Analysis and assessment of urban flash floods on areas with limited available altimetry data (Arouca, NW Portugal): a methodological approach

The flooding of small water courses is a key issue in numerous urban areas. In Arouca village (NW Portugal) the urbanisation process in the past three decades promoted a complex urban hydrological response which has been reflected on the increase of urban flooding events. Therefore, a methodological approach was applied to assess problematic areas to urban flash floods taking into account the lack of altimetry data. For this purpose, the precipitation events and the rainfall contributing to the urban hydrodynamics were analysed and an inventory of the surface runoff arrangement was performed through fieldwork established by the urban fabric structure and based on the analysis of the village topography. The critical places selected from this inventory were then confirmed by historical and recent records of urban flooding events. The results show that there are two main contributory areas that deliver around 192 m³/s, in 1 h and 45 min, for a return period of 10 years. The values of precipitation related with flood events show that there are four different scenarios: rapid peaks (I), gradual peaks (II), spaced peaks (III) and a mixed situation between the first two situations (IV). The runoff survey reveals a significant slope for most of the streets, which promotes a rapid concentration of runoff in the ponds. The exposed elements identified are located precisely in the old alluvial plain, where the Arda River flowed freely until the 1950s. These results show how rapid urbanisation of a traditional rural area contributes towards the occurrence and the increasing number of flood events.