Energy Dissipation in Drop Manhole Cascades

Drop manholes are hydraulic devices typically employed to dissipate flow energy in high slope, urban drainage systems. The use of several drops of small height placed in series (drop manhole cascade) is a more economical and effective method for controlling velocity and kinetic energy than a single drop of large height. An experimental campaign was specifically designed to investigate hydraulics of drop manhole cascades at the LIA Laboratory, University of Cassino and Southern Lazio. Results were summarized in equations that quantify overall dissipation performance and describe the hydraulics of the downstream flow in terms of outflow depth and energy head. The performance of single drops and a three-drop cascade are compared. The data show how the energy of the stream passing through the dropshaft cascade quickly achieves a new equilibrium condition downstream from the second manhole. Finally, the energy dissipation of a drop manhole cascade was compared with that of smooth and roughened chutes to highlight similarities and differences between different dissipation structures.