Abstract
The study of scour mechanism downstream of low-head control structures is a fundamental topic for hydraulic engineers. Generally, the analysis of the scour process is conducted under steady flow conditions, assuming that the maximum discharge is occurring for sufficient time to reach the equilibrium scour configuration. Nevertheless, in rivers the scour process generally occurs in correspondence with a flood event, which is characterized by discharge varying with time. This last condition is still less studied and analyzed in terms of effects on bed morphology. Researchers mainly focused on the maximum scour depth assuming that it occurs in correspondence with the peak discharge, but they rarely took into account the evolution of the scour process under unsteady flow conditions. The aim of the present paper is to analyze the evolution of scour morphology under unsteady flow conditions, and compare it with that obtained under steady flow conditions. In particular, three structure typologies were tested: a stepped gabion weir with upstream impermeable filtering layer, a straight rock sill, and a curved rock sill. The results showed that the scour phenomenon deeply depended on inflow conditions. Nevertheless, it was also shown that the equilibrium morphology of the downstream stilling basin is essentially the same under both unsteady and steady flow conditions if the duration of the unsteady event is enough long.
1. Introduction
The scour mechanism occurring downstream of low-head control structures is an important topic that has been widely analyzed in recent decades. In particular, the analysis mainly focused on the hydraulics and the scour characteristics in the stilling basin. Lowhead structures have been found effective in controlling sediment transport and, at the same time, they are able to guarantee a reduced impact on the ecosystem. Therefore, many traditional structures have been re-converted into more eco-friendly anthropic works, such as block ramps, rock grade control structures, stepped gabion weirs, cross-vane, W-weirs, etc.
4. Conclusion
This paper analyzed the effect on maximum scour depth of the inflow conditions. Namely, the analysis was conducted in the presence of both stepped gabion weirs and rock sills. References tests, under steady flow conditions, were conducted in order to obtain the reference values of the maximum scour depth occurring in the downstream stilling basin. The same tests were repeated under unsteady flow conditions. The hydrograph adopted to simulate the unsteady flow conditions was characterized by a decreasing discharge phase whose duration is double of the increasing discharge phase.
