Hydraulic features of supercritical flow along prismatic side weirs

Side weirs are key structures often located in combined sewer systems, provided the upstream flow is subcritical. At present, knowledge of the outflow process when the flow along the weir is subcritical can be considered satisfactory. The same cannot be said when, as often happens in practice, the upstream flow is subcritical while the flow along the weir is supercritical. In this paper the local flow features along prismatic side weirs in a circular channel were investigated. In particular, the condition of supercritical flow along the weir was analysed. Based on experimental work and on data available in the literature, some characteristics of both the flow that remains in the main channel and the flow that leaves it were investigated. These include: the distribution of the discharge in the main channel, the lateral outflow angle and the lateral outflow velocity. The results combine to provide a better understanding of the outflow process and are also readily applicable for design. Finally, a theoretical global approach is presented to estimate the head loss due to the flow partition.