In the United States, the labyrinth spillway, a "folded" weir, has become a relatively common approach to upgrade dams with inadequate spillway capacity. The Piano Key Weir (PKW) is a variation of the labyrinth concept. The PKW has a smaller upstream to downstream footprint than a labyrinth weir, due to a cantilevered apex geometry, making the weir a more practical alternative for applications where the footprint is limited (e.g. the crest of gravity dams). Each of these passive spillways provides significant discharge capacity within a given spillway width, often comparable to that of gated spillways. It is interesting to note that while the labyrinth concept has become relatively commonplace for dam construction and rehabilitation in the United States, the PKW has not yet gained popularity. By contrast, the PKW is being widely used in France, while there are few labyrinth spillways. The vast majority of the labyrinth spillways have been constructed for earth embankment dams while PKW spillways are more common for concrete gravity dams. This paper includes a discussion of the similarities and the differences between these unique spillways and draws upon the authors' experience with numerous labyrinth spillways in the United States and several PKW projects in France. This discussion includes key geometric parameters, hydraulic modeling and performance, additional beneficial research and lessons learned in design and construction.
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