Protected stilling basins downstream of low-head river training structures: energy dissipation

Low-environment impact structures have become very popular in the last decades. They are widely used in river restoration both to control sediment transport and, at the same time, to create optimal conditions for fish species. They generally improve water quality due to higher oxygenation. Nevertheless, these are complex structures which require a particular attention to the effects of their presence on the local eco-system. Among this structure typology, block ramps are probably the most common. They are characterized by a quite complex flow regime and by significant erosive processes occurring downstream of them. Therefore, the knowledge of hydraulic characteristics is a fundamental aspect for a correct functioning of the structure. In particular, the downstream stilling basin morphology strongly depends on both hydraulic and geometric parameters. Namely, the stilling basin geometry has to be carefully analyzed in order to prevent structural collapse and to create opportune pools where fishes can find an optimal natural habitat. This paper focuses on energy dissipation process occurring in correspondence with block ramps and downstream stilling basins. The analysis of the dissipative phenomenon is conducted in the presence of protection structures (sills) located in the stilling basin in order to minimize scour characteristics. Two stilling basin configurations were compared: prismatic configuration having the same width of the ramp and symmetrically enlarged configuration. The energy dissipation for both the tested configurations was analyzed and discussed emphasizing the effect of spatial positions of the sills on it.