Nat. Hazards Earth Syst. Sci., 16, 311–331, 2016 www.nat-hazards-earth-syst-sci.net/16/311/2016/ doi:10.5194/nhess-16-311-2016 © Author(s) 2016. CC Attribution 3.0 License. Assessment of physical vulnerability of buildings and analysis of landslide risk at the municipal scale: application to the Loures municipality, Portugal C. Guillard-Gonçalves, J. L. Zêzere, S. Pereira, and R. A. C. Garcia RISKam, CEG, Institute of Geography and Spatial Planning, Universidade de Lisboa, Lisbon, Portugal Correspondence to: C. Guillard-Gonçalves ([email protected]) Received: 5 August 2015 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 10 September 2015 Revised: 28 December 2015 – Accepted: 13 January 2016 – Published: 3 February 2016 Abstract. This study offers a semi-quantitative assessment 1 Introduction of the physical vulnerability of buildings to landslides in a Portuguese municipality (Loures), as well as the quantita- Landslides are natural phenomena that can cause costly dam- tive landslide risk analysis computed as the product of the age when occurring in or impacting constructed areas. Land- landslide hazard by the vulnerability and the economic value slide risk analysis is used to estimate the risk of landslide of the buildings. The hazard was assessed by combining the hazard to individuals, populations, properties, or the envi- spatiotemporal probability and the frequency–magnitude re- ronment (Fell et al., 2008; Corominas et al., 2014, 2015) lationship of the landslides. The physical vulnerability as- and generally contains five main steps: (i) hazard identifi- sessment was based on an inquiry of a pool of European land- cation, (ii) hazard assessment, (iii) inventory of elements at slide experts and a sub-pool of landslide experts who know risk and exposure, (iv) vulnerability assessment, and (v) risk the study area, and the answers’ variability was assessed with estimation. Landslide risk analysis is useful to locate the standard deviation. The average vulnerability of the basic ge- zones where the risk is highest, but it is a complex and time- ographic entities was compared by changing the map unit consuming task, especially when the study is conducted at and applying the vulnerability to all the buildings of a test the municipal scale. site, the inventory of which was listed on the field. The eco- During the last three decades, the landslide risk (R) has nomic value was calculated using an adaptation of the Por- been considered as the product of the landslide hazard (H), tuguese Tax Services approach, and the risk was computed the vulnerability (V), and the value of the elements at for different landslide magnitudes and different spatiotempo- risk (EV) (Varnes and the IAEG Commission on Land- ral probabilities. As a rule, the vulnerability values given by slides and other Mass-Movements, 1984; Michael-Leiba et the sub-pool of experts who know the study area are higher al., 1999; Cardinali et al., 2002; Remondo et al., 2005; than those given by the European experts, namely for the Uzielli et al., 2008; van Westen et al., 2008; Zêzere et al., high-magnitude landslides. The obtained vulnerabilities vary 2008): R D H × V × EV, where R is the risk (annual loss of from 0.2 to 1 as a function of the structural building types and property value). Landslide hazard (H) is the probability of the landslide magnitude, and are maximal for 10 and 20 m occurrence within a specified period of time and within a landslide depths. However, the highest risk was found for the given area of a potentially damaging phenomenon (Varnes landslides that are 3 m deep, because these landslides com- and the IAEG Commission on Landslides and other Mass- bine a relatively high frequency in the Loures municipality Movements, 1984) having a given magnitude (Jaiswal et al., with a substantial potential damage. 2011a), which is typically measured with the landslide area or the landslide volume (Lee and Jones, 2004; Li et al., 2010). The vulnerability (V) concept is defined in physical terms as the “degree of loss” of a given element or set of elements at risk exposed to the occurrence of a landslide of a given Published by Copernicus Publications on behalf of the European Geosciences Union. 312 C. Guillard-Gonçalves et al.: Assessment of physical vulnerability of buildings and analysis of landslide risk magnitude, expressed in a scale ranging from 0 (no loss) (e.g. structure type, construction material, maintenance state) to 1 (total loss) (e.g. Varnes and the IAEG Commission on (e.g. Uzielli et al., 2008, 2015; Li et al., 2010; Du et al., Landslides and other Mass-Movements, 1984; Remondo et 2013; Peng et al., 2015). Most of the time, landslide inten- al., 2008). The value of the elements at risk is the economic sity parameters can be quantified (e.g. landslide velocity), value (EV) of the elements at risk, which in this study corre- while proposed values for resistance or susceptibility of the spond to the built environment. exposed buildings are usually assigned based on expert opin- Whereas the landslide susceptibility and the landslide haz- ion (Peng et al., 2015; Uzielli et al., 2015), which may in- ard have been extensively studied in the last two decades, crease the subjectivity and uncertainty of the vulnerability whether with heuristic, statistic-probabilistic, or determinis- estimation. In addition, expert surveys can be used to esti- tic methods (e.g. Fell et al., 2008; Corominas et al., 2014), mate physical vulnerability using the standard deviation of less work has been done, for various reasons, on the spa- the expert answers to measure the variability of the average tial assessment of landslide vulnerability and on the assess- vulnerability (Winter et al., 2014). ment of the value of the elements at risk (e.g. Zêzere et Physical vulnerability assessment has several sources of al., 2007, 2008; Papathoma-Köhle et al., 2012a; Silva and uncertainty that can be either epistemic or aleatory (Ciurean Pereira, 2014). et al., 2013). Epistemic uncertainties can come from the use First, for most types of landslide, very limited damage of proxies for the landslide intensity assessment (e.g. veloc- data are available (van Westen et al., 2005; Papathoma-Köhle ity, depth of affected material, volume), or from the charac- et al., 2012a), which hamper the creation and validation of terization of elements at risk (e.g. structural-morphological any reliable vulnerability model. Second, different physical characteristics, state of maintenance, strategic relevance), mechanisms are associated with different types of landslides, from the vulnerability model (e.g. selection of parameters, which mean that the same elements at risk have different mathematic model, calculation limitations), or from expert vulnerability to different types of landslides. Therefore, the judgement about building resistance parameters and land- method used for assessing rockfall vulnerability would not slide damaging potential (Ciurean et al., 2013). Aleatory un- be directly transferable to the slow slide vulnerability as- certainties come from the spatial variability of parameters sessment (Alexander, 2005; Papathoma-Köhle et al., 2011; (e.g. landslide intensities, population density) (Ciurean et al., Ciurean et al., 2013). Third, the vulnerability of the elements 2013). For instance, the position of the element at risk (e.g. a at risk depends on the landslide intensity, which is usually building) on the track of a landslide is a source of aleatory associated with the landslide velocity (Hungr, 1997; Lateltin uncertainty as the damage would not be the same if it is lo- et al., 2005) that may range from some millimetres per year cated on the crown of the landslide or on its run-out zone to several metres per second (Cruden and Varnes, 1996). (van Westen et al., 2005). Moreover, methods used to assess vulnerability should Some examples of non-site-specific studies on landslide be selected according to the scope and the scale of the risk to buildings are available in the technical literature study, which influences the level of spatial detail requested (e.g. Michael-Leiba et al., 1999; Cardinali et al., 2002; Re- (Papathoma-Köhle et al., 2011). A vulnerability study con- mondo et al., 2008; Uzielli et al., 2008; Zêzere et al., 2008; ducted at the municipal level typically implies the existence Jaiswal et al., 2010, 2011b; Uzielli et al., 2015). Despite of a large number of elements at risk (e.g. buildings) and the progress already made, major limitations persist on the details about building characteristics and landslide damage. reliable assessment of landslide frequency and magnitude Due to this reason, landslide vulnerability assessment is usu- (which are both critical for the hazard assessment), and on ally performed in small study areas with a reduced number of the quantification of the buildings’ vulnerability, which is exposed elements in order to ease the methodology demon- frequently based on expert opinion. This work aims to con- stration (e.g. Uzielli et al., 2015). tribute to the fulfilling of a research gap on the physical vul- Previous studies have attempted to assess the landslide nerability assessment based on expert opinion. The main pur- vulnerability and to analyse the landslide risk. Some of poses of the study are to develop and apply a method for them are qualitative, focusing on human lives (e.g. San- building vulnerability assessment in a Portuguese municipal- tos, 2003) and in both buildings and human lives (Mac- ity (Loures), and to analyse the landslide risk to buildings in quarie et al., 2004). Other physical vulnerability studies are this study area. semi-quantitative, assigning empirical weighting of a set of Following the previous work of Guillard and building resistance parameters to buildings exposed to land- Zêzere (2012), the susceptibility of the slopes was modelled slides (e.g. Silva and Pereira, 2014), or applying vulnerability for deep-seated and shallow slides, and the hazard was curves to buildings exposed to hydrometeorological hazards assessed, considering the magnitude probability of the land- (e.g.