Hazard zonation, vulnerability and losses

Two studies are presented within this subproject:

Quantitative Estimation of Regional-Scale Vulnerability to Landslides
Hazard zonation in Møre og Romsdal

Quantitative Estimation of Regional-Scale Vulnerability to Landslides

A methodology for scenario-based, quantitative estimation of regional-scale vulnerability is proposed. The vulnerability model parameterises the severity of landslide action (landslide intensity) and the capacity of vulnerable elements to withstand an action of a given degree of severity (susceptibility of vulnerable elements) is proposed here:

V = I · S                                                                                                                                

in which V indicates vulnerability; I indicates landslide intensity and S indicates the susceptibility of elements at risk.

Susceptibility refers to the lack of inherent capacity of the elements in the spatial extension under investigation to preserve their physical integrity and functionality in the course of the physical interaction with a generic sliding mass. The susceptibility defined herein is independent of the characteristics of the acting agent, i.e. the landslide.

The susceptibility S of any category of elements at risk ranges from 0 (no susceptibility) to 1 (maximum susceptibility). A general susceptibility model is proposed:

in which is the i-th of ns susceptibility factors (each defined in the range [0,1]) contributing to the category susceptibility. Such factors are category-dependent, and may be defined on the basis of existing literature or by other criteria set by the user. Tentative models for the calculation of the susceptibility for a number of categories of elements at risk are proposed i.e. susceptibility of buildings and susceptibility of persons. The quantification of susceptibility of persons includes: Susceptibility of persons in open spaces and vehicles (considering population density, income and age) and Susceptibility of persons in structures.

Figure 1 Susceptibility model for persons in structures, (SSTR = susceptibility of structures, SPST = susceptibility for persons in structures)

Landslide intensity
Hungr (1997) concisely and effectively defined landslide intensity as ¿a set of spatially distributed parameters describing the destructiveness of a landslide¿. While such qualitative definition of intensity can be readily accepted, a univocal quantitative definition is not available at present. Landslide intensity has been addressed and defined quantitatively using a variety of parameters. Hungr (1997) reports use of maximum velocity, total displacement, differential displacement (relative to points adjacent to the point under consideration), depth of the moving mass, depth of deposits after the movement ceases, depth of erosion, unit discharge, kinetic energy per unit area, maximum thrust, impact pressure, maximum normal or shear strain at or below ground surface.

A composite landslide intensity parameter, accounting for kinetic and kinematic characteristics of the interaction between the sliding mass and the reference area is proposed.

Kinetic characteristics are related to the kinetic energy of the sliding mass, i.e. to the damage caused by impact on the vulnerable elements. Kinematic intensity accounts for the effects of size-linked features of a reference landslide. The damage caused by a slow-moving landslide on a building is mainly due to the displacement (i.e. kinematic parameter), while kinetic characteristics could be predominant in case of a rapid movement.

Any kinetic intensity parameter may be defined by the user provided it is defined in the range [0,1] with unit values indicating maximum intensity. Models for kinetic intensity (as function of landslide velocity) and kinematic intensity (as functions of displacements) are proposed.

Figure 2 Proposed kinetic intensity function

Figure 2 Proposed kinetic intensity function

Figure 3 Study region

Here susceptibility expresses the likelihood that a landslide will occur in an area on the basis of the local terrain conditions. Return period or annual probability of occurrence is not considered. The main difference between susceptibility and hazard is therefore that the latter considers the temporal factor, by estimating the probability of occurrence of the phenomenon within a specified period of time.

Method of study
An empirical approach was used to map and evaluate landslide susceptibility. In this approach a grid based Geographic information system (GIS) was used to construct a landslide hazard map for east part of Norway Romsdal region. Five layers of data with 30 × 30 m resolution grid were superimposed to create the landslide susceptibility map. Slope was given the most emphasis, followed by, topological index, density of vegetation, density of lineament and proximity to road networks. A numerical rating system was applied and each of the five factors was grouped into three categories, and each category was assigned a value between 1 and 3, with 1 being least susceptible and 3 most susceptible to landslides.

Figure 4 The method of the study

Figure 5 Example of resulting susceptibility map. Previous landslides are with red dots.

The picture above shows a resulting susceptibility map. Comparison with previous landslides shows that most of the previous landslides (70% - 95%) are within the high or medium susceptible areas.