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 × 109m3 worldwide). A newly evolved shape of free-flow spillway (the "Piano Key weir") has the potential to increase the specific flow as high as fourfold at lower discharges. It could substantially increase, at low cost, the safety and the storage and/or the flood control efficiency of many existing as well as new dams. It is thus very advantageous to increase the specific flow as much as possible. Some tens of existing spillways have been designed accordingly as vertical walls on a flat bottom, with a trapezoidal labyrinth layout which is much longer than the spillway length (often four times). They usually double the specific flow of an Ogee Spillway. Piano-Key Weir derives its name from its plan-form shape which looks like a 'piano-key'. It is configured as polygonal walls such as to provide a much longer crest length than the conventional ogee spillways. This paper presents experimental data analysis on this new (not patented) solution in order that: Increasing the storage of many irrigation dams is justified by the increased needs or due to the sedimentation of reservoirs. Increasing the spillage capacity is often justified by a better knowledge of hydrology. Improving flood mitigation by existing dams may be very cost effective. Positive outcome of this study will be very much relevant for dam safety concerns worldwide in the context of adverse hydrological consequences due to ongoing global warming phenomenon.