José Luís Zêzere

DISASTER: a GIS database on hydro-geomorphologic disasters in Portugal

AbstractIn the last century, Portugal was affected by several natural disasters of hydro-geomorphologic origin that often caused high levels of destruction. However, data on past events related to floods and landslides were scattered. The Disaster project aims to bridge the gap on the availability of a consistent and validated hydro-geomorphologic database for Portugal, by creating, disseminating and exploiting a GIS database on disastrous floods and landslides for the period 1865–2010, which is available in http://riskam.ul.pt/disaster/en. Data collection is steered by the concept of disaster used within the Disaster project. Therefore, any hydro-geomorphologic case is stored in the database if the occurrence led to casualties or injuries, and missing, evacuated or homeless people, independently of the number of people affected. The sources of information are 16 national, regional and local newspapers that implied the analysis of 145,344 individual newspapers. The hydro-geomorphologic occurrences were stored in a database containing two major parts: the characteristics of the hydro-geomorphologic case and the corresponding damages. In this work, the main results of the Disaster database are presented. A total of 1,621 disastrous floods and 281 disastrous landslides were recorded and registered in the database. These occurrences were responsible for 1,251 dead people. The obtained results do not support the existence of any exponential increase in events in time, thus contrasting with the picture provided to Portugal by the Emergency Events Database. Floods were more frequent during the period 1936–1967 and occurred mostly from November to February. Landslides were more frequent in the period 1947–1969 and occurred mostly from December to March.

The role of conditioning and triggering factors in the occurrence of landslides : a case study in the area north of Lisbon (Portugal)

Abstract This study of the recent landslide activity in the region north of Lisbon is based on detailed field mapping (1:2000 scale) in five sample areas. A total of 574 landslides were identified in an area of 61.6 km 2 , i.e., about 10 slope movements/km 2 . Thirty-six percent of the slope movements were shallow translational slides, affecting Upper Pleistocene and Holocene unconsolidated deposits. The deeper and larger translational and rotational slides developed in marls and clays of different lithological units. About 20% of the total number of landslides were triggered by human activity (slope cutting, artificial fills and river channel diversion) as a consequence of urban development around Lisbon. The most important triggering factor was rainfall. Although moderate-intensity rainfall episodes may be responsible for a few small landslides on river banks and artificial cuts (return period of about 3 years), two main situations were identified: (1) high-intensity rainfall episodes, which caused flash floods and gave rise to numerous slides and falls, due to bank erosion (return period of over 55 years); and (2) long-lasting rainfall periods, which caused the deeper translational, rotational and complex slides (return period of about 25 years).

Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors

Detailed geomorphological mapping carried out in 5 sample areas in the North of Lisbon Region allowed us to collect a set of geological and geomorphological data and to correlate them with the spatial occurrence of landslide. A total of 597 slope movements were identified in a total area of 61.7 km2, which represents about 10 landslides per km2. The main landslide conditioning factors are: lithology and geological structure, slope angle and slope morphology, land use, presence of old landslides, and human activity. The highest landslide density occurs in Cretaceous marls and marly limestones, but the largest movements are in Jurassic clays, marls and limestones. The landslide density is higher on slopes with gradients above 20 °, but the largest unstable area is found on slopes of 10 ° to 15 °, thus reflecting the presence of the biggest slope movements. There is a correlation between landslides and topographical concavities, a fact that can be interpreted as reflecting the significance of the hydrological regime in slope instability. Concerning land use, the highest density of landslides is found on slopes covered with shrub and undergrowth vegetation. About 26% of the total number of landslides are reactivation events. The presence of old landslides is particularly important in the occurrence of translational slides and complex and composite slope movements. 20% of the landslide events were conditioned by anthropomorphic activity. Humans intervention manifests itself in ill-consolidated fills, cuts in potentially unstable slopes and, in a few cases, in the changing of river channels. Most slope movements in the study area exhibit a clear climatic signal. The analysis of rainfall distribution in periods of recognised slope instability allows the distinction of three situations: 1) moderate intensity rainfall episodes, responsible for minor slope movements on the bank of rivers and shallow translational slides, particularly in artificial trenches; 2) high intensity rainfall episodes, originating flash floods and most landslides triggered by bank erosion; 3) long-lasting rainfall periods, responsible for the rise of the groundwater table and triggering of landslides with deeper slip surfaces.

Application of Social Vulnerability Index (SoVI) and delineation of natural risk zones in Greater Lisbon, Portugal

Social Vulnerability Index (SoVI) was applied to Greater Lisbon (Portugal). Based on the concepts used for the SoVI assessments in the US, 46 variables representing social vulnerability of the 149 civil parishes of Greater Lisbon were chosen. Thirty-eight variables were selected after application of correlation tests. They were standardized, and a Principal Component Analysis and a Varimax rotation were applied to them. Seven factors were extracted using the Kaiser criterion, which explain 79.5% of the variance, and the SoVI scores were then mapped using a standard deviation classification. Twelve of the 149 civil parishes of Greater Lisbon have a very high social vulnerability and 24 of them have a high social vulnerability. The map of SoVI was then integrated with susceptibility maps of earthquakes, floods, flash floods, landslides, tsunami, and coastal erosion, thus delineating risk zones. Twenty-two civil parishes of Greater Lisbon have a very high risk; among them, 17 belong to Lisbon Municipality, four belong to Loures Municipality, and one belongs to Vila Franca de Xira Municipality. Finally, exposed population was considered and combined with risk zones map in order to assess the number of people being potentially exposed to risk and their location.

Mapping landslide susceptibility using data-driven methods

Most epistemic uncertainty within data-driven landslide susceptibility assessment results from errors in landslide inventories, difficulty in identifying and mapping landslide causes and decisions related with the modelling procedure. In this work we evaluate and discuss differences observed on landslide susceptibility maps resulting from: (i) the selection of the statistical method; (ii) the selection of the terrain mapping unit; and (iii) the selection of the feature type to represent landslides in the model (polygon versus point). The work is performed in a single study area (Silveira Basin - 18.2km2 - Lisbon Region, Portugal) using a unique database of geo-environmental landslide predisposing factors and an inventory of 82 shallow translational slides. The logistic regression, the discriminant analysis and two versions of the information value were used and we conclude that multivariate statistical methods perform better when computed over heterogeneous terrain units and should be selected to assess landslide susceptibility based on slope terrain units, geo-hydrological terrain units or census terrain units. However, evidence was found that the chosen terrain mapping unit can produce greater differences on final susceptibility results than those resulting from the chosen statistical method for modelling. The landslide susceptibility should be assessed over grid cell terrain units whenever the spatial accuracy of landslide inventory is good. In addition, a single point per landslide proved to be efficient to generate accurate landslide susceptibility maps, providing the landslides are of small size, thus minimizing the possible existence of heterogeneities of predisposing factors within the landslide boundary. Although during last years the ROC curves have been preferred to evaluate the susceptibility models performance, evidence was found that the model with the highest AUC ROC is not necessarily the best landslide susceptibility model, namely when terrain mapping units are heterogeneous in size and reduced in number.

The record precipitation and flood event in Iberia in December 1876: description and synoptic analysis

The first week of December 1876 was marked by extreme weather conditions that affected the south-western sector of the Iberian Peninsula, leading to an all-time record flow in two large international rivers. As a direct consequence, several Portuguese and Spanish towns and villages located in the banks of both rivers suffered serious flood damage on 7 December 1876. These unusual floods were amplified by the preceding particularly autumn wet months, with October 1876 presenting extremely high precipitation anomalies for all western Iberia stations. Two recently digitised stations in Portugal (Lisbon and Evora), present a peak value on 5 December 1876. Furthermore, the values of precipitation registered between 28 November and 7 December were so remarkable that, the episode of 1876 still corresponds to the maximum average daily precipitation values for temporal scales between 2 and 10 days. Using several different data sources, such as historical newspapers of that time, meteorological data recently digitised from several stations in Portugal and Spain and the recently available 20th Century Reanalysis, we provide a detailed analysis on the socio-economic impacts, precipitation values and the atmospheric circulation conditions associated with this event. The atmospheric circulation during these months was assessed at the monthly, daily and sub-daily scales. All months considered present an intense negative NAO index value, with November 1876 corresponding to the lowest NAO value on record since 1865. We have also computed a multivariable analysis of surface and upper air fields in order to provide some enlightening into the evolution of the synoptic conditions in the week prior to the floods. These events resulted from the continuous pouring of precipitation registered between 28 November and 7 December, due to the consecutive passage of Atlantic low-pressure systems fuelled by the presence of an atmospheric-river tropical moisture flow over central Atlantic Ocean.

Mortality Patterns of Hydro-Geomorphologic Disasters

Social impacts caused by floods and landslides in Portugal in the period of 1865-2010 are gathered in the DISASTER database. This database contains 1,902 hydro-geomorphologic cases that caused 1,248 fatalities (81% and 19% associated with floods and landslides, respectively). The use of the DISASTER database allowed for: (i) the analysis of the frequency and the temporal evolution of fatal floods and landslides; (ii) the analysis of the spatio-temporal distribution of fatalities; (iii) the identification of the most deadly flood and landside types; (iv) the verification of gender tendencies in mortalities; and (v) the evaluation of individual and societal risk. The highest number of flood and landslide cases and related mortalities occurred in the period of 1935-1969. After this period, the number of flood and landslide mortalities decreased, although landslide fatalities remained higher than those registered in the period of 1865-1934. The occurrence of flood fatalities was widespread in the country, with an important cluster in the Lisbon region and in the Tagus valley, while fatalities caused by landslides mainly occurred in the north of the Tagus valley. Flash floods caused the majority of fatalities associated with floods, while falls and flows were responsible for the highest number of fatalities associated with landslides. Males were found to have the highest frequency of fatalities. In the case of floods, the majority of fatalities were found both outdoors and inside of buildings in rural areas while fatalities inside buildings were dominant in landslide cases, mostly in rural areas.

Impacts of the North Atlantic Oscillation on Landslides

Western Iberia landslides are mostly triggered by rainfall, as, in fact, are most landslides worldwide. Results obtained using empirical relationships between rainfall amount and duration, and slope instability show that critical rainfall conditions for failure are not the same for different types of landslides. While rapid debris flows are usually triggered by very intense showers concentrated in just a few hours, shallow translational soil slips are most commonly triggered by intense precipitation falls within the 1–15 days long range. On the contrary, activity of the more deeply-seated landslides of rotational, translational and complex types is related to successive weeks of nearly constant rainfall, over periods of 30–90 days. Large-scale patterns such as the El Nino and the North Atlantic Oscillation (NAO) change slowly and have been shown to have an impact in both the precipitation regime and the temporal occurrence of different landslide types in different areas of the world. In this work a particular attention is devoted to the impact of NAO on the landslide events that have occurred in the region located just north of Lisbon between 1956 and 2010. Results show that the large inter-annual variability of winter precipitation observed in Portugal is largely modulated by the NAO mode. The application of a 3-month moving average to both NAO index and precipitation time series allowed the identification of many months with landslide activity as being characterized by negative average values of the NAO index and high values of average precipitation (above 95 mm/month). Landslide activity in the study area is related to both intense, short duration precipitation events (1–15 days) and long-lasting rainfall episodes (1–3 months). The former events trigger shallow translational slides while the later episodes are usually associated with deeper and larger slope movements. The association between the NAO and landslide activity is shown to be more evident for the group of deep seated landslide events.

Structure and Characteristics of Landslide Input Data and Consequences on Landslide Susceptibility Assessment and Prediction Capability

The main objective of this study is to assess the influence of landslide representation format (i.e. landslide represented as points or areas) in landslide susceptibility results, especially at scales that can directly interfere with spatial planning. For the study area of Rio Grande da Pipa basin, Arruda dos Vinhos, Portugal, the Information Value method is used to statistically integrate two rotational slides groups (deep and shallow) and a dataset of independent predisposing geo-environmental factors. For both landslide groups, landslides were represented by: (i) the landslide area; (ii) the landslide depletion area; (iii) the centroid of landslide area; and (iv) the centroid of landslide depletion area. Additionally each group was randomly partitioned in two equivalent landslide sub-groups (50–50%), one for modeling and the other for independent validation of the landslide susceptibility maps. The evaluation of the landslide representation format on the prediction capacity of each landslide susceptibility model was based on Receiving Operating Characteristic curves and in the calculation of Area Under the Curve. As main results this work points out the sensitivity of landslide susceptibility models prediction capability to the landslide representation format. Consistently, for both landslide groups, the better predictive results were achieved by modeling with the landslide depletion area and validating with landslide depletion area and the worst results by modeling with landslide centroid and validating with the landslides area. Furthermore the same hierarchy of landslide representation formats regarding the prediction capability of the landslide susceptibility models was recorded independently of being deep or shallow rotational slide types.

Coastline at Risk: Methods for Multi-Hazard Assessment

ABSTRACT SOLDATI, M., MAQUAIRE, O., ZEZERE, J. L., PIACENTINI, D and LISSAK, C, 2011. Coastline at Risk: Methods for Multi-Hazard Assessment. In: Micallef, A. (ed.), MCRR3–2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 335–339. Grosseto, Tuscany, Italy, ISSN 0749-0208. In recent years the interest for coastal hazards has increased significantly due to extreme events, sometimes related to climate change, that frequently occur in different parts of the world, often inducing high risk situations. This paper outlines the objectives, perspectives and preliminary results of a research project entitled “Coastline at risk: Methods for multi-hazard assessment” which aims at providing useful knowledge for the mitigation of coastal instability. The project is managed by the European Centre on Geomorphological Hazards (Centre Européen sur les Risques Geomorphologiques, CERG) and funded by the EUR-OPA Major Hazards Agreement. Investigations are focused on coastal landslides in three different morpho-climatic European environments: Malta (Mediterranean coastline), Lower Normandy (Channel coastline) and Central Portugal (Atlantic coastline). The research outputs are expected to provide a significant opportunity for scientific discussion based on the comparison of data regarding instability situations in the context of multi-hazards assessment. The latter has until now dealt slightly with the coasts of Malta, Normandy and Portugal on which the investigations will be focused, despite significant risk issues present there, as evidenced from a series of accidents and damages recorded after landslide events. The aims of the project will be pursued through multidisciplinary investigations which foresee geomorphological and engineering-geological approaches. Integrated avant-garde research techniques, both traditional and innovative, will be applied with special reference to mapping, monitoring and modelling of coastal instability phenomena. The final objective is to propose a method for coastal multi-hazard assessment that can be used to face and manage coastal hazards.