Safety and Reliability of Complex Engineered Systems – Podofillini et al. (Eds) © 2015 Taylor & Francis Group, London, ISBN 978-1-138-02879-1 Historical flood events in the Tagus estuary: Contribution to risk assessment and management tools A. Rilo & P. Freire National Civil Engineering Laboratory, Lisbon, Portugal P.P. dos Santos & A.O. Tavares Centre for Social Studies and Earth Sciences Department, University of Coimbra, Coimbra, Portugal L. Sá National Authority for Civil Protection, Carnaxide, Portugal ABSTRACT: Flood risk assessment in estuaries requires validated approaches that are able to integrate the multiplicity of hazard forcings. Historical events are valuable in improving the knowledge about flood characteristics and their impacts. The present study aims to present a preliminary analysis of flood event triggers based on historical data. A geodatabase was constructed based on national newspapers, databases and photographic archives from national institutions, and includes 236 occurrences corresponding to 46 events. Flood events are particularly distributed between October and February and associated to multi- ple flood triggering factors particularly rainfall combined with deficient urban drainage conditions. This study provided a good insight about the contribution of different forcing mechanisms in Tagus estuary flood events which information is both useful for flood forecasting validation, and management purposes as future land use planning. In future work, comparison with instrumental data will enable the assessment of historical data sources uncertainty. in estuaries sea level rise and growing storminess (IPCC 2013) will intensify flood events (Bilskie et al. 2014). Moreover, human pressure develop- ment in estuarine margins will enhance the impact of extreme water levels and risk exposure. Information of past flood events can contrib- ute to improve the knowledge about flood driv- ers, event characteristics and associated impacts with particular emphasis on the pre-instrumental period (Barrientos & Rodrigo 2006, Bayliss & Reed 2001). Several databases including historical floods events are available, differentiated by the scale of approach and the inclusion criteria (Mysiak et al. 2013, Santos et al. 2014), mainly for risk manage- ment (eg. Kron et al. 2013) and emergency pur- poses (eg. EM-DAT 2013). In Portugal, about 75% of the population lives in the coastal region. Most of the major cities, including the capital, are located in the vicinity of estuaries. Located in the west coast, the Tagus estuary (Fig.1) has an area of about 320 km 2 , housing 18 municipalities of the metropolitan area of Lisbon with about one million inhabitants (INE 2012). Estuarine hydrodynamic conditions and water 1 INTRODUCTION Flood risk management and adaptation strategies require accurate hazard prediction to support the assessment of potential losses and consequences (Simonović 2012). Successful efforts have been made in recent decades to develop accurate and timely flood predictors, particularly for river floods and coastal floods along open shores (Vanderkimpen et al. 2009, Ghimire 2013, Fortunato et al. 2013). In estuaries, flood forecasting is still a challeng- ing task due to the lack of suitable methodologies to cope with the interaction of different haz- ard sources as tides, river flows, wind and waves (Cluckie et al. 2000). Hosting important urban areas along its margins, estuaries present high risk to flooding (Morris et al. 2013, Carrasco et al. 2012), well demonstrated by the economic and human impacts of recent events, such as the hur- ricane Katrina (2005) in New Orleans (Miller et al. 2015), the storm Xynthia (2010) along the French coast (Bertin et al. 2014, Andre et al. 2013) and the hurricane Sandy (2012) in New York (Aerts et al. 2013). Flood risk is expected to increase with changing climate (Allison et al. 2009), and