Landslides are mostly seen at certain critical slope angles (Lee and Min, 2001; Öztürk, 2002; Rozos et al., 2010; Yılmaz et al., 2012; Özşahin, 2015; Avcı, 2016; Jayanthi et al., 2016; Laldintluanga et al.; 2016). Many researchers have asserted that an increase in the slope increases susceptibility to landslides (Pachauri and Pant, 1992; Gökçeoğlu and Ercanoğlu, 2001; Lee and Min, 2001; Öztürk, 2002; Lee, 2005; Özşahin, 2013; Özşahin and Kaymaz, 2013; Taşoğlu et al., 2016), while a portion of researchers have specified that landslides can also form on low slope angles (Ayenew and Barbieri, 2015). The general opinion is that landslides drop after a certain value, increasing with the slope value (Milewski et al., 2009; Chen et al., 2017; Hong et al., 2017). Conversely, Van Westen et al. (2003), in a landslide susceptibility evaluation study that they conducted in Italy, reported that the slope was not an effective parameter for the study area because the possibilities of landslides in all of the slope groups were close to one another.
Landslides are evaluated in different groups. In the literature, there have been intervals preferred by different researchers, while landslide slope values do not have definitive slope intervals. The most fundamental cause of this is that slope values are different for each land condition. Table 1 provides the slope values that were generally preferred in the literature review.
Table 1
The realized landslide slope values and groups in the literature
Milewski et al [220] | 0–10° | 10–30° | > 30° | | |
low slope | moderate slope | steep slope |
Guo et al. [104] | 0–10 ° | 10–20° | 20–30° | 30–40° | > 40° |
very soft | soft | moderate | steep | very steep |
Kayastha [45] | < 15° | 15–25 ° | 25–35 ° | 35–45 ° | (> 45 ° |
flat | moderate slope | rather moderate slope | steep slope | very steep slope |
Ercanoğlu et al [79] | < 6 | 6 and 16 | 16–25 | 25–33 | > 33 |
very soft slopes | light slopes | moderately steep slopes | steep slopes | segments |
Duo et al [111] | 0 ° -15° | 15 ° − 30 ° | 30 ° -45 ° | > 45 ° | |
soft slope | moderately steep slope | steep slope | cliff | |
Özdemir [222] | 0–2 | 2–15 ° | 15–25° | 25–45° | > 45 (68) |
very small slope (flat) | small slope | light slope (moderate) | steep | very steep |
Özşahin and Kaymaz [188] | 0–3 ° | 3–10 ° | 10–20 ° | 20–30° | > 30 |
very small slope | small slope | moderate slope | steep slope | very steep slope |
Table 1. Realized landslide slope angles and groups in the literature.
The literature evaluates landslide groups in classes 3, 4, and 5. Values grouped as very low slope, low slope, moderate slope, high slope, and very high (steep) slope, based on the conditions of the studied land, were categorized based on low, moderate, and high slope values to be able to make generalizations in this study.
5.1. Effect of high value (> 35°) slope angles on landslides.
The reason for the rarity of landslides in the very high slope range is that the width of the weathering zone is small and this reduces the susceptibility of landslides. It reduces normal stress and facilitates the activation of materials by increasing the shear stress of the soil, such as an increase in slope values and the strain of land shear. Landslides with high slope angles exceeding 45° comprise sturdy rocks rather than weathered materials in the nature of ground, and these types of rocks are stable. However, any increase in the slope leads to an increase in the possibility of breaking away (Wilson and Gallant, 2000; Gökçeoğlu and Ercanoğlu, 2001; Lee and Min, 2001; Öztürk, 2002; Özşahin and Kaymaz, 2013; Lee, 2005; Sadr et al., 2014; Jayanthi et al., 2016; Laldintluanga et al., 2016).
Some researchers have stated that landslides were more common in steep areas when compared to moderate and soft-slope areas in their study areas (Rozos et al., 2010; Alexakis et al., 2013; Laldintluanga et al., 2016; Abedini et al., 2017). Nagarajan et al. (2000), in a study conducted in the Konkan region of India, determined that rockfall-type landslides occurred on steep slopes with a slope angle of > 80°, and that mudslide-type landslides occurred on slopes with a slope angle of > 35°. They also reported that landslide susceptibility increased with an increase in the slope.
Conversely, there are studies in which a sharp drop was seen in landslide intensity when slope angle reached 45° (Tangestani, 2004; Jaafari et al., 2014; Chen et al., 2018). Although Chen et al. (2015) observed in their study that sloped fractures on high-angled slopes were more common than on low angle slopes, they stated that the frequency of landslides did not decrease because the high slope transitions could not support the accumulation of soil above a certain threshold. Similarly, Gökçeoğlu and Ercanoğlu (2001) reported that, in order to see landslides in ground soil, the thickness of the present soil had to be at least 1–2 m and, most of the time, it would be difficult to see landslide activity because it was not possible to reach these thickness values. Jayanthi et al. (2016) did not encounter any landslide in areas at slope angles > 25° because vegetation is unimportant in the very high slope category. Additionally, no rockfall of any kind was encountered in the research area despite the ability of rockfalls to form at great heights.
5.2. Effect of moderate value (15–35°) slope angles on landslides
This situation is valid for slopes with moderate slope angles. The increasing angles in these types of slopes negatively affects the susceptibility of slopes because it will increase the shear stress of the soil. Most researchers have reported that most landslides in their area of research formed on slopes with angles ranging from 15 to 35° (Lee and Min, 2001; Ercanoğlu et al., 2004; Hong et al., 2017; Pham et al., 2017).
However, the literature contains research that has claimed the opposite of this. Erener and Lacasse (2007) did not report a relationship regarding landslides at locations where the slope angle was > 15°. Chau and Chan (2005) reported that landslides developed at higher angles, despite the fact that slopes with angles of 25–30° are more suitable for landslides.
5.3. Effect of low value (< 15°) slope angles on landslides
Because slopes with high angles form from rock units and the thickness of the weathering zone increases in low-angle slopes, it is thought that slopes with this slope angle are more susceptible to landslides. There are studies that have demonstrated that landslides occur at low slope angles (Milewski et al., 2009; Yılmaz et al., 2012).
There are studies regarding landslides that have occurred on slopes with low slope angles, below 15°, in the literature. Conversely, there are studies that have claimed that, because shear stress would be lower in areas with much lower slope angles, landslides would not form at these slope angles, and that they encountered few or no landslides in areas with slope angles below 15° (Lee and Min, 2001; Ercanoğlu et al., 2004; Dağ, 2007; Pham et al., 2017).
5.4. Classification of slope angles
An important issue is the classification of the slope factor. Landslide susceptibility evaluations report which slope classification intervals landslides are concentrated on by classifying different degrees of the slope to determine the slopes at which the landslide frequency is the greatest. There is no slope classification used as a standard interval with regards to the slope angle. In most studies, the classification intervals belonging to the slope angles are taken differently. Generally, researchers prefer automatic classification because it is a fast and easy solution. On the other hand, because each study has different intervals according to the properties of the land, researchers can use their own classifications based on the land conditions and the concentrations of landslides in the research area (Biçer-Tetik, 2017). For this purpose, slope angle maps are separated into groups (e.g., 0–5°, 5–10°, 10–15°, and > 15°). These groups can later be reclassified as low slope, medium slope, and high slope. Other than this, there have also been other classifications created in the literature. One of these is the natural fracture optimization technique by Jenks (1967). Some researchers have observed that this classification was effective in describing information content-specific to the soil regarding the vulnerability of the slope and landslides, and used it in their studies (Balamurugan et al., 2016). Most researchers have applied equal intervals to determine the lower classifications (Yılmaz et al., 2012). Some have used these classifications again at 5° intervals (Özşahin, 2013; Chen et al., 2015). Differently, there have been researchers who used certain standard classification intervals. For example, while Özşahin and Kaymaz (2013) noted the slope classifications made by Mcdonald (1975) in the classification of slope angles in their study area, Özşahin (2013) examined the effect of the slope according to the slope classification explained by Bijukchhen et al. (2013). Özşahin (2015) reported the effect of the slope in the examination field in another study according to the slope classification explained by Varnes (1976). Constantin et al. (2011) made classifications according to the field observations of Balteanu (2010). Mahanta et al. (2016) used the BIS classification, which defines the slope map and slope classifications according to the formation frequency of the specific slope angles.
The evaluation of the reviewed studies was done in an attempt to determine the slope angle intervals in which the study areas were generally found (Fig. 1), how many classes a slope could be divided into (Fig. 2), at which slope angles landslides were observed (Fig. 3), and at which slope angles were no landslides observed (Fig. 4).
From the literature, 50 study areas were randomly selected and drawn (Fig. 1). It was seen that most of the studies were conducted in areas with a slope angle of 0–90° and following that, were areas with a slope angle of 0–70°.
Figure 1. Slope interval graphs for 50 areas at which landslide research was conducted.
The classification intervals used in the studies varied. The literature research provided the slope classification intervals prepared from randomly selected studies. Classification interval graphics were drawn for 125 selected studies and given in Fig. 2.
Figure 2. Graphs for the classification groups of the selected studies.
As seen from Fig. 2, the most frequently preferred classification intervals in the literature are 5 and 6. It can also be seen that the classification interval selection in the literature offers a wide array, from 3 to 20°. The classification interval selection varies based on the slope angles and mass motion type of the studied area.
The slope intervals at which most of the landslides occurred were separated into 3 classifications, and graphs were drawn and are presented in Fig. 3.
Figure 3. Realized landslide slope interval graphics.
The literature mentioned the slope intervals at which landslides occurred or rarely occurred and as can be seen in Fig. 3, most of the landslides occurred at slope angles below 30°.
Graphs for slope angles at which no landslides occurred (non-landslide areas) were also prepared (Fig. 4). These were evaluated in 3 groups, as < 15°, 15–45°, and > 45°. It can be seen that there was no increase in the number of landslides encountered on slopes with angles below 15° or above 45°.
Figure 4. Slope angle intervals at which there were non-landslides or landslides rarely occurred.