Impact of topographic obstacles on the discharge distribution in open-channel bifurcations

When simulating urban floods, most approaches have to simplify the topography of the city and cannot afford to include the obstacles located in the streets such as bus stops, trees and parked cars. The aim of the present paper is to investigate the error made when neglecting such singularities in a simple flooded 3-branch crossroad configuration with a specific concern regarding the error in discharge distribution to the downstream streets. Experimentally, the discharge distribution for 14 flows in which nine obstacles occupying 1/6 of the flow section are introduced one after the other is measured using electromagnetic flow-meters. The velocity field for one given flow is obtained using horizontal-PIV. Additionally, all these flows are computed using a CFD methodology. It appears that the modification in discharge distribution is mostly related to the location of the obstacles with regards to the intersection, the location of the separating interface and is strongly impacted by the Froude number of the inflow while the influence of the normalized water depth remains very limited. Overall, the change in discharge distribution induced by the obstacles remains lower than 15% of the inflow discharge even for high Froude number flows.