Flood impact assessment in urban settings with porous buildings. Insights from a fine-scale hydraulic-economic model

Classical hydraulic approaches of urban floods consider buildings as obstructions to water flow, without considering the flow exchanges between streets and buildings. Since the hydraulic behavior within the building is not considered, there exists a gap on the available information regarding the fine characterization of material damage and human exposure within buildings. Because of this gap, damage estimation usually assumes that the floodwater depths inside and outside the buildings are the same. However, as flood damage functions are very elastic with respect to floodwater depth, especially in the lower values, relatively small differences in water depth can lead to large differences in the assessment of material damage. Not considering street-building flow exchanges might be introducing a bias in the estimation of property damage. In this paper, we propose to analyse how fine-scale hydraulic and economic modelling approaches considering street-building flows can influence the characterization of material damage at a larger scale (district). To do so, we couple a hydraulic model with an economic model to simulate water depths and to estimate flood damage under: i) a classical approach using non-porous buildings and ii) an alternative approach explicitly considering street-building flow exchanges. The results obtained show rather notable differences in floodwater depth and economic damage at both building and district level.