Methodology for qualitative urban flood risk assessment

Abstract

Urban pluvial flooding has the potential to cause significant damage and disruption as it often occurs in highly urbanized areas (residential, commercial or industrial occupation). Flooding are already frequent events and their frequency is expected to increase due not only due to urbanisation but also due to expected climate changes (Ugarelli et al., 2011). Recent developments in urban flood modelling (e.g. Leitão, 2009; Maksimović et al., 2009) allow carrying out reliable simulations of the whole drainage system, including both sewer and surface drainage systems. Several references can be found in the literature (e.g. Apel et al., 2009; Douglas et al., 2010) with examples of usage of urban drainage modelling results to estimate the consequences of flooding and consequently assess flooding risk. In this study, 1D/1D model results are used to assess urban flooding risk caused by intense rainfall and subsequent surface water runoff and limited sewer hydraulic capacity. The risk approach used is based on the combination of likelihood and consequence levels, using a risk matrix - a qualitative risk assessment method. Five levels of both likelihood and consequence are defined by the risk analyst. Based on the risk level, different actions may be considered to tackle the flooding problem and mitigate its impacts. The methodology was applied to a real case, a small, densely urbanised catchment located in Lisbon (Portugal) that has experienced frequent pluvial flooding events in recent years. Two rainfall events (Storm A and Storm B) and three consequence dimensions were considered: flooding effects on public transportation services, properties and pedestrians safety.