Could Piano key weirs be subject to nappe oscillations?

Nappe oscillation may affect a variety of free overflow hydraulic structures operating at low heads such as crest gates and various types of weirs. This phenomenon produces oscillations in the thin nappe cascading downstream of the control structure. These oscillations can result in a high noise levels that may affect people and structures located nearby. With long linear and thin overtopping walls, piano key weirs might be subject to strong nappe oscillation risks, as a few documented cases with labyrinth weirs. However, the specific geometric features of piano key weirs, such as the outlet ramps or the limited width of the inlet apex, could perhaps prevent the development of nappe oscillation. In this paper, a prototype-scale linear weir is operated to assess the risk of nappe oscillations on a piano key weir like configuration (sidewall). In particular, the effect of the weir width and inclined apron is analyzed. The experimental results show that nappe oscillation development should not occur on piano key weir, contrary to what might be observed on a labyrinth weir with the same crest.