Tested since 15 years and implemented on many dams in various countries since 10 years, this improved labyrinth design appears very cost efficient, optimizing hydraulic efficiency as well as structural requirements and construction facility.
For cost or safety reasons the fully gated spillways will probably be avoided for most future dams and free flow spillways will likely be the most used solution. There is thus a great future for labyrinth weirs, either as Piano Keys Weirs or by optimization of traditional labyrinths. Fuse devices may also be attractive.
For most dams built worldwide in last century, free flow spillways were used for small or medium catchment areas and gated spillways were used for large catchments corresponding to significant floods.
For new dams compared to Creager type weir profiles, Labyrinth Weir may divide by a factor of 3 the spillway length or by 2 the upstream head. For existing free overflow spillways they may be used to increase safety or to increase storage at low cost.
Compared to Creager type weir profiles, P.K. Weirs may divide by a factor of about 3 the spillway lengths required at fill dams or by 2 the upstream head required at any weir. For existing free overflow spillways they may be used to enhance freeboard and increase safety or to increase storage at low cost. They may be used as emergency spillways associated with an existing or new gated spillway, using the freeboard for discharging part of any exceptional floods. For instance a freeboard of 4 m could be used to discharge an additional 40 to 50 m3/s per metre of spillway. As there is also much air entrainment in flows from P.K. Weir and other labyrinth structures, energy dissipation is enhanced and, for moderate discharges, any downstream erosion may be much reduced.
The piano keys weir (PK Weir) represents an effective solution for the increase in the storage capacity and/or the capacity of evacuation of floods of the majority of the existing dams. It can be an economic solution for the new dams.
Free-flow spillways are simpler and safer than gated ones, but the low specific flow of their traditional shapes requires high spilling nappe depths and thus huge losses of storage (100 x 109m3 worldwide). A new shape of free-flow spillway (the “Piano Keys weir”) can increase the specific flow fourfold or more. It could substantially reduce the cost of most new dams.