分布广泛,近年来在气候变暖背景下快速发展。热喀斯特湖的形成和发展与地下冰含量及气候变化有着密切关系,强烈影响多年冻土的热稳定性。为了更深入理解在气候变暖背景下热喀斯特湖的发展及其对下伏多年冻土的影响,以青藏高原北麓河地区一个典型热喀斯特湖的长期监测数据为资料,发展了耦合大气—湖塘—冻土三个过程要素的一维热传导模型,模拟了四种不同深度热喀斯特湖在气候变暖背景下的发展规律及其对多年冻土的热影响。结果表明:浅湖(<1.0m)在目前稳定气候背景下处于较稳定状态,湖冰能够回冻至湖底,对下伏多年冻土影响较小;较深湖塘(≥1.0m)冬季不能回冻至湖底,湖深不断增加,且底部在50年内将会形成不同深度的融区。随着气候变暖,热喀斯特湖的热效应显著,深度快速增加,较深湖塘的最大湖冰厚度减小,底部多年冻土快速融化形成开放融区。研究将有助于理解气候变化对青藏高原多年冻土区地貌演化及水文过程的影响。     

Numerical investigation into the effects of geometrical and loading parameters on lateral spreading behavior of liquefied layer

Numerical simulation of liquefaction-induced lateral spreading in gently sloped sandy layers requires fully coupled dynamic hydro-mechanical analysis of saturated sandy soil subjected to seismic loading. In this study, a fully coupled finite element model utilizing a critical-state two-surface-plasticity constitutive model has been applied to numerically investigate the effects of surface/subsurface geometry on lateral spreading. Using a variable permeability function with respect to excess pore pressure ratio is another distinctive feature of the current study. The developed code has been verified against the results of the well-known VELACS project. Lateral spreading phenomenon has been the focus of an extensive parametric study using the developed code. Numerical modeling of three different geometrical forms of surface and subsurface of liquefiable layer indicates that the amount and direction of lateral spreading are noticeably affected by the geometrical conditions. Also, parametric studies by the developed numerical model show that the effects of layer height, maximum ground acceleration, and loading frequency on the level of lateral spreading are significantly different for the examined geometrical condition.     

Influence of hydrate exploitation on stability of submarine slopes

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.

     

基于计算流体力学的寒区土壤水热耦合模型研究

土壤水文过程(水分运移和传热)及其对气候变化的响应是寒区水文学的前沿问题.然而,冻土的存在使得寒区土壤水文过程变得极其复杂.此外,寒区自然环境恶劣,较难获取长时间序列和高分辨率的野外观测资料.近年来,充分利用已有的观测数据,构建寒区土壤水热耦合模型,并开展相应的数值模拟研究,已成为理解寒区土壤水文物理过程,揭示其动力学...     

Analysis of groundwater flow through low-latitude alpine permafrost by model simulation: a case study in the headwater area of Yellow River on the Qinghai-Tibet Plateau,China

Warming climate and thawing permafrost have profound impacts on groundwater flow regimes in cold regions because of the shrinkage or disappearance of the confining unit formed by the permafrost layers and improving hydraulic connections. Numerical simulations of coupled groundwater flow and heat transfer are often used to characterize the changing permafrost hydrogeology. In this study, a number of scenarios for different hydraulic gradients and lake-water depths have been used to simulate the concordant permafrost evolution and groundwater movement using a two-dimensional cylindrical coordinate model at time scales of decades to centuries in response to a warming climate. The model is applied to a representative headwater catchment in the south-central headwater area of the Yellow River on the northeastern Qinghai-Tibet Plateau, China. The results show that the presence and movement of groundwater and the deeper subpermafrost aquifer can substantially accelerate permafrost degradation, and the disappearance of residual permafrost at depth can result in the sudden establishment of deep groundwater flow paths. All hydrological impacts will become evident after the stabilization of the hydrothermal and flow fields at 100–200 years. The stable discharge rate of groundwater flow varies from 8.0 to 12.4 m³ s⁻¹, and the stable velocity of groundwater flow varies from 1.6 × 10⁻⁷ to 4.4 × 10⁻⁷ m s⁻¹ under different scenarios within the model domain. The modeling results also demonstrate that flow velocity and discharge rate in local groundwater flow systems can be enhanced by an increased hydraulic conductivity, leading to an accelerated degradation of isolated permafrost bodies.

     

Multi-dimensional chemo-osmotic consolidation of clays

A model of chemo-osmotic consolidation of clays in multi-dimensional domains is presented, with volumetric strains induced by both changes in the chemistry and osmotically driven pore water flow considered. Three fully coupled governing equations considering force equilibrium, pore water transport and chemical transport are presented and solved using the transient finite element method. The proposed approach is verified via consideration of chemo-osmotic consolidation of a compacted clay landfill liner and then applied to investigation of a hypothetical case with a local leachate leak in the compacted clay liner. An assessment of the impact of the two-dimensional nature of the system is made. The consolidation process is found to be dominated by osmotic consolidation in the early stages and subsequently by chemical consolidation. It is found that the surface settlement and the leachate concentration in the compacted clay liner may be highly overestimated by a one-dimensional analysis. Moreover, the peak negative excess pore pressure predicted by the two-dimensional solution remains in a shallow region under the leak while in the one-dimensional solution it moves progressively downwards.     

Thermo-hydro-mechanical modeling of fracturing porous media with two-phase fluid flow using X-FEM technique

In this paper, a fully coupled thermo-hydro-mechanical model is presented for two-phase fluid flow and heat transfer in fractured/fracturing porous media using the extended finite element method. In the fractured porous medium, the traction, heat, and mass transfer between the fracture space and the surrounding media are coupled. The wetting and nonwetting fluid phases are water and gas, which are assumed to be immiscible, and no phase-change is considered. The system of coupled equations consists of the linear momentum balance of solid phase, wetting and nonwetting fluid continuities, and thermal energy conservation. The main variables used to solve the system of equations are solid phase displacement, wetting fluid pressure, capillary pressure, and temperature. The fracture is assumed to impose the strong discontinuity in the displacement field and weak discontinuities in the fluid pressure, capillary pressure, and temperature fields. The mode I fracture propagation is employed using a cohesive fracture model. Finally, several numerical examples are solved to illustrate the capability of the proposed computational algorithm. It is shown that the effect of thermal expansion on the effective stress can influence the rate of fracture propagation and the injection pressure in hydraulic fracturing process. Moreover, the effect of thermal loading is investigated properly on fracture opening and fluids flow in unsaturated porous media, and the convective heat transfer within the fracture is captured successfully. It is shown how the proposed computational model is capable of modeling the fully coupled thermal fracture propagation in unsaturated porous media.     

Progress in Numerical Simulation of Long-Term Impact of Thermokarst Lakes on Permafrost Thermal Regime

Thermokarst lakes are a major heat source for the adjacent permafrost and a significant source of atmospheric methane. These lakes have important impacts on the physical, chemical, biological, geomorphological and hydrological processes occurring in the ground under and around thermokarst lakes, and seriously affect the local environment and the stability of the structures constructed in permafrost regions. Numerical simulation methods provide an effective method for quantitative analysis of the long-term impact of thermokarst lakes and their evolution on permafrost surrounding the lakes, and have deepened our knowledge about the impact of thermokarst lakes immensely. Summarizing the research progresses in numerical simulation of long-term impact of thermokarst lakes on thermal regime of surrounding permafrost has an important guiding function to improve mathematical models and develop more effective models. In this study, the components, functions, advantages and defects of several typical mathematical models having developed over the past ten years or so were reviewed, such as the heat conduction model with phase change, thaw slumping model, the coupled lake-permafrost model, thaw lake expansion model combining thermal processes with mass wasting and thaw-driven subsidence, the coupled heat conduction and moisture migration model, and the moving mesh method based thermokarst lake dynamic evolution model. Several issues deserving to be paid further attention in the future researches were proposed, including creating more effective models, determining the more realistic initial condition, lucubrating thermal and physical parameters of the typical soils, consider the impact of lake water replenishment, quantitative analysis of the thermal effect of supra-permafrost water flow around the thermokarst lakes, creating the coupled governing equation of heat conduction with phase change and convective heat transfer, embed ding the effect of climate warming in the model, numerical investigation of the long-term influence of thermokarst lake drainage on the environment change in permafrost regions, analyzing the long-term joint impact of multiple lakes on adjacent permafrost, simulating the near-shore talik development process and feature beneath shallow water in expanding thermokarst lakes, and continuing to do the systemic and comprehensive field measurements.     

Comparing the performance of TRIGRS and TiVaSS in spatial and temporal prediction of rainfall-induced shallow landslides

This study compares the performance of transient rainfall infiltration and grid-based regional slope stability (TRIGRS) model and time-variant slope stability (TiVaSS) model in the prediction of rainfall-induced shallow landslides. TRIGRS employs one-dimensional (1-D) subsurface flow to simulate the infiltration rate, whereas a three-dimensional (3-D) model is utilized in TiVaSS. The former has been widely used in landslide modeling, while the latter was developed only recently. Both programs are used for the spatiotemporal prediction of shallow landslides caused by rainfall. This study uses the July 2011 landslide event that occurred in Mt. Umyeon, Seoul, Korea, for validation. The performance of the two programs is evaluated by comparison with data of the actual landslides in both location and timing by using a landslide ratio for each factor of safety class (\({\text{LR}}_{\text{class}}\) index), which was developed for addressing point-like landslide locations. Moreover, the influence of surface flow on landslide initiation is assessed. The results show that the shallow landslides predicted by the two models are highly consistent with those of the observed sliding sites, although the performance of TiVaSS is slightly better. Overland flow affects the buildup of the pressure head and reduces the slope stability, although this influence was not significant in this case. A slight increase in the predicted unstable area from 19.30 to 19.93% was recorded when the overland flow was considered. It is concluded that both models are suitable for application in the study area. However, although it is a well-established model requiring less input data and shorter run times, TRIGRS produces less accurate results.     

Reliability Analysis of Earth Slopes Considering Spatial Variability

The paper presents a computational procedure for reliability analysis of earth slopes considering spatial variability of soils under the framework of the Limit Equilibrium Method. In the reliability analysis of earth slopes, the effect of spatial variability of soil properties is generally included indirectly by assuming that the probabilistic critical slip surface is the same as that determined without considering spatial variability. In contrast to this indirect approach, in the direct approach, the effect of spatial variability is included in the process of determination of the probabilistic critical surface itself. While the indirect approach requires much less computational effort, the direct approach is definitely more rigorous. In this context this paper attempts to investigate, with the help of numerical examples, how far away are the results obtained from the indirect approach from that obtained from the direct approach. In both the approaches, it is required to use a model of discretization of random fields into finite random variables. A few such models are available in the literature for one-dimensional (1D) as well as two-dimensional (2D) spatial variability. The developed computational scheme is based on the First Order Reliability Method (FORM) coupled with the Spencer Method of Slices valid for limit equilibrium analysis of general slip surfaces. The study includes bringing out the computational advantages and disadvantages of the three commonly used discretization models. The sensitivity of the reliability index to the magnitudes of the scales of fluctuation has also been studied.     

多年冻土区热融湖研究现状与展望

热融湖是多年冻土区地下冰融化形成的典型地貌单元,热融湖及其变化对多年冻土热状态、水文过程、生态环境、冻土工程稳定性等有着重要的影响. 热融湖还是重要的温室气体源,与全球气候系统存在着复杂而显著的互馈过程,是气候与环境变化的指示器. 因此,开展热融湖的相关研究是近年来冻土学研究的热点之一. 通过文献综述,从以下方面评述了热融湖研究的现状与进展：1) 热融湖形态特征及其演化过程;2) 热融湖热状况及其热效应研究;3) 热融湖和多年冻土区土壤-植被生态系统的相互作用研究;4) 热融湖对大气中温室气体的贡献. 最后,讨论了该领域目前面临的主要问题,提出了应在热融湖演化的基础理论、动态变化过程的预测、热融湖的冻土水文效应、区域尺度热融湖与冻土、生态、水文和气候耦合过程的综合等方面进一步开展研究,为定量预测和评价冻土区热融湖环境工程效应,应发展不同尺度的计算模型.     

A low-dimensional subsurface model for saturated and unsaturated flow processes: ability to address heterogeneity

A low-dimensional model that describes both saturated and unsaturated flow processes in a single equation is presented. Subsurface flow processes in the groundwater, the vadose zone, and the capillary fringe are accounted for through the computation of aggregated hydrodynamic parameters that result from the integration of the governing flow equations from the bedrock to the land surface. The three-dimensional subsurface flow dynamics are thus described by a two-dimensional equation, allowing for a drastic reduction of model unknowns and simplification of the model parameterizations. This approach is compared with a full resolution of the Richards equation in different synthetic test cases. Because the model reduction stems from the vertical integration of the flow equations, the test cases all use different configurations of heterogeneity for vertical cross-sections of a soil-aquifer system. The low-dimensional flow model shows strong consistency with results from a complete resolution of the Richards equation for both the water table and fluxes. The proposed approach is therefore well suited to the accurate reproduction of complex subsurface flow processes.     

陆面水文过程与大气模式的耦合及其在黑河流域的应用

陆面过程模式对陆面水文过程有比较详尽的描述,然而,目前的陆面水文过程只考虑了垂直方向的水分运移,比较适合平坦地区的模拟,而在地形坡度较大的山区只考虑垂直方向的水分移动是不够的,尤其是目前随着计算机条件提高,分辨率越来越高,地表水以及土壤水的侧向流动成为山区水文过程必须考虑的部分。同时,目前的陆面过程模式中的径流量是作为诊断量处理,不参与运算。针对以上问题,对Noah陆面过程模式进行了改进,增加了地表积水和积水蒸发、坡面汇流方案、次表面流方案,并且将Routing模块通过次网格过程与大气中尺度模式MM5耦合,发展了高分辨率大气—水文耦合模式。运用发展的高分辨率大气—水文耦合模式,对黑河流域中上游2003年6月23~25日降水过程进行了模拟,研究了陆面水循环过程对大气场的影响。结果表明陆面水循环过程对近地层大气影响很大,首先影响了土壤的湿度与蒸发,进而对边界层稳定性,云结构、云水、雨水含量产生影响,对区域降水也有一定影响。     

基于水动力学模型的沿海城市洪水实时演进模拟

为了准确分析洪涝灾害对防洪体系现状的影响，做出相应的防洪减灾措施，以浙江省台州市灵江下游流域为研究区域，构建了基于Saint-Venant方程的水动力学耦合模型，对河道溃决洪水过程进行实时仿真模拟。综合考虑研究区域地形、气象、水文资料、水利工程、下垫面条件等因素，在一维河网模型和二维水动力学模型耦合衔接中，最大程度还原真实地形中河槽内外的水流交互淹没，借助研究区域内典型台风暴雨资料，率定验证本文建立的一维-二维耦合水动力学模型，检验后的模型可实现灵江下游沿岸城市不同量级设计洪水及历史洪水的实时淹没过程模拟。模型计算结果表明，该模型模拟复杂地形条件情况下流域洪水实时演进过程达到了较高精度，在水系沿程典型断面水位计算值与实测值误差不超过0.1 m。