Effect of the inlet-to-outlet key width ratio of Piano Key Weir on its hydraulic behaviour

The Piano Key Weir (PKW) is an ungated type of spillway, i.e., a novel evolution over the traditional labyrinth weir. It allows the reservoirs to operate with elevated supply levels without causing any damage to the dam structures, thereby providing additional storage. It is designed to improve the hydraulic performance of linear weirs by increasing pass discharge and energy dissipation. In this study, an experimental investigation has been carried out to assess the effect of the inlet-to-outlet width ratio (Wi/Wo) on PKW hydraulic behaviours viz hydraulic efficiency and energy dissipation. To this end, nine different width proportions (1 ≤ Wi/Wo ≤ 2) type-A PKW models were tested and examined. The findings revealed that the Wi/Wo ratio significantly impacts the hydraulic performance of PKW, and the results indicate that the efficiency of the PKW increases as the width ratio increases at a certain limit and then starts decreasing. The discharge coefficient was the highest for the given discharge and head, resulting in the best hydraulic performance with a Wi/Wo ratio between 1.25 and 1.30. However, the energy dissipation across the PKW decreases as the width ratio increases. Moreover, the discharge coefficient of different width ratios (Wi/Wo) ranging between 1.28 and 1.30 is 7–17% higher than Wi/Wo = 1 and 8–13% higher than Wi/Wo = 2.0. However, the energy dissipation across the weir for Wi/Wo = 2.0 indicates 15–29% less energy dissipation than Wi/Wo = 1. It means the energy dissipation across the weir decreases as the Wi/Wo ratio increases.