Numerous annual slope failures are induced by heavy rainfall during the monsoons, especially in developing countries in Asia. The authors have developed a simple method to predict rising groundwater levels in natural slopes at a relatively shallow depth based on parametric studies conducted using the finite element method. An assumption of a semi-infinite homogeneous slope was adopted in the analysis. Addtionally, the authors numerically modelled the vertical infiltration and the lateral seepage flow. Using this method implies that the finite element analysis is not mandatory in the evaluations of practical slopes. Such a simplified approach helps avoid time-consuming tasks in rigorous computations. However, a semi-infinite assumption used in the developed method may provide us with unsuitable solutions, particularly in cases where the slopes include heavily terraced topography with local small cliffs, because theoretically, the first slope failure tends to occur in steep slopes, like the edge of a rice terrace, even though they are very small cliffs. Nevertheless, these local solutions do not affect the conclusions for disaster risk reduction. Moreover, such unsuitable alternatives must be eliminated during analysis. To address this matter, the current study proposes a novel concept of specific lengths. This procedure provides a representative length within the specified length range. The averaged slope gradient is defined by focusing on the secant lines between each topographical grid, while those defined outside the specified range – for example, local cliff angles – are ignored in the slope stability calculation. Consequently, the proposed concept was confirmed to be efficient and can be applied to evaluate the terraced rice fields in Sapa, northern Vietnam. In the past, this area had experienced rainfall-induced slope failures; hence, the proposed method may be able to simulate these occurences. The proposed concept’s effectiveness when applied to terraced fields should continue to be verified through case studies conducted in areas with extensive smallterraced topography.

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