**4. Conclusions**

The initial conditions such as the saturation degree and the groundwater table are the first important factors for riverbank slope stability in the rainy season. Setting the initial conditions leads to different mechanisms of infiltration, seepage, and changes of groundwater pressure.

In highly saturated conditions, the FOS decreases with the increase in rainfall intensity and accumulation. During a rainfall event, the rainwater infiltrates and affects the riverbank stability through two processes—changes in pore-water pressure and a wetting front controlled by the rainfall intensity and the hydraulic conductivity.

When the rainfall intensity is lower than soil hydraulic conductivity, the rainwater infiltrates, and transient seepage occurs through the unsaturated area, causing the groundwater table to rise. In soil with a higher hydraulic conductivity and rainfall intensity, the groundwater table rises quickly, leading to a higher potential of riverbank failure. Riverbank failure often occurs under high-intensity rainfall.

When the rainfall intensity is slightly higher than the soil hydraulic conductivity, the groundwater table rises slowly, and the FOS decreases slowly in the early period after raining. With a pond on the surface, the wetting front appears when there is excess rainfall water. The FOS decreases more quickly by the development of both the wetting front and the groundwater table convergence.

When the rainfall intensity is much higher than the soil hydraulic conductivity, the groundwater does not change in a short time. The wetting front descends slowly, and the height of the pond increases quickly with higher rainfall intensity. The wetting front and pond loading area are the main factors causing FOS changes.

In cases with no pond, the wetting front is on a shallow surface and descends very slowly, and the rainfall water transient seepage occurs slowly. In a short time, the groundwater table and the wetting front do not significantly increase; thus, the FOS slightly decreases. The decreasing of FOS is due to transient seepage changes of unsaturated soil properties as well as losing soil suction.

During long-term events with low-intensity rainfall, i.e., less than 10 mm/h, the FOS primarily depends on the river water level. A riverbank with a high slope angle and cracks with high hydraulic conductivity will have a higher potential of riverbank failure.

**Author Contributions:** Conceptualization, T.T.D. and D.M.D.; Methodology, T.T.D.; Software, T.T.D.; Validation, T.T.D., D.M.D. and K.Y.; Formal Analysis, T.T.D.; Investigation, T.T.D., D.M.D.; Resources, T.T.D.; Data Curation, T.T.D.; Writing-Original Draft Preparation, T.T.D.; Writing-Review & Editing, T.T.D.; Visualization, T.T.D.; Supervision, K.Y.; Project Administration, T.T.D.; Funding Acquisition, T.T.D.

**Funding:** This research was funded by the project Code 105.08-2015.24, which was sponsored by Nafosted, Ministry of Science and Technology, Vietnam.

**Acknowledgments:** This paper was completed with the support of the project Code 105.08-2015.24, which was sponsored by Nafosted, Ministry of Science and Technology, Vietnam, and the support by Geotechnical Laboratory, Ibaraki University, Japan to determine unsaturated soil properties. The authors express our sincere gratitude for these supports.

**Conflicts of Interest:** The authors declare no conflict of interest.
