Author(s): Cheng Zeng; Chi-Wai Li; Anuel Mignot; Nicolas Riviere
Keywords: Open-channel dividing flow; Obstacles; Numerical modeling; Drag force method; Discharge distribution
Abstract: For practical numerical modeling of urban floods, the computational domain is large and generally the grid resolution is relatively coarse to save computational effort. The complex topography of a city has to be simplified and the obstacles located in the streets such as bus stops, trees, parked cars are neglected. Mignot el al. (2013) showed experimentally that for dividing flows in a street junction, the presence of one or two obstacles of size equal to 1/6 of the street width will induce a change in the discharge distribution within 15% of the incoming discharge rate. This implies that the impact of obstacles is not very substantial and a simplified modeling of the obstacle effect may be sufficient for accurate determination of the discharge distribution and the associated water levels. In the present work a 3D numerical model is developed by adopting the drag force method (DFM) to simulate the blocking effects of obstacles. A quadratic law is used to relate the resistance effect to the squares of the flow velocity, and a large resistance coefficient (frk) is used to restrict the flow into the obstacle. The experiments conducted by Mignot et al. are replicated in the numerical simulation. A total of 126 cases are simulated. The impact of the frk-value on the flow pattern, water surface elevation and discharge distribution is firstly verified. The results show that the change in discharge distribution by the presence and location of the obstacles can be accurately predicted, although the fine flow features around the obstacle are not very accurate. The drag force method is concluded to be effective and efficient to simulate the resistance effects of obstacles on flow distribution in street junctions.