The process of evaporation interacts with the soil, which has various comprehensive mechanisms. Multiphase flow models solve air, vapour, water, and heat transport equations to simulate non-isothermal soil moisture transport of both liquid water and vapor flow, but are only applied in non-vegetated soils. For (sparsely) vegetated soils often energy balance models are used, however these lack the detailed information on non-isothermal soil moisture transport. In this study we coupled a multiphase flow model with a two-layer energy balance model to study the impact of non-isothermal soil moisture transport on evaporation fluxes (i.e., interception, transpiration, and soil evaporation) for vegetated soils. The proposed model was implemented at an experimental agricultural site in Florida, US, covering an entire maize-growing season (67 days). As the crops grew, transpiration and interception became gradually dominated, while the fraction of soil evaporation dropped from 100% to less than 20%. The mechanisms of soil evaporation vary depending on the soil moisture content. After precipitation the soil moisture content increased, exfiltration of the liquid water flow could transport sufficient water to sustain evaporation from soil, and the soil vapor transport was not significant. However, after a sufficient dry-down period, the soil moisture content significantly reduced, and the soil vapour flow significantly contributed to the upward moisture transport in topmost soil. A sensitivity analysis found that the simulations of moisture content and temperature at the soil surface varied substantially when including the advective (i.e., advection and mechanical dispersion) vapour transport in simulation, including the mechanism of advective vapour transport decreased soil evaporation rate under wet condition, while vice versa under dry condition. The results showed that the formulation of advective soil vapor transport in a soil-vegetation-atmosphere transfer continuum can affect the simulated evaporation fluxes, especially under dry condition.

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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.

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The topic of this thesis is the quantification of the influence of preferential flow on landslide-triggering in potentially unstable slopes. Preferential flow paths (e.g., cracks, macropores, fissures, pipes, etc.) commonly exists in slopes. Flow velocities in preferential flow paths can be significantly larger than in the matrix. Under large rainfall or snow-melt events, preferential flow can bypass the adjacent soil matrix and directly reach the groundwater table. The fast pressure build-up caused by preferential flow can reduce the effective stress and shear strength, which is an important triggering factor for landslides. Single-permeability models can not appropriately simulate preferential flow. Hence, hydro-mechanical models of landslide need the inclusion of preferential flow. Preferential flow also affects tracer transport in subsurface flow systems. The celerity in unsaturated flow represents the maximum water velocity in a soil, and it may be used to predict the first arrival time of a conservative tracer. The celerity function is derived from the soil hydraulic conductivity function for unsaturated flow, and is used to derive the breakthrough curve of a conservative tracer under advective transport. Analysis of the bimodal hydraulic function for a dual-permeability model shows that different parameter sets may result in similar soil hydraulic conductivity behavior, but distinctly different celerity behavior. In Chapter 4, a 2D hydro-mechanical model is developed using COMSOL multi-physics modeling software to couple a dual-permeability model with a linear-elastic model. Numerical experiments are conducted for two different rainfall events on a synthetic slope. The influence of preferential flow on slope stability is quantified by comparing the simulated slope failure area for single-permeability model and dual-permeability models. The single-permeability model only simulate regular wetting fronts propagating downward without representing the preferential flow. In contrast, the dual-permeability model can simulate the influence of preferential flow including the enhanced drainage that facilities pressure dissipation under low-intensity rainfall, as well as the fast pressure build-up that may trigger landslides under high-intensity rainfall. The dual-permeability model resulted in a smaller failure area than the corresponding single-permeability model under low-intensity rainfall, while the dual-permeability model resulted in a larger failure area and earlier timing than the corresponding single-permeability model for high-intensity rainfall. In Chapter 5, a parsimonious 1D hydro-mechanical model is developed for field application by coupling a 1D dual-permeability model with an infinite slope stability analysis approach. The numerical model is benchmarked against the HYDRUS-1D for the simulation of non-equilibrium flow. In Chapter 6, the model is applied to simulate the pressure response in a clay-shales slope located in northern Italy. In the study area, preferential flow paths such as tension cracks and macropores are widespread. Intense rain-pulses in the summer can cause nearly-instant pressure responses which may reactivate landslide movement. The water exchange coefficient of the dual-permeability model is calibrated for two single-pulse rainfall-events in the summer, while all other parameters are obtained from field investigations. Results from the dual-permeability model are compared to previously published outcomes using a linear-diffusion equation, where the diffusion coefficient was calibrated for each rainfall event separately. The dual-permeability model explicitly accounts for the influence of both matrix flow and preferential flow on water flow and pressure propagation in variably saturated soils, and is able to simulate the measured pressure response to multi-pulse rainfall-events quite well even in the winter time. Results indicate that the dual-permeability model may be more appropriate for the prediction of landslide-triggering when the pore water pressure response is influenced by preferential flow under high-intensity rainfall. @en

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.

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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.@en