Effects of Two-Phase Flow of Water and Air on Shallow Slope Failures Induced by Rainfall: Insights from Slope Stability Assessment at a Regional Scale
2. Materials and Methods
2.1. Study Area
2.2.1. Landslide Inventories and Spatial Data
2.2.2. Material Properties
2.3.1. Rainfall Infiltration Analysis
2.3.2. Slope Stability Analysis
2.3.3. Slope Geometry and Boundary/Initial Conditions
2.3.4. Performance Evaluation of Slope Stability Assessment
3. Results and Discussion
3.1. Validation with Comparison to Field Measurements
3.2. Infiltration Analysis
3.3. Slope Stability Assessment
- Considering that air flow changes the rate of increase in Pw on slopes with a low infiltration capacity when ponding occurs during heavy rainfall, and that the saturated hydraulic conductivity is relatively marginal in the mountainous areas of Korea (i.e., less than 2.78 × 10−5 m/s according to the National Disaster Management Institute ), it is necessary to apply the two-phase flow model to accurately interpret rainfall infiltration.
- The initial safety factor prior to rainfall depends on the shear strength parameters (i.e., internal friction angle and cohesion); however, variations in the safety factor are strongly dependent on an increase in the Pw rate. Slopes in the single-phase flow model rapidly become saturated during heavy rainfall because of the high rainfall infiltration rates without the interruption of air. Thus, safety factors decrease rapidly compared to those in the two-phase flow model.
- Landslide susceptibility maps change depending on an increase in Pw, which depends on relative permeability. It is also sensitive to the model type. The MSR and confusion matrix yield the highest performance for the two-phase flow model with an appropriate range of stable cell coverage for the best simulation. Thus, it is concluded that infiltration and slope stability analyses using the two-phase flow model have good applicability in evaluating landslide events in Umyeon Mountain. However, it is necessary to additionally evaluate the applicability of this model for other landslide cases.
- We performed infiltration and slope stability analyses focusing on the geotechnical characteristics of unsaturated soils in the upper layer. Besides, we excluded geological characteristics given that most of the slope failure in the study area occurred at the colluvium layer with shallow depths up to 2 m. The two-phase flow model can be usefully applied to the region where shallow slope failure occurs primarily. However, further study is required to examine the effects of the geological structure for improving the applicability of the two-phase flow model to the region where deep slope failure occurs primarily, which is affected not only by geotechnical characteristics but also by geological structure.
- The performance of slope stability assessments at a regional scale is greatly affected by the uncertainty and variability of geotechnical and hydrological input parameters when physically based models are applied. Geotechnical and hydrological properties have probabilistically or statistically been characterized based on types of soil and lithology to properly consider the uncertainty and variability of them [58,59], whereas we used representative constant soil properties at each of the five zones as averaged from field measurements. The applicability of the two-phase flow model to a regional scale can be improved by further study applying the probabilistic approach for characterizing the uncertainty and variability of geotechnical and hydrological input parameters.
Conflicts of Interest
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|Zone No.||(°)||c (kPa)||ks (m/s)||Van Genuchten SWCC Coefficient|
|1||25.3||9.6||18.1||15||7.15 × 10−6||0.051||0.35|
|2||28.5||5.8||17.7||14.9||3.87 × 10−6||0.051||0.35|
|3||37.6||7.7||17||14||1.80 × 10−6||0.038||0.63|
|4||30.9||7.6||17.3||14.5||9.70 × 10−6||0.038||0.63|
|5||28.2||6.3||18.2||15||3.69 × 10−6||0.038||0.63|
|Water density,||1000 kg/m3|
|Air density,||1.25 kg/m3|
|Bulk modulus of water, Kw||2 × 109 Pa|
|Bulk modulus of air, Ka||1 × 105 Pa|
(Total Cells: 63,157)
Actual (Observed) Class
Actual (Observed) Class
|Predicted class||Positive||True positive 148 (0.23%)||False positive 12,498 (19.79%)||True positive 150 (0.24%)||False positive 15,913 (25.2%)|
|Negative||False negative 13 (0.02%)||True negative 50,498 (79.96%)||False negative 11 (0.02%)||True negative 47,083 (74.55%)|
(= (TP + TN)/N)
|Positive predictive power|
(= TP/(TP + FP))
(= TP/(TP + FN))
(= TN/(FP + TN))
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Kang, S.; Cho, S.-E.; Kim, B.; Go, G.-H. Effects of Two-Phase Flow of Water and Air on Shallow Slope Failures Induced by Rainfall: Insights from Slope Stability Assessment at a Regional Scale. Water 2020, 12, 812. https://doi.org/10.3390/w12030812
Kang S, Cho S-E, Kim B, Go G-H. Effects of Two-Phase Flow of Water and Air on Shallow Slope Failures Induced by Rainfall: Insights from Slope Stability Assessment at a Regional Scale. Water. 2020; 12(3):812. https://doi.org/10.3390/w12030812Chicago/Turabian Style
Kang, Sinhang, Sung-Eun Cho, Byungmin Kim, and Gyu-Hyun Go. 2020. "Effects of Two-Phase Flow of Water and Air on Shallow Slope Failures Induced by Rainfall: Insights from Slope Stability Assessment at a Regional Scale" Water 12, no. 3: 812. https://doi.org/10.3390/w12030812