CHAPTER 2 LITERATURE REVIEW
2.1 Landslide Hazard Studies: Terminologies and Associated Concepts
As stated earlier, the term landslide means the movement of a mass of rock, debris or earth down a slope as defined by Cruden [1]. The term also includes slope failures as added by Highland and Bobrowsky [34]. The movement of a mass of rock can be a) falling, b) toppling, c) sliding, d) spreading, or e) flowing as shown in Fig. 2.1.
In studying landslide hazard, there are some terminologies and concepts associated with this matter. The terms used in this research are mainly taken from Gilbert, et al. [35] and Guzzetti [36]. Some importance definitions are given below:
1. Landslide susceptibility refers to the likelihood of a landslide occurring in an area on the basis of local terrain or environmental conditions. Susceptibility does not consider the temporal probability or time frame of failures. Most of approaches in assessing and mapping landslide susceptibility are based on accurate evaluation of the spatial distribution of both factors, i.e. geo- environmental factors and landslides.
Fig. 2.1 Type of mass movement Souce: Sassa [37]
2. Landslide hazard refers to the potential for occurrence of a damaging landslide within a given area and within a specified period of time. Such damage could include loss of life or injury, property damage, social and economic disruption, or environmental degradation. Guzzetti [36] extended the definition that includes ‗the magnitude of the event‘. Thus, the definition incorporates the concepts of location, time, and magnitude of landslide hazard. Landslide hazard (HL) is formulated by International Association for Engineering Geology and the environment (IAEG) and Varnes [38] as multiplication of probability of landslide size (AL), probability of temporal occurrence (NL), and landslide susceptibility (LS). The equation is as follows:
A P N LSP
HL L L (2.1)
Landslide susceptibility (LS) is a component of landslide hazard (HL).
3. Landslide risk refers to the probability of harmful consequences, the expected number of lives lost, persons injured, extent of damage to property or ecologic systems, or disruption of economic activity, within a landslide prone area. The risk may be individual or societal in scope, resulting from the interaction between hazard and individual or societal vulnerability.
4. Element at risk refers to the population, public and private infrastructure, economic activities, ecologic values, etc., at risk in a given area.
5. Specific landslide risk means the expected degree of loss due to a particular landslide, based on risk estimation—the integration of frequency analysis and consequence analysis. Specific landslide risk (RS) is formulated by IAEG and Varnes [38] as follows:
L L
S H V
R (2.2)
To determine risk, one needs to know landslide hazard (HL) and landslide vulnerability (VL).
6. Landslide vulnerability reflects the degree of loss to a given element (or set of elements) within the area affected by the hazard, expressed on a scale of 0 (no loss) to 1 (total loss); vulnerability is shaped by physical, social, economic, and environmental conditions.
7. Slope instability hazard zonation is defined by Varnes [38] as the mapping of areas with an equal probability of landslide occurrences within a specified period of time. Landslide instability hazard zonation is simply called as landslide hazard zonation (LHZ) as suggested by Gilbert, et al. [35] and most of relevant literature.
In term of mapping, there are various types of landslide hazard maps as the result of stage of landslide studies. The definitions of landslide hazard maps are adopted from AGS [39], UN [40], and Varnes [38]. Some important landslide map definitions are given below:
1. A landslide inventory map shows the locations and outlines of landslides. A landslide inventory is a map represents a single event or multiple events.
Small-scale maps may show only landslide locations, whereas large-scale maps may distinguish landslide sources from deposits, classify different kinds of landslides, and show other relevant data.
2. A landslide susceptibility map ranks slope stability of an area into categories that range from stable to unstable. Many susceptibility maps use a color scheme that relates warm colors (red, orange, and yellow) to unstable and marginally unstable areas and cool colors (blue and green) to more stable areas.
3. A landslide hazard map indicates the annual probability of landslides occurring throughout an area. An ideal landslide hazard map shows not only the chances that a landslide may form at a particular place, but also the chances that a landslide from farther upslope may strike that place.
4. A landslide risk map shows the expected annual cost of landslide damage throughout an area. Risk maps combine the probability information from a landslide hazard map with an analysis of all possible consequences (property damage, casualties, and loss of service).
In most of literature the terms ‗susceptibility‘ and ‗hazard‘ are often used incorrectly as synonymous terms as explained by Guzzetti [36] whereas both terms have different meaning. ‗Landslide susceptibility‘ is more in probability of ‗spatial‘
occurrence of slope failures based on terrain parameter conditions or in other words it is an estimate of ‗where‘ landslides likely to occur. Meanwhile, besides predicting
‗where‘ a slope failure will occur, the term ‗landslide hazard‘ forecasts ‗when‘ or
‗how frequently‘ it will occur, and ‗how large‘ it will be. Thus, landslide hazard is more difficult to deal with than landslide susceptibility. Moreover, susceptibility is the spatial component of the hazard.
The reason that landslide hazard is more difficult to deal with than landslide susceptibility has made the number of publications on the study of landslide susceptibility abundance such as studies done by Agostoni, et al. [28], Ahmad, et al.
[41], Anbalagan [2], Ayalew and Yamagishi [5], Chuanhua and Xueping [42],
Gahgah, et al. [43], Hong, et al. [44], Komac [45], Lee and Pradhan [10], Lee and Pradhan [7], Lee and Sambath [46], and Liao [47], even though these investigations are mostly entitled as landslide hazard study or assessment. This is what Brabb [48]
found in most of literatures that landslide hazard (map) is often represented as landslide susceptibility (map). Guzzetti [36] added that literature on landslide hazard frequently discusses methods and techniques to model landslide susceptibility rather than landslide hazard. From this point of view, the author would like to emphasize that this research deals with landslide susceptibility mapping rather than landslide hazard mapping.