Effects Of Preferential Flow Research Articles

A new method and apparatus for measuring in situ air permeability of unsaturated soil

A new apparatus and an interpretation method simplified from existing solutions of spherical air flow in soil were developed to measure in situ air permeability (ka) of unsaturated soil. By pumping air with a controlled volumetric flux (qv) into a spherical aeration bulb and measuring the corresponding pseudo-steady-state air gauge pressure (Δp0), kawas determined based on the slope of a qv–Δp0relationship. The method was applied to measure kain a full-scale landfill cover. The kameasurements were verified by finite-element modelling of air flow. The measurements show good agreement with the computed results. Sensitivity analyses reveal that using a smaller bulb radius (r0) would increase the measurement accuracy by minimising the boundary effects associated with atmospheric conditions and contrasting kabetween neighbouring soils. Yet, the use of a small bulb would result in more prominent effects of preferential flow and hence substantial overestimation of ka. For a small bulb size (r0= 0.01 m), the measurement accuracy of kacould be within 15% (i.e., 2.4 × 10−13to 6.3 × 10−13m2) of the true values when the ratio of vertical-to-horizontal kais between 0.4 and 1.

Numerical Simulation of Unstable Preferential Flow during Water Infiltration into Heterogeneous Dry Soil

Water infiltration and unsaturated flow through heterogeneous soil control the distribution of soil moisture in the vadose zone and the dynamics of groundwater recharge, providing the link between climate, biogeochemical soil processes and vegetation dynamics. Infiltration into dry soil is hydrodynamically unstable, leading to preferential flow through narrow wet regions (fingers). In this paper we use numerical simulation to study the interplay between fingering instabilities and soil heterogeneity during water infiltration. We consider soil with heterogeneous intrinsic permeability. Permeabilities are random, with point Gaussian statistics, and vary smoothly in space due to spatial correlation. The key research question is whether the presence of moderate or strong heterogeneity overwhelms the fingering instability, recovering the simple stable displacement patterns predicted by most simplified model of infiltration currently used in hydrological models from the Darcy to the basin scales. We perform detailed simulations of constant-rate infiltration into soils with isotropic and anisotropic intrinsic permeability fields. Our results demonstrate that soil heterogeneity does not suppress fingering instabilities, but it rather enhances its effect of preferential flow and channeling. Fingering patterns strongly depend on soil structure, in particular the correlation length and anisotropy of the permeability field. While the finger size and flow dynamics are only slightly controlled by correlation length in isotropic fields, layering leads to significant finger meandering and bulging, changing arrival times and wetting efficiencies. Fingering and soil heterogeneity need to be considered when upscaling the constitutive relationships of multiphase flow in porous media (relative permeability and water retention curve) from the finger to field and basin scales. While relative permeabilities remain unchanged upon upscaling for stable displacements, the inefficient wetting due to fingering leads to relative permeabilities at the field scale that are significantly different from those at the Darcy scale. These effective relative permeability functions also depend, although less strongly, on heterogeneity and soil structure.

CFD modeling of residence time distribution and experimental validation in a redox flow battery using free and porous flow

Abstract The successful use of clean energy sources will rest, in part, on the availability of efficient energy storage systems. Thus, the present work examines two critical factors (flow patterns and residence time distribution, RTD) affecting the performance of one of the most promising energy storage systems: redox flow batteries (RFBs). The goal is to develop a methodology for analyzing the hydrodynamic and mass transport behavior within a commercial redox flow battery (CRFB). This methodology includes modeling of the flow behavior as well as its experimental validation with RTD. The CRFB consisted of a square geometry with an interdigitated flow field design. Experimental residence time distribution curves for this device were obtained at four different flow rates using the stimulus-response technique with step signal. Theoretically, the flow behavior within the CRFB was approximated by an axial dispersion model (ADM), and a plug dispersion exchange model (PDE), as well as by solving the hydrodynamic equations (Navier–Stokes and Brinkman equations) and the mass transport equation (convection-diffusion) using COMSOL Multiphysics® 5.3. The calculated RTD curves are in good agreement with the experimental RTD curves. Our theoretical and experimental analysis resulted in better approximations of the axial dispersion, and the presence of stagnant zones, and channeling and by-pass (i.e., preferential flow) effects at low and intermediate Reynolds numbers (Re). The experimental RTD curves show that the liquid flow pattern in the CRFB deviates considerably from the axial dispersion model at low Re, conditions under which the CRFB exhibits significant channeling effects, as well as stagnant and dead zones. The PDE model was able to describe the deviation from ideal flow pattern caused by channeling, recycling, or by the presence of stagnant regions in the CRFB. The mass transport in the CRFB was determined by measuring the distribution of tracer molecules at different time points after the initial (step) tracer injection. The interdigitated flow field geometry exhibited a homogeneous flow distribution, featuring an increased mean electrolyte velocity inside the electrode. This simulation allowed locating stagnant zones that would locally affect the mass transport properties and globally lead to a reduction of the RFB conversion efficiency.

Effects of preferential flow on snowmelt partitioning and groundwater recharge in frozen soils

Abstract. Snowmelt is a major source of groundwater recharge in cold regions. Throughout many landscapes snowmelt occurs when the ground is still frozen; thus frozen soil processes play an important role in snowmelt routing, and, by extension, the timing and magnitude of recharge. This study investigated the vadose zone dynamics governing snowmelt infiltration and groundwater recharge at three grassland sites in the Canadian Prairies over the winter and spring of 2017. The region is characterized by numerous topographic depressions where the ponding of snowmelt runoff results in focused infiltration and recharge. Water balance estimates showed infiltration was the dominant sink (35 %–85 %) of snowmelt under uplands (i.e. areas outside of depressions), even when the ground was frozen, with soil moisture responses indicating flow through the frozen layer. The refreezing of infiltrated meltwater during winter melt events enhanced runoff generation in subsequent melt events. At one site, time lags of up to 3 d between snow cover depletion on uplands and ponding in depressions demonstrated the role of a shallow subsurface transmission pathway or interflow through frozen soil in routing snowmelt from uplands to depressions. At all sites, depression-focused infiltration and recharge began before complete ground thaw and a significant portion (45 %–100 %) occurred while the ground was partially frozen. Relatively rapid infiltration rates and non-sequential soil moisture and groundwater responses, observed prior to ground thaw, indicated preferential flow through frozen soils. The preferential flow dynamics are attributed to macropore networks within the grassland soils, which allow infiltrated meltwater to bypass portions of the frozen soil matrix and facilitate both the lateral transport of meltwater between topographic positions and groundwater recharge through frozen ground. Both of these flow paths may facilitate preferential mass transport to groundwater.

Crab burrows as preferential flow conduits for groundwater flow and transport in salt marshes: A modeling study

Crab burrows can act as preferential flow conduits for pore water-surface water interactions in salt marshes, but the effect of preferential flow on subsurface transport in these tidally-influenced systems is not fully understood. We used numerical models based on salt marshes of North Inlet, South Carolina, to investigate the impacts of crab burrows on porewater salinity. This modeling effort was inspired by field results from North Inlet, where prior field studies that used a combination of tension samplers and passive diffusion samplers measure salinity in crab burrows and in the adjacent sediment matrix found that the minimum salinity (28 PSU) reported by the tension samplers was larger than the maximum salinity (26 PSU) reported by passive diffusion samplers. Two kinds of numerical models were developed to investigate the effect of crab burrows on tidally-driven groundwater flow and salt transport. In the equivalent-continuum model (ECM), crab burrows were included via a shallow surface layer with hydraulic properties representing a bulk average of sediment matrix and crab burrow properties. In the preferential flow model (PFM), an independent high-permeability material was embedded in the surface muddy layer to explicitly simulate preferential flow conduits. The simulated results showed that both models can depict the effect of crab burrow on soil saturation and salt transport in salt marshes. The presence of crab burrows can greatly increase tidally-driven water exchange, improve the intensity and duration of soil aeration and enhance salt transport in salt marshes. The effect of crab burrows on groundwater flow and salt transport varied spatially from the creek bank to marsh interior. PFM models demonstrated that salinity is likely to differ between crab burrows and the sediment matrix, which supports observed differences in results between tension samplers and passive diffusion samplers. These findings may have important implications for practical pore water sampling and hydrochemical investigation in coastal wetlands.

Assessment of Nitrogen and Phosphorus Pathways at the Profile of Over-fertilised Alluvial Soils. Implications for Best Management Practices

Contrasting soil profiles (coarse-textured and fine-textured) treated with brilliant blue (BB) dye tracer, inorganic nitrogen (N) and phosphorus (P) concentrations along and between stained preferential flow pathways were examined for an irrigated and overfertilised maize monoculture system at the Mediterranean central Chile. The PO4-P concentrations were 2- to 10-fold higher in areas with BB than in areas without BB below 0.5-m soil depth in both soils. This elevated concentration was attributed to transport through cracks in fine-textured soil and finger flow in coarse-textured soil. The highest PO4-P value (13 mg kg−1) was found in areas with BB at the fine-textured soil. There were no significant differences in inorganic N concentration between areas with and without BB for both soils, which suggest that the effects of preferential flow are less important for inorganic N forms. There was a strong significant (p < 0.01) positive correlation between PO4-P and NH4-N concentrations in the fine-textured soil, and the amounts retained were clearly proportional to the clay content. Strategies for reducing N and P losses must be placed on good agronomic management of irrigated maize cropping system including accurate calculation of N and P fertiliser rates and establishment of suitable mitigation measures such as cover cropping.

Analysis of plant root–induced preferential flow and pore-water pressure variation by a dual-permeability model

Vegetation can affect slope hydrology and stability via plant transpiration and induced matric suction. Previous work suggested that the presence of plant roots would induce preferential flow, and its effects may be more significant when the planting density is high. However, there is a lack of numerical studies on how planting density affects soil pore-water pressure and shear strength during heavy rainfall. This study aims to investigate the impact of plant root–induced preferential flow on hydromechanical processes of vegetated soils under different planting densities. Two modelling approaches, namely single- and dual-permeability models, were integrated with an infinite slope stability approach to simulate pore-water pressure dynamics and slope stability. Laboratory tests on soils with two different planting densities for a plant species, Schefflera heptaphylla, were conducted for numerical simulations. The single-permeability model overestimated the pore-water pressure in shallow soil and underestimated the infiltration depth. The dual-permeability model, which is able to model the effects of preferential flow, can better capture the observations of rapid increase of pore-water pressure and deeper pressure response in the vegetated soil. However, caution should be taken on the choice of pore-water pressure when using the dual-permeability model to assess the factor of safety. The dual-permeability model using the pore-water pressure in the preferential flow domain and that in the matrix domain would result in a lower and higher factor of safety, respectively.

Modified Well-Field Configurations for Improved Performance of Contaminant Elution and Tracer Tests.

Contaminant elution and tracer (CET) tests are one method for characterizing the impact of mass transfer, transformation, and other attenuation processes on contaminant transport and mass removal for subsurface systems. The purpose of the work reported herein is to explore specific well-field configurations for improving CET tests by reducing the influence of preferential flow and surrounding-plume effects. Three injection-extraction well configurations were tested for different domain conditions using a three-dimensional numerical model. The three configurations were the traditional configuration with a single pair of injection-extraction wells, modified configuration I with one extraction well located between two injection wells, and modified configuration II with two pairs of injection-extraction couplets (one nested within the other). Elution curves for resident contaminant and breakthrough curves from simulated tracer tests were examined for specific landmarks such as the presence and extent of steady-state (relatively high concentrations) and asymptotic (asymptotic decrease to low concentrations) phases, as well as distinct changes in slope. Temporal-moment analysis of the breakthrough curves was conducted to evaluate mass recovery. Effective diffusion coefficients were obtained by fitting selected functions to the elution curves. Based on simulation results for a homogeneous domain, full isolation of the inner extraction well from the surrounding plume was obtained for the modified configuration II, whereas the extraction wells are impacted by the surrounding plume for the other two configurations. Therefore, configuration II was used for additional simulations conducted with layered and heterogeneous domains. Tracer-test simulations for homogeneous and layered domains indicate 100% mass recovery for the inner extraction well. For the heterogeneous domain, decreasing the distance between the inner injection-extraction well couplet and adjusting the pumping-rate distribution between the two extraction wells increased the mass recovery from 69% to 99%.

Parameterizing Deep Water Percolation Improves Subsurface Temperature Simulations by a Multilayer Firn Model

Deep preferential percolation of melt water in snow and firn brings water lower along the vertical profile than a laterally homogeneous wetting front. This widely recognized process is an important source of uncertainty in simulations of subsurface temperature, density and water content in seasonal snow and in firn packs on glaciers and ice sheets. However, observation and quantification of preferential flow is challenging and therefore it is not accounted for by most of the contemporary snow/firn models. Here we use temperature measurements in the accumulation zone of Lomonosovfonna, Svalbard, done in April 2012 – 2015 using multiple thermistor strings to describe the process of water percolation in snow and firn. Effects of water flow through the snow and firn profile are further explored using a coupled surface energy balance - firn model forced by the output of the regional climate model WRF. In situ air temperature, radiation and surface height change measurements are used to constrain the surface energy and mass fluxes. To account for the effects of preferential water flow in snow and firn we test a set of depth-dependent functions allocating a certain fraction of the melt water available at the surface to each snow/firn layer. Experiments are performed for a range of characteristic percolation depths and results indicate a reduction in root mean square difference between the modeled and measured temperature by up to a factor of two compared to the results from the default water infiltration scheme. This illustrates the significance of accounting for preferential water percolation to simulate subsurface conditions. The suggested approach to parameterization of the preferential water flow requires low additional computational cost and can be implemented in layered snow/firn models applied both at local and regional scales, for distributed domains with multiple mesh points.

In-situ experiments on bentonite-based buffer and sealing materials at the Mont Terri rock laboratory (Switzerland)

Repository concepts in clay or crystalline rock involve bentonite-based buffer or seal systems to provide containment of the waste and limit advective flow. A thorough understanding of buffer and seal evolution is required to make sure the safety functions are fulfilled in the short and long term. Experiments at the real or near-real scale taking into account the interaction with the host rock help to make sure the safety-relevant processes are identified and understood and to show that laboratory-scale findings can be extrapolated to repository scale. Three large-scale experiments on buffer and seal properties performed in recent years at the Mont Terri rock laboratory are presented in this paper: The 1:2 scale HE-E heater experiment which is currently in operation, and the full-scale engineered barrier experiment and the Borehole Seal experiment which have been completed successfully in 2014 and 2012, respectively. All experiments faced considerable difficulties during installation, operation, evaluation or dismantling that required significant effort to overcome. The in situ experiments show that buffer and seal elements can be constructed meeting the expectations raised through small-scale testing. It was, however, also shown that interaction with the host rock caused additional effects in the buffer or seal that could not always be quantified or even anticipated from the experience of small-scale tests (such as re-saturation by pore-water from the rock, interaction with the excavation damaged zone in terms of preferential flow or mechanical effects). This led to the conclusion that testing of the integral system buffer/rock or seal/rock is needed.

Macropores and preferential flow—a love‐hate relationship

AbstractPreferential flow is of high relevance for runoff generation, transport of chemicals and nutrients, and the transit time distribution of water in the soil or watershed. However, preferential flow effects are generally ignored in lumped hydrological models. And even most physically‐based models ignore macropores and preferential flow features at the soil and hillslope scale. Keith Beven was never satisfied with this situation and he tried again and again to convince the scientific community to focus their research on the complex topic of macropore and preferential flow. Although he recognized how difficult it is to correctly include preferential flow in hydrological models, he made substantial progress defining and describing macropore flow and showing its relevance, developing models to simulate preferential flow, and in particular, the interaction between macropores and the soil matrix. In this short commentary, I reflect on these achievements and outline a vision for research in preferential flow experiments and modeling.

Simulating ice layer formation under the presence of preferential flow in layered snowpacks

Abstract. For physics-based snow cover models, simulating the formation of dense ice layers inside the snowpack has been a long-time challenge. Their formation is considered to be tightly coupled to the presence of preferential flow, which is assumed to happen through flow fingering. Recent laboratory experiments and modelling techniques of liquid water flow in snow have advanced the understanding of conditions under which preferential flow paths or flow fingers form. We propose a modelling approach in the one-dimensional, multilayer snow cover model SNOWPACK for preferential flow that is based on a dual domain approach. The pore space is divided into a part that represents matrix flow and a part that represents preferential flow. Richards' equation is then solved for both domains and only water in matrix flow is subjected to phase changes. We found that preferential flow paths arriving at a layer transition in the snowpack may lead to ponding conditions, which we used to trigger a water flow from the preferential flow domain to the matrix domain. Subsequent refreezing then can form dense layers in the snowpack that regularly exceed 700 kg m−3. A comparison of simulated density profiles with biweekly snow profiles made at the Weissfluhjoch measurement site at 2536 m altitude in the Eastern Swiss Alps for 16 snow seasons showed that several ice layers that were observed in the field could be reproduced. However, many profiles remain challenging to simulate. The prediction of the early snowpack runoff also improved under the consideration of preferential flow. Our study suggests that a dual domain approach is able to describe the net effect of preferential flow on ice layer formation and liquid water flow in snow in one-dimensional, detailed, physics-based snowpack models, without the need for a full multidimensional model.

The influence of preferential flow on pressure propagation and landslide triggering of the Rocca Pitigliana landslide

The fast pore water pressure response to rain events is an important triggering factor for slope instability. The fast pressure response may be caused by preferential flow that bypasses the soil matrix. Currently, most of the hydro-mechanical models simulate pore water pressure using a single-permeability model, which cannot quantify the effects of preferential flow on pressure propagation and landslide triggering. Previous studies showed that a model based on the linear-diffusion equation can simulate the fast pressure propagation in near-saturated landslides such as the Rocca Pitigliana landslide. In such a model, the diffusion coefficient depends on the degree of saturation, which makes it difficult to use the model for predictions. In this study, the influence of preferential flow on pressure propagation and slope stability is investigated with a 1D dual-permeability model coupled with an infinite-slope stability approach. The dual-permeability model uses two modified Darcy-Richards equations to simultaneously simulate the matrix flow and preferential flow in hillslopes. The simulated pressure head is used in an infinite-slope stability analysis to identify the influence of preferential flow on the fast pressure response and landslide triggering. The dual-permeability model simulates the height and arrival of the pressure peak reasonably well. Performance of the dual-permeability model is as good as or better than the linear-diffusion model even though the dual-permeability model is calibrated for two single pulse rain events only, while the linear-diffusion model is calibrated for each rain event separately. In conclusion, the 1D dual-permeability model is a promising tool for landslides under similar conditions.

Impact of Preferential and Lateral Flows of Water on Single‐Ring Measured Infiltration Process and Its Analysis

Core Ideas A special device was developed to visually display the infiltration process. Impacts of preferential and lateral flow on infiltration rates were measured. Mathematical models were derived to correct the measured infiltration rates. The corrected infiltration processes agreed well with the 1D values. To improve the measurement accuracy of the soil infiltration rate, we studied the effects of preferential flow caused by the gap between the soil profile and the ring wall during its installation and lateral flow induced by the three‐dimensional impact of a single ring on the infiltration process. Laboratory experiments using silt loam taken from Beijing were conducted with a device designed to produce a soil profile that could visually display water movement during infiltration in the soil. Observations of the wetted soil profile inside and under the halved ring during the infiltration process were used to partition rates from the soil surface, preferential flow, and lateral movement of water. Mathematical models were derived to correct the measured infiltration rates affected by preferential and lateral flows during the initial and steady infiltration stages. Results indicated that in the initial infiltration stage (0–17 min), the preferential flow significantly increased the measured infiltration rates. Error caused by preferential flow ranged from 207 to 121%. In the following infiltration stage (17–120 min), the contribution of lateral flow ranged from 21 to 77%. Steady infiltration rates estimated with the modified model were 7% higher than those of one‐dimensional infiltration rates, and saturated hydraulic conductivities measured by single ring infiltrometer were 15% higher than rates estimated by the constant head method, which may have been caused by higher static water pressure inside the ring caused by preferential flow passage. This study offers a new analysis of ponded single‐ring infiltrometer data.

Effects of Anisotropy of Preferential flow on the Hydrology and Stability of Landslides

Infiltration is one of the most important landslides triggering mechanisms and it is controlled by the hydraulic characteristics of the soil, which depend on degree of saturation, existence of preferential flow paths and anisotropy. In order to account for preferential flow that can have place in macro-pores and fissures, it is common to represent the soil matrix by means of the superimposition of two different domains: a soil matrix domain, which mainly accounts for the flow in the porous matrix, and preferential flow domain representing the flow through macro-pores and fissures. There have been recent investigations on the influences of preferential flow on slope stability; however, the combined effects of anisotropy and preferential flow on infiltration processes and on rainfall induced landslide mechanisms have not been studied yet, at our knowledge. Aiming at better understanding the effects that anisotropy combined with preferential flow has on the infiltration process, we investigated the stability of a hillslope using a numerical modelling approach. Results indicate that anisotropy affects the slope stability and its failure area.

Coupling a 1D Dual-permeability Model with an Infinite Slope Stability Approach to Quantify the Influence of Preferential Flow on Slope Stability

In this study, a 1D hydro-mechanical model was developed by coupling a dual-permeability model with an infinite slope stability approach to investigate the influence of preferential flow on pressure propagation and slope stability. The dual-permeability model used two modified Darcy-Richards equations to simultaneously simulate the matrix flow and preferential flow in a slope. The simulated pressure head was sequentially coupled with the soil mechanics model. The newly-developed numerical model was codified with the Python programming language, and benchmarked against the HYDRUS-1D software. The benchmark example showed that the proposed model is able to simulate the non-equilibrium phenomenon in a heterogeneous soil. We further implemented the model to conduct a synthetic experiment designing a slope with heterogeneous soil overlying an impermeable bedrock as a combined analysis of hydrology and slope stability, the results shows that the occurrence of preferential flow can reducing the time and rainfall amount required for slope failure. The proposed model provides a relatively simple and straightforward way to quantify the effect of preferential flow on the pressure propagation and landslide-triggering in heterogeneous hillslope.

Verification of the multi-layer SNOWPACK model with different water transport schemes

Abstract. The widely used detailed SNOWPACK model has undergone constant development over the years. A notable recent extension is the introduction of a Richards equation (RE) solver as an alternative for the bucket-type approach for describing water transport in the snow and soil layers. In addition, continuous updates of snow settling and new snow density parameterizations have changed model behavior. This study presents a detailed evaluation of model performance against a comprehensive multiyear data set from Weissfluhjoch near Davos, Switzerland. The data set is collected by automatic meteorological and snowpack measurements and manual snow profiles. During the main winter season, snow height (RMSE: &lt; 4.2 cm), snow water equivalent (SWE, RMSE: &lt; 40 mm w.e.), snow temperature distributions (typical deviation with measurements: &lt; 1.0 °C) and snow density (typical deviation with observations: &lt; 50 kg m−3) as well as their temporal evolution are well simulated in the model and the influence of the two water transport schemes is small. The RE approach reproduces internal differences over capillary barriers but fails to predict enough grain growth since the growth routines have been calibrated using the bucket scheme in the original SNOWPACK model. However, the agreement in both density and grain size is sufficient to parameterize the hydraulic properties successfully. In the melt season, a pronounced underestimation of typically 200 mm w.e. in SWE is found. The discrepancies between the simulations and the field data are generally larger than the differences between the two water transport schemes. Nevertheless, the detailed comparison of the internal snowpack structure shows that the timing of internal temperature and water dynamics is adequately and better represented with the new RE approach when compared to the conventional bucket scheme. On the contrary, the progress of the meltwater front in the snowpack as detected by radar and the temporal evolution of the vertical distribution of melt forms in manually observed snow profiles do not support this conclusion. This discrepancy suggests that the implementation of RE partly mimics preferential flow effects.

Enhanced transport of CeO2 nanoparticles in porous media by macropores

This is the first study to investigate the effect of macropores on the transport of CeO2 nanoparticles (nCeO2) in quartz sand and soil. The artificial macropore types are the vertical continuous macropore (O–O), and the vertical discontinuous macropore (O–C). The results indicated that the mobility of nCeO2 was significantly enhanced by the macropore in both quartz sand and soil, and the enhancement was greater in the continuous macropore than in the discontinuous macropore. Compared with the homogeneous column, both the O–O and O–C macropores in quartz sand favored an earlier breakthrough and a larger initial effluent recovery rate of nCeO2. However, there was little influence on the plateau concentration and the total effluent recovery rate. In soil, both types of macropores significantly shortened nCeO2 breakthrough time, and favored a higher plateau concentration, and a larger initial and total effluent recovery rate. The O–O macropore which accounted for only 1% of the total pore volume had doubly increased the total mobility of nCeO2 in soil; even the mobility was increased by 30% with the O–C macropore. It was found that the effect of preferential flow on nCeO2 transport was greater in soil than it was in quartz sand.

Viscoelastic surfactants for diversion control in oil recovery

Hydrocarbon recovery is significantly improved in heterogeneous systems by injecting viscoelastic surfactant solutions. These solutions retard the effect of preferential flow through so called ‘thief zones’ (high permeability regions). The viscoelastic fluid passively increases flow resistance in regions of high permeability thereby partially blocking thief-zones. The low permeability volume paths in these heterogeneous reservoirs are swept more efficiently since less of the injected flooding fluid is lost. The produced water cut is significantly reduced which increases the overall effectiveness of the recovery process. The solutions we used are self-regulating, making them universally applicable without extensive knowledge of the reservoir properties.

Quantification of the influence of preferential flow on slope stability using a numerical modelling approach

Abstract. The effect of preferential flow on the stability of landslides is studied through numerical simulation of two types of rainfall events on a hypothetical hillslope. A model is developed that consists of two parts. The first part is a model for combined saturated/unsaturated subsurface flow and is used to compute the spatial and temporal water pressure response to rainfall. Preferential flow is simulated with a dual-permeability continuum model consisting of a matrix domain coupled to a preferential flow domain. The second part is a soil mechanics model and is used to compute the spatial and temporal distribution of the local factor of safety based on the water pressure distribution computed with the subsurface flow model. Two types of rainfall events were considered: long-duration, low-intensity rainfall, and short-duration, high-intensity rainfall. The effect of preferential flow on slope stability is assessed through comparison of the failure area when subsurface flow is simulated with the dual-permeability model as compared to a single-permeability model (no preferential flow). For the low-intensity rainfall case, preferential flow has a positive effect on drainage of the hillslope resulting in a smaller failure area. For the high-intensity rainfall case, preferential flow has a negative effect on the slope stability as the majority of rainfall infiltrates into the preferential flow domain when rainfall intensity exceeds the infiltration capacity of the matrix domain, resulting in larger water pressure and a larger failure area.