Flood impact assessment in urban context. How important is it to couple hydraulic and economic models at a fine scale?

Estimating the impacts of flooding in urban areas is of primary importance when qualifying the exposure of territories. One of the difficulties encountered is that the processes at stake involve fine scales (building scale). Classical hydraulic approaches of urban floods consider buildings as obstructions to water flow, without taking into account the flow exchanges between streets and buildings. However, according to the literature, it seems necessary for characterizing material damage and human exposure in terms of buildings' dangerousness.

Regarding the estimation of material damage, it is usual to assume that the water levels inside and outside the buildings are the same. This practice does not take into account the hydrostatic effects of flooding. As flood damage functions are very elastic with respect to water levels, especially in the lower values, relatively small differences in water levels can lead to large differences in the assessment of material damage. For these reasons, not taking into account the flow exchanges between streets and buildings may introduce a bias in the estimation of property damage.

We propose to analyse how fine-scale approaches (building,) taking into account flows, can influence the characterization of material damage at a larger scale (district). We use the Richelieu district of the city of Nîmes, in the south of France to illustrate our work, taking as reference the 1988 event. We couple hydraulic and economic models to simulate water depths following i) a classical approach of building treatment and ii) an alternative approach explicitly taking into account street-building flow exchanges. The simulated water depths are then fed to the economic model, allowing us to determine, by comparing scenarios, possible bias and its magnitude.

The Preliminary results show significant differences in water levels inside buildings compared to outside. In terms of damage, at the district level, not taking into account street-building flows leads to an overestimation of material damage. These results invite us to carry out complementary analyses at higher levels of resolution, by considering the dynamics of the flows inside the buildings and their repercussion in terms of material damage but also of danger.