CHAPTER 2 LITERATURE REVIEW
2.2 Landslide Triggering and Causative Factors
2.2.2. Geomorphology
The presence or absence of former landslides is an important key if one would consider geomorphology factors in LHA as suggested by Varnes [38]. Past landslides would be the best guide to investigate the future behavior of the study area. Among the important aspects of geomorphology are slope and slope aspect. Carrara, et al.
[61] added curvature factor as another aspect while DeGraff and Romesburg [62]
included elevation factor in the landslide hazard model.
More practical and visual descriptions on slope, slope aspect, and curvature terms can be found in ESRI [63]. Slopes of a hill describe the steepness of hill slope. Slope aspect is defined as a compass direction a hill faces while curvature is used to explain the physical characteristics of drainage basin such as erosion and runoff process.
Slope contributes the overall rate of movement downslope the hill. Slope aspect
shows the direction of flows. Curvature of terrain is shown as convex, concave, peaks, and flat surface. Sloping surfaces that are convex in the cross-sectional direction are called as ridges. Those having convex cross-section and convex longitudinal direction are called peaks. Meanwhile, pits are indicated by concave curvatures as described by Kumar, et al. [64]. Curvature affects the acceleration and deceleration of flow and, therefore, influences erosion and deposition. The curvature forms, i.e. convex or concave, influence the convergence and divergence of flow.
This information is useful to identify the areas where the flow will accumulate and possible landslide or debris locations.
2.2.2.1 Slope gradient
Slope gradient is an important causative factor. The shape of slopes affects the direction of and the amount of surface runoff or subsurface drainage down the slopes.
For hazard zoning purpose, Varnes [38] stated that the steepness of slopes is important to consider due to its relation with the strength of slope-forming material.
Slope gradient is usually classified into ranges of degree or percent. However, the interrelation between slope gradient and slope instability is complex. The steepest slope gradients do not always mean as the most prone slopes to landslide. Steep slopes are usually occupied by very resistant/competent rock causing the slopes more stable than comparatively gentle slopes of weak material. Havenith, et al. [65] added that landslide body and scarp usually occur in areas with relatively small slope angels.
The preferential location of landslides at small slope angles is not caused by the slope angel itself but related with complex interaction between slope angle and the environment.
The ranges of high risk slopes vary from one place to another. For examples, most of landslide cases in Lantau Island Hongkong occurred within a slope range of 250- 350 in the middle and western parts of the island and 300-350 in the eastern part respectively from investigations conducted by Zhou, et al. [51]. Slopes above 400 and above 450 were set as the highest risk slopes referring to the works conducted by Anbalagan [2] and Pachauri and Pant [53] respectively. Meanwhile, Lee and Pradhan [7] found that slope range of 160-250occupied by most of landslide occurrences in
Selangor, Malaysia. Landslides may also occur at gentle slope e.g. 90-120 as reported by Jäger and Wieczorek [66]. The different ranges of risky slope indicate that the typical risky slopes are different from place to place. Hence, the risky slope should be carefully recognized for LHA purpose.
2.2.2.2 Slope Aspect
Slope aspect and terrain curvature are frequently used as causative factors in LHA.
Carrara, et al. [61] has conducted a study on analysis of digital terrain for slope stability. The study found that slope aspect and its curvature can be related to slope stability. However, researchers had different opinions about the importance role of slope aspect in causing slope instability. Greenbaum, et al. [67] concluded that slope aspects have no significant influence on landsliding. On the contrary, some authors such as Suzen [57] and Van Westen, et al. [68] agreed that there is a relationship between landslide and slope aspect. Dai and Lee [69] added that the condition of moisture retention and vegetation of slopes are affected by the orientation of slopes and in turn, these conditions can affect the soil strength. In spite of the existence of the relationship between slope aspect and landslides, researchers found that there is no single relationship between landslide and slope aspect. Moreover, the facts that some studies were carried in various places, e.g. some were in northern hemisphere countries, some were in the southern, and some were in equatorial belt, would surely produce different results considering the sun exposure (intensity).
Investigators found different relation between slope aspect and landslides.
DeGraff and Romesburg [62] pointed out that slope aspect contains information about the structural and basic condition of a slope including fault planes and climatic factors respectively. The author further made assumptions that slopes which are facing the sun, particularly the afternoon sun, tend to have higher soil temperature, lower soil moisture, less vegetation, and therefore tend to have higher erosion rate. Further investigation carried out by Caiyan, et al. [70] in Gorges reservoir, China, found that landslides mostly occurred at slopes facing south, southwest and southeast. Lineback, et al. [71] found that the number of landslide occurrences is larger in the wetter north- facing slopes than in drier south facing slopes when assessing landslide potential
zones in Payette River, Idaho, United States. Marston, et al. [72] reported that soil exposed on south facing slopes tends to undergo wetting and drying cycle so that it contributes the increase of the number of landsides in Himalaya. Meanwhile, the investigation conducted by Lee and Pradhan [7] showed that high frequency of landslides in Selangor, Malaysia, took place in north and northwest facing slopes.
2.2.2.3 Curvature
Curvature also has a contribution to landsliding. The acceleration or deceleration of colluviums, the loose bodies of sediment, is affected by curvature. The shape of hill slope, i.e. concave or convex, will determine where colluviums will accumulate.
Ahmad and McCalpin [73] stated that the abundance of colluvial slides was found in
‗hollows‘ or concave terrain. Concave slopes behave as a channel. It contains and retains more water gained from rainfall for a longer period than convex slopes as described by Kumar, et al. [64] and Lee and Evangelista [74]. Investigation on the relation of curvature and landslides produced different results. Ohlmacher [75] found that planar plan curvature has the highest probability of landsliding in Appalachian Plateau and scattered regions within the Midcontinent of North America. In Pemalang, Indonesia, investigation on the landslide susceptibility conducted by Oh, et al. [76] showed that most of landslide occurrences took place in convex curvature areas. Similar results were reported by Lee and Talib [55] and Lee and Pradhan [7]
that convex curvature areas were found to have more frequent landslides compared to concave and planar curvature areas in Penang and Selangor, Malaysia, respectively.
Oh, et al. [77] also investigated landslide susceptibility in Pechabun area of Thailand.
The landslide occurrences mostly occupied concave curvature areas.
2.2.2.4 Elevation
Elevation is one of geomorphology factors that affect slope instability. Authors are in different opinions about this. Asfaw [78] considered elevation as an important factor for a reason that precipitation and weathering are inherent factors of elevation. Chau and Chan [79] found that elevation as a causative factor is related to human
developments/activities that are concentrated at certain elevations belt. Dai and Lee [69] argued that at very high elevation, the terrain usually consists of weathered rocks whose shear strength is much higher. Meanwhile, at intermediate elevations, the terrain is covered by thin colluviums causing it more instable. At very low (gentle) elevations, the terrain is usually covered by thick colluviums or residual soils so that the frequency of landslides is low. Although landslides have been found to have a relationship with elevations, Gómez and Kavzoglu [80] argued that the relationship is still unclear. The author further described that the elevation influences the number of biophysical parameters and anthropogenic activities, and the soil characteristics are affected by the elevation. Gao and Lao [81] introduced an empirical formula relating possibility of landslide P(H) at a particular place with height factor as follows:
H
1.28229H2 0.001847H3
/106P (2.3)