Preliminary assessment of flood hazard in Nabão River basin using an analytical hierarchy process

Floods are natural phenomena that have a major impact on society, agriculture and socioeconomic activities. Therefore, flood hazard mapping plays a key role in the decision-making process since it provides essential information to improve the understanding of the nature, risk and characteristics of floods. The application of a GIS-based multi-criteria analysis in the context of flood risk assessment has been recognized as a powerful tool to integrate and analyze data from different sources, thus providing different scenarios. The present work introduces the results of a preliminary assessment of flood hazard areas in the Nabão River basin located in the center of Portugal. Results show that 12.5% of Nabão River basin is prone to moderate flood hazard. It is noticeable that, 15% and 2.2% of this basin is susceptible to high and very high flood risk, respectively. These areas are those closest to Nabão River and its tributaries, as expected and are generally laying at low elevations. A high concentration within settled and paved areas, mainly affecting Tomar city and a relevant area within this municipality was also depicted. However, urban paved surfaces hinder water infiltration of runoff, thus increasing the flood risk, results also show the relevance of land use and soil type within this basin. The lower infiltration and poor planning in the vicinity of alluvial plains and river banks can also be inferred in non-urban areas that revealed to be highly vulnerable to floods.