Overview of Piano Key weir prototypes and scientific model investigations

The interest in Piano Key weirs (PKW) was increasing over the past years, both in terms of application and hydraulic research. Several systematic model test series were performed and published so far, originating from different hydraulic laboratories around the world. Therein, the key geometric parameters dominating the hydraulic capacity of PKWs were identified as developed crest length, transversal weir width, height of the walls and key widths. The present paper compares the available data-sets provided by several laboratories and as constructed on prototypes, partially including a systematic model testing of the relevant parameters. The present overview is based on the geometrical dimensions of the investigated and designed PKWs, without giving any hydraulic examination. The discharge capacity of a PKW is affected influenced by further geometrical parameters, as the upstream approach flow channel conditions, the height and position of eventual parapet walls Ppx, the crest shape, the shape of the upstream diffuser, the height of the foundation Pd, the upand downstream overhang lengths Bo and Bi, and the tailwater conditions. However, the influence of these parameters can be considered as secondary in comparison with the influence of the key parameters as described above. Some attention has been recently directed to the thickness of the walls (Laugier et al. 2011). As PKWs typically operate under low heads, low values of the ratio H/Ts can decrease the discharge coefficient of the walls, then operating as broad-crested weirs instead of sharp-crested weirs.