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Typical urban flood flow features are usually computed using two-dimensional numerical models. How such modelling can be implemented in dense urban areas with obstacles is investigated. A strategy for representing the effect of urban obstacles in various flow conditions is defined. The comparison between the available laboratory measurements and the model results show that if the water depth is high enough and the flow remains subcritical, two-dimensional modelling with constant eddy viscosity provides the effect of the obstacles on the flow distribution accurately, even with a coarse mesh; moreover, an oversimplified representation of the sidewalks averaging the street cross section elevations seems sufficient. Oppositely, if the water depth is low and/or the flow becomes supercritical, the description of the flow is not relevant enough and it generates errors in the flow distribution at the crossroads.

Floods in dense urban areas propagatemainly through the streets, where the flow can be locally affected by elements of urban topography. This study aims at assessing the need of integrating detailed topography in numerical models when simulating urban floods. Acoustic Doppler Velocimetry and Large Scale Particle Image Velocimetry measurements in an experimental three branch junction representing a city crossroad are used to calibrate a numerical model solving the 2D shallow water equations. A constant eddy viscosity model proves to be accurate enough to calculate velocity fields, but such model requires a fine calibration against experimental data. Simulations run with this calibrated model are performed to study the impact of obstacles and sidewalks representative of urban areas. It is found that obstacles located in the downstream branch can highly perturb the velocities distribution downstream of the junction, whereas obstacles located in the upstream branches have less influence. The presence of sidewalks results in reduced flow section and higher velocities, but additional effects occur within and downstream of the junction. Simulations presented here show the need of considering detailed topography and elements of urban furniture if local velocities have to be represented.

Because stakes are concentrated in urban environment, flood modelling should be carried out there with high accuracy. However, uncertainties remain important because of:-the estimate of the inputs, these inputs being either rainfalls, or flows coming from the upstream basin or from the exchanges with the sewage network;-the complexity of the processes, which includes a frequent change of the flow regime-from subcritical to supercritical and oppositely-, the difficulty in predicting the flow distribution at the street junctions, the oscillations created by waves reflecting against , for instance, obstacles or walls of houses;-the parameters of the flooded area: variable roughness, very irregular topography, moving obstacles in the streets, water entering the built-up areas, etc. An assessment of these uncertainties is presented for a few examples coming from laboratory experiments or from the two-dimensional modelling of flood events in French towns. These latter results can be used in order to estimate the range of uncertainty during field studies.

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