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Assessing the Effects of Rainfall Intensity and Hydraulic Conductivity on Riverbank Stability

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Assessing the Effects of Rainfall Intensity and Hydraulic Conductivity on Riverbank Stability water Article Assessing the Effects of Rainfall Intensity and Hydraulic Conductivity on Riverbank Stability Toan Thi Duong 1,*, Duc Minh Do 1 and Kazuya Yasuhara 2 1 Department of Geotechnics and Infrastructure Development, VNU University of Science, Vietnam National University, Hanoi 100000, Vietnam; [email protected] 2 Institute for Global Change Adaptation Scienceicas (ICAS), Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan; [email protected] * Correspondence: [email protected]; Tel.: +84-093-454-3261 Received: 15 March 2019; Accepted: 3 April 2019; Published: 10 April 2019 Abstract: Riverbank failure often occurs in the rainy season, with effects from some main processes such as rainfall infiltration, the fluctuation of the river water level and groundwater table, and the deformation of transient seepage. This paper has the objective of clarifying the effects of soil hydraulic conductivity and rainfall intensity on riverbank stability using numerical analysis with the GeoSlope program. The initial saturation condition is first indicated as the main factor affecting riverbank stability. Analyzing high-saturation conditions, the obtained result can be used to build an understanding of the mechanics of riverbank stability and the effect of both the rainfall intensity and soil hydraulic conductivity. Firstly, the rainfall intensity is lower than the soil hydraulic conductivity; the factor of safety (FOS) reduces with changes in the groundwater table, which is a result of rainwater infiltration and unsteady state flow through the unsaturated soil. Secondly, the rainfall intensity is slightly higher than the soil hydraulic conductivity, the groundwater table rises slowly, and the FOS decreases with both changes in the wetting front and groundwater table. Thirdly, the rainfall intensity is much higher than the soil hydraulic conductivity, and the FOS decreases dominantly by the wetting front and pond loading area. Finally, in cases with no pond, the FOS reduces when the rainfall intensity is lower than hydraulic conductivity. With low hydraulic conductivity, the wetting front is on a shallow surface and descends very slowly. The decreasing of FOS is only due to transient seepage changes of the unsaturated soil properties by losing soil suction and shear strength. These obtained results not only build a clearer understanding of the filtration mechanics but also provide a helpful reference for riverbank protection. Keywords: riverbank stability; rainfall intensity; hydraulic conductivity 1. Introduction Rainfall is one of the main factors causing slope failure in tropical areas. The effects of rainfall properties on slopes have been studied in large amounts of research that have analyzed slope stability by both investigation and simulated models. By the simulation of different conditions of slopes and rainfall boundaries, previous research has indicated the changes of soil unsaturated properties, such as the reduction of suction, shear strength, and the increasing of hydraulic conductivity and pore-water pressure [1–20]. Finally, these changes cause slope failure. Before rain, the slope area above the groundwater table is considered as being in a partly unsaturated and dry state near the surface [1–4]. The unsaturated area reduces by rainfall water infiltration during and after the rainfall event. The change of the area from unsaturated to saturated is caused by the advancement of the wetting front from the surface [4,5,9,13,21–23] and groundwater table from depth [6–8,10–20]. Those processes were found to be the primary factors controlling the instability of slopes due to Water 2019, 11, 741; doi:10.3390/w11040741 www.mdpi.com/journal/water Water 2019, 11, 741 2 of 16 rainfall and were greatly affected by rainfall intensity (RI) and soil properties, especially by unsaturated soil hydraulic conductivity [4,17–36]. The unsaturated soil hydraulic conductivity (HC) controls the transient seepage, the depth of rainfall infiltration, the changes in pore pressure during the rainfall event, and finally, affects the FOS. The effects of soil hydraulic conductivity on slope stability in the rainy season are usually assessed from the point of view of three topics: (1) the effects of changes in the value of soil hydraulic conductivity and rainfall intensity [17–30]; (2) the soil anisotropic HC and hydraulic hysteresis [31–35]; (3) the failure delay phenomenon due to the defense of HC and RI [36]. This research focuses on the overview and analysis of the first of these topics. The effects of the HC on the FOS were specifically simulated by [25,26]. Those research works reported that high hydraulic conductivity led to rapid saturation. Additionally, infiltration causes the wetting front to quickly shift downward. This shift causes water to contact the underlying impermeable soil, leading to a rapid rise in pore-water pressure and the formation of a perched water table. The slope reaches full saturation and experiences a reduction in soil resistance. Consequently, the factor of safety rapidly decreases [26]. High-intensity rainfall has large effects on the slope if the soil slope has high hydraulic conductivity (Ks ≥ 10−4 m/s) and when the slope has poorly drained soils (i.e., Ks ≤ 10−6 m/s) [25]. The simulated analyses were also carried out with different boundaries of rainfall. When building the relationship of rainfall intensity and unsaturated hydraulic conductivity by the ratio of RI/HC, the previous research analyzed the RI = HC [5], RI ≥ HC [19,22]; and RI < HC [8,17,21,23,26,29] cases. Setting up a boundary with RI = HC or RI ≥ HC and no pond [5,19,22], the development of a wetting front from the crest of the slope and the reduction of soil suction during rainfall were found to dominantly affect the slope stability. The development of the defense of the wetting front under increased rainfall intensity shortens the time required for the wetting front to reach the pore-water pressure and moisture content sensors. The growing of groundwater has not been mentioned much. The same trend was also found in [17,21,23], which had the boundary of RI < HC. The larger the coefficient of permeability is, the greater the depth of the wetting front is. The opposite mechanism is seen in [8,29] with a boundary of small RI and high initial saturation and hydraulic conductivity, and the pore-water pressure increases gradually from the deep part to the crest of the slope [8,29]. For a slope with a larger HC, the slope failures possibly take place under rainfall with a shorter duration and a greater intensity [8]. Those research works also concluded that when the rainfall intensity is greater than the saturated hydraulic conductivity of the surface soils, a runoff occurs along the slope surface [8,22]. Although those previous research works had unobvious objectives focusing on building the effects of hydraulic conductivity, the results clearly showed that the slope stability was significantly affected by the soil hydraulic conductivity and the rainfall intensity boundary. The ratios of RI/HC were divided into three cases; however, each previous research work only concentrated on one of those cases. Moreover, there was a difference regarding the development of the pore-water pressure by the rising groundwater table [8,29] or by a wetting front from the surface [21,23] when those analyses had the same initial conditions. Further research should cover all three cases of different RI/HC ratios. A riverbank is a special slope that relates closely to hydraulic dynamics, not only from rainfall infiltration from the surface concerning the space above the slope, but also significantly from the fluctuation of the river water level and groundwater table [27,37–43]. Past research works assessed the effects of rainfall on riverbank stability using the indirect process of transient seepage due to water level changes and assumed that rainfall has no effect on the riverbank surface. By reviewing previous papers, it was obvious that the hydraulic conductivity has great effects on slope stability during rainfall events. However, some limitations were found in the reviewed research. The effects of the different RI/HC values were not specific to a researcher, and there was a difference in the previous discussion on seepage mechanics, such as the changes in the wetting front and the groundwater table. Moreover, research on the effects of hydraulic conductivity on riverbank slope stability was performed in only a few papers. Therefore, the objective of this paper is to build Water 2019, 11, 741 3 of 16 Water 2019, 11, x FOR PEER REVIEW 3 of 16 the mechanics of riverbank failure with different rainfall infiltration and soil hydraulic conductivity models.models. The The numerical numerical analyses analyses by by the the GeoSlope GeoSlope program with both SEEP/WSEEP/W and and SLOPE/W SLOPE/W moduli moduli areare applied applied in in this this study. study. 2. Materials and Methods 2. Materials and Methods The case study is a riverbank area in the Red River in Hanoi city, Vietnam. The Red River The case study is a riverbank area in the Red River in Hanoi city, Vietnam. The Red River originatesoriginates in Yun-nan, China,China, and and flows flows through throug northwesth northwest Vietnam. Vietnam. The The total total length length of the of Red the RiverRed Riverin Hanoi in Hanoi is about is about 90 km 90 with km with about about 40 km 40 ofkm riverbank of riverbank inside inside the the urban urban area area of Hanoi,of Hanoi, where where the thepopulation population density density and and density density of of houses houses built built near near the the natural natural riverbankriverbank areare highest. These These areas areas areare located located in in high high river river terraces, terraces, outside outside the the river river dike, dike, and and near near natural natural banks.
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