Uncertainties in laboratory modelling of urban flooding

In the first part of this communication, we will discuss the main uncertainties affecting laboratory scale modelling of urban flooding and their relative importance. These include uncertainties in hydrological data, on roughness representation, on exchanges between overland flow and the drainage network, measurement uncertainties as well as the influence of overlooked processes.
In a second part, we will focus on the issue of scaling, particularly for lab experiments representing urban flooding at the district level. Laboratory scale models of urban flooding tend to use distinct scale factors (ratio between prototype and model dimensions) along the horizontal and vertical directions (to avoid millimetre-scale water depths in the lab, or giant lab setups hardly possible to fit in a hydraulic lab). This leads to so-called geometrically distorted scale models. It is believed that this strategy ensures improved accuracy and representativeness of the measurements; but specific artefacts also arise from the model distortion. Here, we provide first quantitative insights into the effects of model distortion, based on a recent reanalysis of existing experimental datasets. In the tested configurations, the influence of model distortion on the predicted values of water depth and street discharges is found of the order of 10%.
We will conclude with recommendations on the way to go to make the optimal use of laboratory scale modelling for addressing the needs of the flood risk community.