Labyrinth Weirs: Nappe Interference and Local Submergence

The interaction of nappes from adjacent weir walls near an upstream apex can cause local submergence, which reduces labyrinth weir discharge efficiency. Local submergence effects can cause the head-discharge relationships for labyrinth weirs of equivalent lengths but different cycle numbers to differ. Empirical labyrinth weir discharge coefficients implicitly account for apex effects and local submergence; however, the uncertainty associated with applying hydraulic design data developed from specific weir geometries to comparable but different labyrinth weir geometries is not well understood. This study investigates the concept of labyrinth weir nappe interference and identifies labyrinth weir flow characteristics that decrease discharge efficiency, including local submergence. Parametric methods for quantifying nappe interference region size as a function of weir geometry (e.g., sidewall angle and crest shape) and flow conditions (e.g., headwater and nappe aeration) are presented. A comparison between empirical methods found in the literature for determining the size of the nappe interference region and laboratory-scale experimental data showed a limited correlation. The data presented in this study are recommended as a more accurate alternative to predict the extent of the nappe interference region.