Subcritical Side-Weir Flow at High Lateral Discharge

Side weirs count among the important hydraulic structures for flow derivation in flood schemes or sewer networks. In this paper, the prismatic side weir in the rectangular channel is considered for entirely subcritical flow. Based on the momentum and energy equations, relationships are developed for the lateral outflow intensity, the local lateral outflow angle, the energy and momentum correction coefficients, and the relationship between the local streamwise component of the lateral outflow velocity and the average channel flow velocity. The governing equations are then numerically solved and the results compared with hydraulic laboratory observations, indicating that the momentum approach may be considered superior to the energy approach. The role of the momentum and energy correction coefficients is thereby particularly analyzed, stating that their correct inclusion both as absolute values and their local variation along a side weir have to be accounted for if the local discharge relative to the approach flow discharge tends to zero.