Supercritical low-crested bilateral weirs: hydraulics and design procedure

Sideweirs are hydraulic structures commonly used in combined sewer systems for separating excess storm water from urban sewage. In the case of low-crested sideweirs, the design is difficult because supercritical flows may establish along the overflow reach. This can lead to transcritical conditions not easily modelled in many cases. The present paper analyses the supercritical outflow process over a low-crested bilateral sideweir in a circular channel, based on both experimental and numerical investigations. Experiments were conducted to define the approach water depth and the free-surface profiles. The measurements were compared with the numerical values resulting from the application of energy and momentum conservation principles. The paper shows that the formulas used for estimating the end depth at free overfalls in circular channels can be applied to evaluate the approach water. The numerical investigation proves that the energy conservation approach allows for the computation of free-surface profiles with reasonable accuracy.