Rainfall is considered a key factor for triggering slope failure, and more than thousands of rainfall-induced slope failures occur around the world every year (Rahardjo et al., 2005; Zhang et al., 2014; Van Asch et al., 2018; Zhu et al., 2020a). Particularly, the different rainfall conditions/types cause slope failure to occur in different patterns, for example, heavy rainfall like rainstorms or torrential rain will make the surface layer of the soil saturated in a short time (Zhu et al., 2020b). The pore air between the saturated surface layer and the phreatic layer is enclosed in the soil, thereby preventing the downward infiltration of water. In this case, the water that has not infiltrated into the soil produces runoff on the ground surface, which will cause erosion of the ground surface and cause shallow slope failure/landslides (Larsen and Simon, 1993). On the other hand, during light rain, the weak rainfall intensity will not quickly saturate the surface layer of the soil. This makes it difficult to create the closed region of pore air. The infiltrated water replaces the pore air in the soil, and the pore air is expelled from the soil through the soil surface (Kuang et al., 2013). Accordingly, water can continuously infiltrate into the deep soil layer, resulting in the continuous increase of the water content in the deep soil layer, thereby reducing the matric suction and subsequently inducing the occurrence of deep-seated slope failure/landslide (Larsen and Simon, 1993). Therefore, it cannot be ignored that the type of rainfall has a significant influence on the infiltration behavior of soil moisture and the failure pattern of the slope.
In the past few decades, many scholars have made great efforts to establish the link between rainfall and slope instability (e.g., Larsen and Simon, 1993; Guzzetti et al., 2008; Saito et al., 2010; Segoni et al., 2014; Chen et al., 2015; Guo et al., 2016; Abraham et al., 2020; Marin et al., 2021; Kim et al., 2021). A considerable part of their research focuses on a widely used method, namely intensity-duration (I−D) threshold models, as the I-D threshold is considered an effective indicator for rainfall-induced slope failure/landslides (Kim et al., 2021). Among them, the most famous one is the power function of rainfall intensity as rainfall duration (i.e., I=aDb, I is rainfall intensity; D is rainfall duration; a and b are fitting parameters). For example, Marin et al., (2021) evaluated the applicability of the two methods (physically-based model and observed landslides) to define rainfall I-D thresholds in individual basins and emphasized the merits of both methods. Abraham et al. (2020) pondered the effect of the selection of rainfall parameters for developing a regional scale rainfall threshold in a data-sparse region and pointed out that the approach selecting the rain gauge based on the most extreme rainfall parameters performed better than the other approaches. However, the main focus of discussion in these studies is the method of the determination of the rainfall intensity, i.e., the average rainfall intensity (Saito et al., 2010), the peak rainfall total (Jarosińska, 2018), or the antecedent rainfall (Kim et al., 2021).
For the rainfall types, the concepts of short-duration − high-intensity (SH) type and long-duration − low-intensity (LL) type have been mentioned in many studies (Chen et al., 2015; Perera et al., 2017; Deng et al., 2018; Jin et al., 2021; Marin et al., 2021; Kim et al., 2021), but almost all judgments are arbitrary or subjective. For example, Jin et al. (2021) defined the rainfall event (I=25 mm/h and D=8 h) as SH type rainfall and the rainfall event (I=0.4 mm/h and D=168 h) as LL type rainfall. Otherwise, the criteria for classifying rainfall types are not uniform. For example, Chen et al. (2015) set the criteria of SH type rainfall as short-duration (<2 h) and high-intensity (>16.1 mm/h) and the criteria for LL type rainfall as long-duration (>71 h) and low-intensity (<8.8 mm/h). While Perera et al. (2017) set the criteria of SH type rainfall as short-duration (<2 h) and high-intensity (>54 mm/h) and the criteria for LL type rainfall as long-duration (>8 h) and low-intensity (<25 mm/h). Apart from the above studies, Saito et al. (2010) examined rainfall I-D conditions of 1,174 shallow landslides that occurred during 2006−2008 in Japan and classified the rainfall type into SH type and LL type based on statistical results as shown in Fig. 1. However, from Fig. 1(b), it can be seen that the criterion for classifying rainfall types proposed by Saito et al. (2010) is too small. The main reason may be that the object of investigation in the study of Saito et al. (2010) is shallow landslides, which are usually induced by heavy rainfall or the erosion of the surface caused by long-term light rain.
Therefore, to eliminate the current dilemma of the inconsistent classification of rainfall types, this study attempts to present a determination method of rainfall type based on rainfall-induced slope instability. Through the analysis of the I−D conditions corresponding to 4,752 scenarios of slope instability, and subsequently, according to the effect of rainfall types on the initiation of slope failure, a determination method of rainfall type (SH type and LL type) based on rainfall-induced slope instability is proposed.