Abstract
Urban flooding is a phenomenon which reflects the increasing urbanization process of the territory. Although not intensively studied in Portugal, urban flood episodes are growing throughout the country in frequency and affected areas. This type of flooding, assuming in many cases the flash floods characteristics, is the result of water concentration in topographically depressed areas during episodes of intense rainfall over short time. This floods events are enhanced by soil sealing and by malfunction or undersized pluvial rainwater drainage systems. The most critical phase of these events occurs when the capacity of this underground drainage system is exceeded, causing the inversion in flow direction and consequently, its concentration in nodal points and emergence at the surface. To understand the dynamics of the surface runoff in the Arouca urban area (Portugal), it was analysed the role played by each element of the urban grid towards the surface runoff process. For this, it was considered: (1) the slopes that favor the rapid and intense spread of the flow, (2) the topography and the urban morphology that interfere directly in the surface drainage pattern. By the interpretation of aerial photos and recent orthophotos, it was also analysed the evolution of land use in the last five decades, aiming to understand the effect that gradual sealing promoted in the increase of the superficial flow component. The collected inventory has enabled the identification of the most susceptible areas in the village and the exposed elements to this risk. The most problematic areas corresponds to the lower areas of the village and some streets where occurs a dual concentration of the runoff, i.e., the surface and the pluvial drainage flows. On the other hand, there was an increase of 21% on artificial surfaces, as a consequence of forest and agricultural areas decline between 1958 and 2005. The chosen methodological approach and the obtained results are a contribution to improve the land planning on the Arouca municipality.