Piano Key Weir Submergence in Channel Applications

Weir submergence can influence head-discharge relationships for weirs used in channel applications when high tailwater conditions exist owing to a downstream control. Weir submergence describes a condition in which the water level downstream of the weir exceeds the weir crest elevation. When a weir becomes submerged, the driving head required to pass a specific discharge over the weir can increase significantly relative to a free-flow condition. In this study, the effects of tailwater submergence on laboratory-scale piano key weir head-discharge relationships were evaluated experimentally and compared with previously published data for labyrinth and sharp-crested linear weir submergence. The results of this comparison show that for relatively low levels of submergence, the piano key weir requires less upstream head relative to the labyrinth weir to pass a given discharge. This increase in efficiency was minimal (<6%) and was reversed at higher submergence levels.