Supercritical Flow in Bend Manhole

Supercritical flow in a bend manhole for combined sewer systems is considered for a typical relative curvature and deflection angles of 45° and 90°. The typical features of bend flow in a U-shaped channel are determined, including the surface profiles along the inner and the outer bend walls, impact flow at the manhole outlet into the downstream sewer pipe, air entrainment characteristics, and wave development in the downstream sewer. Also, typical velocity distributions along the manhole are presented, and the capacity of the conventional bend structure is determined. To increase the discharge capacity across a bend manhole, the so-called bend cover is introduced. This cover element finished in steel can be removed for maintenance and inspection purposes. It limits the wave height close to the bend outlet to 90% of the sewer diameter and allows air entrainment into the downstream sewer. The performance and capacity of bend flow may significantly be increased, and the novel element may easily be added to existing manholes.