13When simulating urban floods, most approaches have to simplify the topography of the city and cannot 14 afford to include the obstacles located in the streets such as bus stops, trees, parked cars, etc. The aim of the 15 present paper is to investigate the error made when neglecting such singularities in a simple flooded 3-branch 16 crossroad configuration with a specific concern regarding the error in discharge distribution to the 17 downstream streets. Experimentally, the discharge distribution for 14 flows in which 9 obstacles occupying 18 1/6 of the flow section are introduced one after the other is measured using electromagnetic flow-meters. The

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SUMMARYThe passage of an extreme storm over an urban area can lead to the flooding of the streets if the rainfall intensity exceeds the design value and/or the drainage system is not functional. The study of flow distribution in street networks thus is important for the design of flood protection measures. The flow distribution is affected by the junction flow characteristics, inflow discharges and downstream water depths. To reduce the degree of empiricism, a 3D Reynolds-averaged Navier-Stokes equations model has been implemented in this study to investigate the flow phenomena in a cross junction. The SpalartAllmaras model is used for turbulence closure. The numerical model utilizes the split-operator approach, in which the advection, diffusion and pressure propagations are solved separately. The numerical model predicts accurately the flow distribution in a channel crossing under different subcritical flow conditions, for which experimental data are available. Recirculation zones exist at both the downstream channels and the associated contraction coefficient varies linearly with the ratio of the discharges at the two inlets. Secondary currents are apparent for the flow with strong asymmetric outlet conditions. Under supercritical inflow conditions, the model reproduces the hydraulic jump and hydraulic drop phenomena and predicts accurately the relationship between the input power ratio and the outflow discharge ratio of the street crossing.