Impact of the porosity of an urban block on the flood risk assessment: A laboratory experiment

Abstract

Physical modelling of urban flooding usually considers that the flow is concentrated in the streets, assuming that urban blocks and individual buildings are non-porous structures. This assumption is open to criticism, as potential lateral flow exchanges through openings in the walls of an urban block (e.g. gates, windows and doors) are likely to modify the flow pattern in the block and adjacent streets. This paper reports an experimental study conducted on a 5.4 m long and 3.2 m wide physical model, representing a rectangular urban block and four surrounding streets. The focus of the study was on investigating the lateral flow exchange processes between the porous urban block and its adjacent streets during flooding, for a variable urban block porosity. Eight porosity values were studied, keeping the same boundary conditions. A porosity equals zero, i.e. a flow around a non-porous block, served as a reference flow situation. For each porosity value, the impact of the lateral flow exchanges on the spatial distribution of flow depth and velocity in the streets and within the block was assessed, as well as the risk to pedestrians. First, it was found that, in the streets surrounding the block, flow depth and depth-averaged velocity can vary by 12% and 70%, respectively, when modifying block porosity. Within the block, the most impacted flow parameter is the number and size of horizontal secondary flow cells. Second, noticeable differences in flow depth can be observed from either side of an opening between street and block. Third, when moving away from the block, flow parameters are little affected. Last, the risk to pedestrians, which is both related to local flow depth and velocity, is dependent on the block porosity value.