Energy dissipation on stepped fall manholes

Fall manholes are often necessary for sewer systems in steep catchment areas. A particular type of fall manhole is the one made up of stepped channels. As energy dissipators, stepped spillways can be easily inserted inside a sewer system. This work describes the results of experiments on stepped spillways conducted at the Hydraulic Laboratory of the University of Pisa in a rectangular cross section channel. The tests aimed to evaluate the amount of energy dissipation caused by this type of structure characterised by a low number of steps. In the first part of the_experiments, the model has been equipped with a simple drop; then other tests have been carried out on 9 different model-types, characterised by different slope values and by a different number of steps. The work points out that the design of a structure with a low number of steps is deeply connected with flow conditions over the structure (that is nappe flow or skimming flow). In this way, the work provides graphs and relations that allow the knowledge both of the hydraulic behaviour and of the corresponding amount of lost energy. Concerning the efficiency of the studied structures, tests show that an increasing number of steps causes a greater dissipation, but, at the same time, a smaller range of existence of the nappe flow condition. A comparison among different energy dissipation structures has been finally realised.