| 1. |
- Murekatete, Rachel Mundeli, 1981-
(författare)
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Sensitivity, Variation, and Application of Least-Cost Path Models in Landscape Connectivity Analysis and Corridor Planning
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent decades, Rwanda has been affected by the loss and fragmentation of natural habitats for native species of animals and plants. As a consequence, landscape connectivity—i.e., the degree to which a landscape facilitates or impedes the movement of organisms between resource patches—has considerably weakened or is even completely lost, causing detrimental effects on biodiversity, notably the reduction of populations of key native species. In order to counter this problem, one potential solution currently being explored by local planners in Rwanda consists of establishing conservation corridors for organisms to move safely between their habitat remnants. Specifically, this thesis was inspired by a project initiated by the Dian Fossey Gorilla Fund International, a conservation non-governmental organization (NGO) based in Rwanda, which consists of establishing a conservation corridor for pollinators.For their capabilities of storing, processing, and visualizing landscape data, geographic information systems (GIS) have been increasingly popular among conservation biologists and practitioners. Of particular relevance to connectivity analysis and corridor planning is the least-cost path model. A typical use of this model is such that one first estimates the cost for a certain action (e.g., movement by an organism or acquisition by a government) at each location of a given landscape and represents the results in the form of a raster surface, and then measures the degrees of connectivity between patches of interest in terms of effective distances, which are equated with least-cost path distances over the raster cost surface. While the least-cost path model is easy to use and available in virtually any commercial raster-based GIS, we observe that users of it often overlook some important assumptions, the violation of which might greatly affect the validity of the model’s outcome.The goal of this thesis is to provide a scientific contribution to landscape connectivity analysis and conservation corridor planning by 1) investigating the potential misuse or abuse of the conventional least-cost path model when sufficient information is not available on the underlying cost surface, 2) proposing an alternative model under such a circumstance and 3) demonstrating its relevance to conservation practice. More specifically, for the model to work, it is explicitly or implicitly assumed that, the optimality of a path is evaluated as the sum of the cost-weighted lengths of all its segments—cost-weighted, i.e., multiplied by their underlying cost values. The validity of this assumption must be questioned, however, if cost values are measured on a scale—e.g., an ordinal scale of measurement in Stevens’s typology—that does not permit arithmetic operations. In a typical practice of landscape connectivity analysis and corridor planning, the raster cost surface is created by transforming one or more sets of values (e.g., land cover type, land ownership, and elevation) attributed to cells into another set of such values (representing cost) through a function reflecting one or more criteria. A question arises: how certain can one be about the correctness of such a cost estimation function?There are at least four issues in the application of the least-cost path model to landscape connectivity analysis and corridor planning under uncertainty. First, while it is generally anticipated that different cost estimation functions lead to different least-cost paths (hence to different effective distances or different corridor locations), little is known on how such differences arise (or do not arise). Second, while it is generally recognized that the location and length of a least-cost path are both sensitive to the spatial resolution of the raster cost surface, little is known if they are always sensitive in the same way and to the same degree and if not, what makes them more (or less) sensitive. Third, when it is difficult to establish a fully connected corridor between target habitat areas (e.g., because of surrounding anthropogenic activities), the least-cost path (which is by definition fully connected) may not be useful at least in its original form. Lastly, even if the conventional least-cost path model may have inconsistent results in theory, it may well be continued to be used in practice, unless there is a sound alternative to it.The issues raised above are addressed through four studies corresponding to four respective papers which are appended to this thesis. While the first three studies use artificial landscape data generated by computers with varying spatial and non-spatial characteristics, the fourth study uses data on a real landscape. The first study (Paper 1) evaluates how the locations and lengths of least-cost paths (the latter of which are referred to as least-cost distances) vary with change in cost estimation parameters. This is done through a series of computational experiments, in which each of the artificial landscapes is converted into different cost surfaces by systematically varying parameters of a cost-estimation function, on which least-cost paths are generated. The locations and lengths of those paths are statistically analyzed to find sources of their variation. The second study (Paper 2) investigates how the least-cost distance is affected by the spatial resolution of the corresponding cost surface. This is also done through a series of computational experiments, in which each of the artificial landscapes is converted into a cost surface, which is, in turn, converted into different cost surfaces (different, i.e., only in their spatial resolutions) by systematically aggregating grid cells. Then, the statistical behavior of the ratio of the least-cost distance measured on a lower-resolution cost surface to that measured on a higher-resolution cost surface is analyzed. The third study (Paper 3) proposes the mini-max path model as an alternative to the least-cost path model. Unlike the conventional model (in which the optimality of a path is based on the sum of its length multiplied by the underlying cost values), the alternative model determines the optimality of a path using the length of a segment(s) of the path that intersects the cells having the maximum cost value (with a special tie-breaking rule). The performances of the two models are tested in one of the following two assumptions at a time: the cost values are measured on an ordinal scale or on a ratio scale. The fourth study (Paper 4) applies the model proposed in the third study to an ongoing conservation project of the Dian Fossey Gorilla Fund International that plans to design a ‘stepping-stone’ corridor—which is not fully connected but takes the form of a sequence of fragmented forest patches—between two core habitat areas of pollinator birds between two protected areas in Rwanda. The project does not have complete information on the study area and the target species and thus the project staff can only rank land cover types in terms of their suitability/cost for being part of the corridor. The utility of the model is tested with different assumptions on the behavior of the birds (e.g., minimum stepping stone size) as well as on the cost associated with the implementation of the corridor (e.g., cost for planting shrubs along the corridor to encourage the birds to use it).The first study finds that the same pair of terminal cells may well be connected by different least-cost paths on different cost surfaces though derived from the same landscape data. The variation among those paths is highly sensitive to the forms of spatial and non-spatial distributions of landscape elements (which cannot be controlled by users of the least-cost path model) as well as by those of cost values derived from them (which may be, at least indirectly, controlled by users of the model). The second study finds that least-cost distances measured on lower-resolution cost surfaces are generally highly correlated with—and useful predictors of—effective distances measured on higher-resolution cost surfaces. This relationship tends to be weakened when linear barriers to connectivity (e.g., roads and rivers) exist, but strengthened as distances increase and/or when linear barriers (if any) are detected by other presumably more accessible and affordable sources such as vector line data. The third study confirms the effectiveness of the conventional least-cost path model on ratio-scaled cost surfaces but finds that the alternative mini-max path model is mathematically sounder if the cost values are measured on an ordinal scale and practically useful if the problem is concerned not with the minimization of cost but with the maximization of some desirable condition such as suitability. The fourth study demonstrates the utility of the mini-max path model by effectively casting the stepping stone corridor problem as a special case of it. The model allows for a rapid first delineation of candidate routes for stepping stone corridors and facilitates the early exploratory stages of conservation projects.Major implications of this thesis to the research and practice in landscape connectivity analysis and conservation corridor planning with raster-based GIS are summarized as follows.When sufficient information is available for quantification of cost values, the conventional least-cost path model is a reasonable approach to use.However, it is worth trying or at least acknowledging alternatives that do not rely on the quantitative-cost assumption if the value of each cell only indicates the ordinal category of cost of intersecting that cell. Note in particular that information used for cost estimation in practice (e.g., expert opinions or public surveys) are often of subjective and qualitative nature.The highest-resolution data may not always be most effective—much less, most cost-effective—for the task being undertaken. The choice of spatial resolution of th
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| 2. |
- Nickman, Alireza
(författare)
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Road disasters? Modeling and assessment of Swedish roads within crucial climate conditions
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An efficient maintenance of roads to ensure high accessibility and durability of the transport capacity requires an understanding of how the hydrological response depends on both the road and the landscape characteristics. New methods and data were used to identify and explain interaction between roads and surrounding environment and their influence on hydrologic responses both in watershed scale and road-section scale. In the watershed scale, flood hazard probability was made with reference to the most influential physical catchment descriptors and road characteristics. Additionally, a physical based model was used to estimate the effect of road topography on the hydrological responses of 20 watersheds to storms with different intensities. A simple method was developed and discussed to address flood risk probability in the road-stream crossings concerning the correlation between the quantities of the physical catchment descriptors and occurrence/absence of flooding. The most influential factors in describing the probability of flooding along the roads were topographic wetness index, soil properties, road density and channel slopes. A detailed study of simulated flow duration curves showed differences between the 20 watersheds for three different storms based on topography with and without roads. An increase in peak flow and reduced time to pick occurred with existence of roads and increased storm intensity.In the road-section scale, an uncertainty-based simulation approach was used to identify the most influencing processes in controlling the dynamics of the groundwater level. A model (CoupModel) set up with four different geological stratifications was made to model two positions in a slope upstream of a road with drainage pipes and ditches. Results from the simulations indicate the significance of precipitation rate, road drainage and position in hillslope, and soil properties and stratifications in controlling groundwater levels. The same model was also applied to simulate soil moisture and temperature dynamics in two road sections by using groundwater and climate data. Porous media properties were obtained as statistical distribution function that provided the best performance of moisture and temperature dynamic in the road layers and underlying soil.
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| 3. |
- Acuña, José, 1982-
(författare)
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Distributed thermal response tests : New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
- 2013
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- U-pipe Borehole Heat Exchangers (BHE) are widely used today in ground source heating and cooling systems in spite of their less than optimal performance. This thesis provides a better understanding on the function of U-pipe BHEs and Investigates alternative methods to reduce the temperature difference between the circulating fluid and the borehole wall, including one thermosyphon and three different types of coaxial BHEs.Field tests are performed using distributed temperature measurements along U-pipe and coaxial heat exchangers installed in groundwater filled boreholes. The measurements are carried out during heat injection thermal response tests and during short heat extraction periods using heat pumps. Temperatures are measured inside the secondary fluid path, in the groundwater, and at the borehole wall. These type of temperature measurements were until now missing.A new method for testing borehole heat exchangers, Distributed Thermal Response Test (DTRT), has been proposed and demonstrated in U-pipe, pipe-in-pipe, and multi-pipe BHE designs. The method allows the quantification of the BHE performance at a local level.The operation of a U-pipe thermosyphon BHE consisting of an insulated down-comer and a larger riser pipe using CO2 as a secondary fluid has been demonstrated in a groundwater filled borehole, 70 m deep. It was found that the CO2 may be sub-cooled at the bottom and that it flows upwards through the riser in liquid state until about 30 m depth, where it starts to evaporate.Various power levels and different volumetric flow rates have been imposed to the tested BHEs and used to calculate local ground thermal conductivities and thermal resistances. The local ground thermal conductivities, preferably evaluated at thermal recovery conditions during DTRTs, were found to vary with depth. Local and effective borehole thermal resistances in most heat exchangers have been calculated, and their differences have been discussed in an effort to suggest better methods for interpretation of data from field tests.Large thermal shunt flow between down- and up-going flow channels was identified in all heat exchanger types, particularly at low volumetric flow rates, except in a multi-pipe BHE having an insulated central pipe where the thermal contact between down- and up-coming fluid was almost eliminated.At relatively high volumetric flow rates, U-pipe BHEs show a nearly even distribution of the heat transfer between the ground and the secondary fluid along the depth. The same applies to all coaxial BHEs as long as the flow travels downwards through the central pipe. In the opposite flow direction, an uneven power distribution was measured in multi-chamber and multi-pipe BHEs.Pipe-in-pipe and multi-pipe coaxial heat exchangers show significantly lower local borehole resistances than U-pipes, ranging in average between 0.015 and 0.040 Km/W. These heat exchangers can significantly decrease the temperature difference between the secondary fluid and the ground and may allow the use of plain water as secondary fluid, an alternative to typical antifreeze aqueous solutions. The latter was demonstrated in a pipe-in-pipe BHE having an effective resistance of about 0.030 Km/W.Forced convection in the groundwater achieved by injecting nitrogen bubbles was found to reduce the local thermal resistance in U-pipe BHEs by about 30% during heat injection conditions. The temperatures inside the groundwater are homogenized while injecting the N2, and no radial temperature gradients are then identified. The fluid to groundwater thermal resistance during forced convection was measured to be 0.036 Km/W. This resistance varied between this value and 0.072 Km/W during natural convection conditions in the groundwater, being highest during heat pump operation at temperatures close to the water density maximum.
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| 4. |
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| 5. |
- Nickman, Alireza
(författare)
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Modeling moisture and temperature dynamics in road structure during winter conditions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Excess moisture significantly impact durability and sustainability of road components especially in cold regions. To improve understanding of moisture dynamics in roads with drainage system, hourly measured moisture content, soil temperature and groundwater level data during a 3-year period from a test site in Växjö, Sothern Sweden were utilized. Seasonal and manipulated changes in the groundwater level and moisture dynamics were observed that describe extreme conditions providing extra moisture to the upper layers of road. CoupModel was used to estimate mass and heat balance in four depths of two spots of the road section. Good performances of the model were achieved in calculation of the soil moisture in different depths. Soil water retention curve parameters were sensitive to constrain criteria which demonstrate importance of soil texture in controlling moisture dynamics. Model could properly capture temperature dynamic during winter time but simulated excess evaporation from soil layers reduced performance of model in the estimation of temperature during summer The combined monitoring and modelling of physical conditions in the road structure will be highly relevant to help decision makers and road engineers to avoid moisture in road structures and to also identify crucial events from meteorological data.
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| 6. |
- Nickman, Alireza
(författare)
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Uncertainty based approach to simulate groundwater levels in a hillslope upstream of a road
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Knowledge of the processes governing groundwater dynamics in the areas close to roads is important for sustainable road infrastructure in the face of a changing climate. This study established an uncertainty based approach to simulate groundwater oscillations in a hillslope upstream of a road by using a process based model. Four different soil configurations were analyzed in CoupModel to simulate groundwater dynamics in presence of a road drainage system by implementation of meteorological data as driving factors. An approach similar to GLUE method was applied to analyze the statistical performance of the simulated groundwater level versus high resolution measured groundwater level dynamics. Results of the simulations indicate deviations in simulated results due to different soil stratifications. Uncertainties resulted from the lack of precise information about the geological structure of the site are important contributors to deviated simulation results. Different scenarios showed different model performances in which a simpler soil profile describes better the groundwater dynamics when it is closer to the road drainage system while a more complicated soil profile better describes groundwater dynamics in undisturbed soils. Correlation between hydraulic conductivity of each layer and the model performance was discussed. The results also indicate significance of variables such as physical drainage characteristics of the road in governing level of saturations also the position of the road structure in a hillslope. Texture (hydraulic conductivity) of the soil layers that fluctuation of groundwater occurs in those layers and types of modifications that have been done due to road construction are important driving factors. These factors are suggested as suitable indicators for designing an early warning system based on physical characteristics of a road site.
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| 7. |
- Nickman, Alireza
(författare)
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Uncertainty based approach to simulate groundwater levels in a hillslope upstream of a road
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Knowledge of the processes governing groundwater dynamics in the areas close to roads is important for sustainable road infrastructure in the face of a changing climate. This study established an uncertainty based approach to simulate groundwater oscillations in a hillslope upstream of a road by using a process based model. Four different soil configurations were analyzed in CoupModel to simulate groundwater dynamics in presence of a road drainage system by implementation of meteorological data as driving factors. An approach similar to GLUE method was applied to analyze the statistical performance of the simulated groundwater level versus high resolution measured groundwater level dynamics. Results of the simulations indicate deviations in simulated results due to different soil stratifications. Uncertainties resulted from the lack of precise information about the geological structure of the site are important contributors to deviated simulation results. Different scenarios showed different model performances in which a simpler soil profile describes better the groundwater dynamics when it is closer to the road drainage system while a more complicated soil profile better describes groundwater dynamics in undisturbed soils. Correlation between hydraulic conductivity of each layer and the model performance was discussed. The results also indicate significance of variables such as physical drainage characteristics of the road in governing level of saturations also the position of the road structure in a hillslope. Texture (hydraulic conductivity) of the soil layers that fluctuation of groundwater occurs in those layers and types of modifications that have been done due to road construction are important driving factors. These factors are suggested as suitable indicators for designing an early warning system based on physical characteristics of a road site.
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| 8. |
- Paul, Seema, et al.
(författare)
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Hydrodynamic wind induce model influencing inner Murchison Bay flow circulation
- 2019
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Wind stress is exerted by the wind on the lake surface may be important for lake flow analysis and the Inner Murchison Bay is strongly influenced by urban pollution from Uganda Capital Kampala and bay is highly eutrophic cause of back and forth nutrient movement. The rapid population growth, growing commercial activities and industrialization in Kampala, Uganda coupled with inadequate provision of waste management services which have led to increase volume of urban waste entering the North-Western part of Lake Victoria. The Murchison Bay (MB) is in the Northern part of Lake Victoria in Uganda which has for decades received a daily wastewater load of 0.2 % of its volume from Kampala City of Uganda, through the Nakivubo channel [1]. Kampala City people habitants 1.7 million. The uncontrolled solid waste seen along roadsides and storm water drains enter the Nakivubo Channel. The Nakivubo channel is ended in a papyrus swamp in Uganda that retained a large portion of water pollution before it reached in the inner Murchison Bay [2]. In addition, the channel is widened in 2001-2003 to improve the city drainage which has increased the potential loading of nutrients to the bay. The partially treated effluent from treatment plants are mixed with the water in channel and it contribute a high significant pollution load, and is the most polluted system by organic matter in solid waste and wastewater discharged from slums and un-sewered areas, Luzira Prison and small & large scale industries [3]. There are urgent need to resolve the problem. Nowadays, the inner Murchison Bay (MB) water quality are destroying by several complex mixture processes, Ex.: pollution and nutrients loading, river inflows, un-sewered water, wetland management and flora and fauna populations. Present study is focusing on inner Murchison Bay (MB) water pollution behaviour by using wind speed hydrodynamic model. The model processes are determining the fate and transport of pollutant that are vertically mixing (wind force) and horizontal flow (advection-dispersion) analysis. Vertically transport of wind flow that is forcing on the lake water surface are analysed by the vertically integrated Shallow Water Equation model. Horizontally transport of pollution of water and its flow and speed are analyzing by advectiondispersion and diffusion model. Result shows that the horizontal mixing is continuing with lake nutrients and the vertically wind flow from MB into lake is also determine the mixing of lake water which is not much affecting the lake nutrients.
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| 9. |
- Paul, Seema
(författare)
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Lake Hydrodynamics and Pollution Transport under Climate Change : The Case of Lake Victoria
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A very small part of the total earth’s water is freshwater (only 2.5 %). Unfortunately, due to climate change and pervasive manmade activities, surface freshwater quality in many places of the world has become degraded. This is manifested in the Rift Valley lakes, a series of lakes in Eastern Africa that runs from Ethiopia in the north to Malawi in the south. Water quality degradation in the Rift Valley lakes is driven by various factors, including water quantity and scarcity, pollution and contamination, nutrients loading, and general water use by industry and society. In particular, Lake Victoria, the world’s second-largest freshwater body and the largest tropical lake, has seriously polluted near lakeshore areas, which is a great regional development problem causing misfortune for millions of people.This dissertation contributes new insights into lake hydrodynamic processes and pollution transport in shallow lakes through developing more accurate models to understand the complex processes of water quality degradation. Based on empirical data this thesis developed systematic methods to consider lake bathymetry, lake flow, water level verification, water balance, hydro-climatological processes, transport and dispersion of pollutants and nutrient particles. The data-driven hydrological model of Lake Victoria that is developed in the thesis considers hydro-meteorological and climatological data, river discharges and outflow, wind speed and direction, atmospheric deposition, nutrient loading, concentration of pollutants and nutrients, and remote sensing satellite data. The thesis illustrates the power of numerical and hydrodynamic methods that uses one- and two-dimensional mathematical equations (1D and 2D) to model the three-dimensional (3D) behaviour of shallow lakes over time. The results indicate that the lake hydrodynamics of Lake Victoria are heavily influenced by lake bathymetry and regional weather patterns and are thus connected to increasing climate variation. The hydro-meteorological processes, verified by empirical data on precipitation, lake flow and lake water levels, show that extreme weather events are responsible for changing the characteristics of lake water balance, changing seasonal variations, and exhibiting strong correlations among water level and hydro-meteorological data. The model of the movement of pollutants and nutrient particles shows how pollutants and nutrients travel within Lake Victoria and where they concentrate in the lake and its sediments. The wind hydrodynamic modelling shows that the wind, along with hydrodynamic stability, plays an important role in pollution flow patterns and that pollutants can be transported from shallow parts, when they leave rivers and shorelines, to deeper lake areas. The hydro-climatological model demonstrates the crucial interdependence between hydrodynamic processes and climatological factors at the catchment scale of Lake Victoria. The numerical models and calculation methods that have been developed in this dissertation represent additional contributions to hydrodynamic research and can be used to investigate hydrodynamic processes in other lakes. The thesis contributes to UN Sustainable Development Goals related to water security, drinking water, food, and health. A potential area of application lies in supporting analysis and mitigation of pollution and climate change effects and more generally aid in the natural resource governance of this vital African lake.
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| 10. |
- Zou, Liangchao, 1987-, et al.
(författare)
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Impact of normal stress-induced closure on laboratory-scale solute transport in a natural rock fracture
- 2020
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Ingår i: Journal of Rock Mechanics and Geotechnical Engineering. - : Chinese Academy of Sciences. - 1674-7755.
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Tidskriftsartikel (refereegranskat)abstract
- The impact of normal stress-induced closure on fluid flow and solute transport in a single rock fracture is demonstrated in this study. The fracture is created from a measured surface of a granite rock sample. The Bandis model is used to calculate the fracture closure due to normal stress, and the fluid flow is simulated by solving the Reynold equation. The Lagrangian particle tracking method is applied to modeling the advective transport in the fracture. The results show that the normal stress significantly affects fluid flow and solute transport in rock fractures. It causes fracture closure and creates asperity contact areas, which significantly reduces the effective hydraulic aperture and enhances flow channeling. Consequently, the reduced aperture and enhanced channeling affect travel time distributions. In particular, the enhanced channeling results in enhanced first arriving and tailing behaviors for solute transport. The fracture normal stiffness correlates linearly with the 5th and 95th percentiles of the normalized travel time. The finding from this study may help to better understand the stress-dependent solute transport processes in natural rock fractures.
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