Circular drop manholes have been an important device for energy dissipation and reduction of flow velocities in urban drainage networks. The energy dissipation in a drop manhole depends on the manhole flow patterns, the outflow regimes in the exit pipe and the downstream operation conditions, and is closely related to the hydraulic and geometric parameters of the manhole. In the present work, the energy dissipation of a drop manhole with three drop heights was experimentally investigated under free outflow conditions and constrained outflow conditions. The results demonstrate that the local head loss coefficient is solely related to the dimensionless drop parameter for free surface outflow without a downstream backwater effect, whereas it depends on the dimensionless submerge parameter for constrained outflow. Moreover, it is concluded that the energy dissipation is largely promoted when outlet choking occurs.

A positive surge is an unsteady open channel flow motion characterized by an increase of flow depth. In previous experimental studies, a positive surge was typically induced by either a sudden increase of discharge in a channel or by the rapid closure of a downstream sluice gate, thus leading to a steep initial profile. However, in many instances, the evolution of a positive surge is of a progressive manner (e.g., in the downstream navigation canal during the emptying operation of lock chambers). In the present work, the inception and development of a positive surge induced by a progressive increase of discharge was investigated in a rectangular channel with a smooth bed. Both undular and breaking surges were studied. The results demonstrate that the maximum wave height at the first wave crest of an undular surge is in very close agreement with the McCowan theory. Additionally, the wave amplitude essentially shows a linearly increasing trend with an increasing surge Froude number up to Fr₀ = 1.26 to 1.28, whereas it tends to suggest a power law reduction for larger surge Froude numbers. Moreover, the dispersion of undular surges is consistent with the linear wave theory only for surge Froude numbers close to unity. Overall, the present study demonstrates the unique features of positive surges induced by a progressive increase of discharge.

In this study, physical experiments were performed in a horizontal rectangular channel to investigate the evolution of undular surges that propagate in still water and the reflection of these surges on a vertical wall. The generated undular surges were located in the intermediate water depth wave regime. The results demonstrated that the maximum wave steepness attained by undular surges was close to the breaking limit of progressive waves at intermediate water depths. Nevertheless, the dispersion relationship was inconsistent with the wave dispersion trend for intermediate gravity waves and strongly depended on the initial flow conditions and surge type. Furthermore, it was found that the longitudinal current velocity was governed primarily by water surface displacement. A novel and universal formula based on the experimental results was proposed to describe the velocity distributions in different types of undular surge flows. In addition, a non-breaking undular surge behaved as a solitary wave regarding its reflection on a vertical wall. Overall, the findings highlighted the effects of the initial flow conditions on the hydrodynamic characteristics of undular surges and demonstrated the unique features of undular surges that propagate in still water.