Performance of a shallow-water model for simulating flow over trapezoidal broad-crested weirs

Shallow-water models are standard for simulating flow in river systems during floods, including in the near-field of sudden changes in the topography, where vertical flow contraction occurs such as in case of channel overbanking, side spillways or levee overtopping. These flow configurations are similar to the flow over a broad-crested weir with the trapezoidal profile in the flow direction (i.e. inclined upstream and downstream slopes). Results of shallow-water numerical modelling were compared with seven previous experimental observations of flow over a frontal broad-crested weir, to assess the effect of vertical contraction and surface roughness on the accuracy of the computational results. Three various upstream slopes (1 : 1, 1 : 2, 1 : 3) and 3 roughness scenarios of the broad-crested weir were tested. The results indicate that for smooth surface numerical simulations overestimate by about 2 to 5 % the weir discharge coefficient, in case of rough surface the maximum difference accounts about 10 % for high relative roughness. The conclusions may be extended to 2D shallow simulations for more complicated configurations including e.g. side weir.