Characterization of Nappe Vibration on a Weir

The nappe vibration phenomenon may affect numerous prototype structures, such as gates and spillways, when they operate under low head conditions. This phenomenon, which is far not well understood and poorly controlled, produces frequency oscillations in the thin flow nappe cascading downstream of the weir. These oscillations result in a disturbing noise production that increases the environmental and societal impacts of the hydraulic structure. Given limited information available regarding the physical processes of this phenomenon, a detailed investigation has been undertaken to characterize the flow for free-overfall structures where nappe vibration may be of concern. The research conducted on a prototype-scale linear weir model (weir length of 3.5 m and a fall height of 3 m) at the Engineering Hydraulics laboratory of the University of Liège enable to describe the flow and nappe vibration by using image and sound analysis. The paper presents the first quantitative results of the study.