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- Rainfall thresholds for landslide activity in Portugal: a state of the artPublication . Zêzere, José; Vaz, Teresa; da Silva Pereira, Susana; Oliveira, Sérgio; Marques, R.; Garcia, Ricardo A CRainfall is the most important physical process responsible for the landslide triggering in Portugal. Results obtained worldwide have shown that control of rainfall on landslides differs substantially depending upon landslide depth and kinematics and the affected material. Therefore, the critical rainfall conditions for failure are not the same for different types of landslides, and may be strongly influenced by regional geologic and geomorphologic conditions. Rapid debris flows are typically triggered by very intense showers concentrated in just a few hours, and shallow translational soil slips are usually triggered by intense precipitation falling within the 1–15 days long range. On the contrary, activity of deep-seated landslides of rotational, translational and complex types is related to periods of nearly constant rainfall, lasting from several weeks to several months. The different rainfall intensity–duration conditions are associated with different hydrologic mechanisms for slope failure. The generation of surface run-off and high peak discharges in first-order mountain catchments is a critical triggering mechanism for debris flows. The intense rainfall allows the rapid growth of pore water pressure and the drop of capillarity forces that sustain the apparent cohesion of thin soils. As a consequence, shallow soil slips occur within the soil material or at the contact with the underlying less permeable bedrock. Long lasting rainfall episodes enable the steady rise of the groundwater table and the development of positive pore water pressures into the soil. Consequently, deep-seated failures occur in relation to the reduction of shear strength of affected materials. In this work, we present the state of the art concerning the proposition of empirical rainfall thresholds in Portugal for different types of landslides observed in different zones of the country: the Lisbon region, the Douro Valley and the NW Mountains, and the Povoação Municipality in São Miguel Island (Azores). The empirical thresholds applied in Portugal are based on the identification of past landslide events and include (i) the computation of antecedent rainfall threshold defined by linear regression, (ii) the normalization of rainfall by the mean annual precipitation, (iii) the definition of lower limit and upper limit rainfall thresholds and (iv) the definition of combined rainfall thresholds, which integrates the rainfall event and the antecedent rainfall for different time periods.
- Effects of landslide inventories uncertainty on landslide susceptibility modellingPublication . Zêzere, José; Neves, Mário; Henriques, C. S.; Garcia, Ricardo; Oliveira, Sérgio; Piedade, Aldina
- Integration of spatial and temporal data for the definition of different landslide hazard scenarios in the area north of Lisbon (Portugal)Publication . Zêzere, José; Reis, Eusébio; Garcia, Ricardo A C; Oliveira, Sérgio; Rodrigues, Maria Luísa; Vieira, Goncalo; Ferreira, A. B.A general methodology for the probabilistic evaluation of landslide hazard is applied, taking in account both the landslide susceptibility and the instability triggering factors, mainly rainfall. The method is applied in the Fanhões-Trancão test site (north of Lisbon, Portugal) where 100 shallow translational slides were mapped and integrated into a GIS database. For the landslide susceptibility assessment it is assumed that future landslides can be predicted by statistical relationships between past landslides and the spatial data set of the predisposing factors (slope angle, slope aspect, transversal slope profile, lithology, superficial deposits, geomorphology, and land use). Susceptibility is evaluated using algorithms based on statistical/probabilistic analysis (Bayesian model) over unique-condition terrain units in a raster basis. The landslide susceptibility map is prepared by sorting all pixels according to the pixel susceptibility value in descending order. In order to validate the results of the susceptibility ana- lysis, the landslide data set is divided in two parts, using a temporal criterion. The first subset is used for obtaining a prediction image and the second subset is compared with the prediction results for validation. The obtained prediction-rate curve is used for the quantitative interpretation of the initial susceptibility map. Landslides in the study area are triggered by rainfall. The integration of triggering information in hazard assessment includes (i) the definition of thresholds of rainfall (quantity-duration) responsible for past landslide events; (ii) the calculation of the relevant return periods; (iii) the assumption that the same rainfall patterns (quantity/duration) which produced slope instability in the past will produce the same effects in the future (i.e. same types of landslides and same total affected area). The landslide hazard is present as the probability of each pixel to be affected by a slope movement, and results from the coupling between the susceptibility map, the prediction-rate curve, and the return periods of critical rainfall events, on a scenario basis. Using this methodology, different hazard scenarios were assessed, corresponding to different rain paths with different return periods.
- Flood susceptibility assessment based on Analytical Hierarchy Process: application in mainland PortugalPublication . Santos, Pedro Pinto; Pereira, Susana; Zêzere, José; Reis, Eusébio; Garcia, Ricardo A C; Oliveira, Sérgio; Santos, MónicaAlthough the scientific and computational advances in the hydrological study of flood hazard assessment, a demand still exists concerning more expedite and extensive methods to map flood susceptibility or proneness over large areas. The relevant question is very often to achieve the best equilibrium between the complexity of flood hazard models and the accuracy and quantity of the input data. The presented study aims to assess flood susceptibility in mainland Portugal through the application of an Analytical Hierarchy Process (AHP), using 17 flood susceptibility conditioning factors related with: i) terrain morphology (elevation, slope angle, slope over area ratio, flow accumulation, topographic position index, landform classification - Jenness); ii) basin morphology (hypsometric integral, Gravelius index, circularity ratio and the orographic coefficient of Fournier); iii) permeability (lithology, , alluvial zones, hydraulic conductivity and ecological zones); iv) rainfall (water balance) and v) hydrography (drainage density and Strahler’s fluvial hierarchy). All variables were classified according to empirical knowledge and further defined as raster datasets at the national level with a resolution of 25x25 m cell size. AHP technique was used to achieve the relative weights based on the Saaty’s scale of influence, which ranges between 1 and 9. The weighting of conditioning factors was performed at two levels: initially, between variables and, secondly, between the different classes of each variable. This later process, although consensual regarding most of the variables (e.g., slope angle or flow accumulation) is rather more complex regarding specific variables which require a deeper interpretation of scores (e.g., topographic position index, landform classification and the permeability-related variables). The study also highlights the challenges posed during the data preparation and integration steps, namely, the integration of data from variables that are represented at the basin level (e.g., those related to basin morphometry and hydrologic behavior) with data represented on a cell-by-cell basis, such as flow accumulation or slope angle. Future exploration of the results includes the aggregation of the AHP-based flood susceptibility scores – distinguishing progressive floods from flash floods – at the administrative level, achieving a flood susceptibility municipal index in order to verify to each extend disastrous floods have been controlled by natural constrains at the municipal level. Such indexes will be applied later in decision support systems in order to define public civil protection-related resources.
- Assessment of physical vulnerability of buildings and analysis of landslide risk at the municipal scale: application to the Loures municipality, PortugalPublication . Guillard-Gonçalves, C.; Zêzere, José; da Silva Pereira, Susana; Garcia, Ricardo A CThis study offers a semi-quantitative assessment of the physical vulnerability of buildings to landslides in a Portuguese municipality (Loures), as well as the quantitative landslide risk analysis computed as the product of the landslide hazard by the vulnerability and the economic value of the buildings. The hazard was assessed by combining the spatiotemporal probability and the frequency–magnitude relationship of the landslides. The physical vulnerability assessment was based on an inquiry of a pool of European landslide experts and a sub-pool of landslide experts who know the study area, and the answers’ variability was assessed with standard deviation. The average vulnerability of the basic geographic entities was compared by changing the map unit and applying the vulnerability to all the buildings of a test site, the inventory of which was listed on the field. The economic value was calculated using an adaptation of the Portuguese Tax Services approach, and the risk was computed for different landslide magnitudes and different spatiotemporal probabilities. As a rule, the vulnerability values given by the sub-pool of experts who know the study area are higher than those given by the European experts, namely for the high-magnitude landslides. The obtained vulnerabilities vary from 0.2 to 1 as a function of the structural building types and the landslide magnitude, and are maximal for 10 and 20m landslide depths. However, the highest risk was found for the landslides that are 3m deep, because these landslides combine a relatively high frequency in the Loures municipality with a substantial potential damage.
- Abadia Basin (Torres Vedras, Portugal) a case study of landslide susceptibility assessment and validationPublication . Garcia, Ricardo; Zêzere, JoséThis study aims to assess and validate the landslide susceptibility on a small catchment (Abadia Basin), using a data-driven approach and GIS tools. Two types of landslides were selected for analysis: rotational slides and shallow translational slides.