Experimental study of discharge coefficient for trapezoidal piano key weirs

• Discharge coefficient of the TPKW is about 22% higher than the rectangular plan form.
• Among the geometrical ratios, Wi/Wo and L/W were found to have the lowest and highest influence on increasing the Cd value respectively.
• Regardless of the L/W value, the peak value for Cd corresponds to H/P ratio near 0.2.
• The presented design criteria for TPKW can be helpful for practical purposes.

Piano Key Weirs (PKW) have been invented in the last decade to increase discharge capacity of hydraulic structures. Despite extensive studies on this type of weir with a rectangular plan form (RPKW), there are only a few pieces of research addressing trapezoidal piano key weirs (TPKW). In this experimental study, geometrical parameters of TPKW models were varied under different flow conditions and effects on discharge coefficient (Cd) were investigated. The Cd values were found to be mostly influenced by L/W whereas Wi/Wo had the least effect. Results also showed that TPKW has higher discharge efficiency in comparison with RPKW. This was believed to be related to formation of an “interference wedge” over the TPKW. Finally, quantitative values for distinguishing three flow regimes (i.e. nappe, transition and submergence) as well as criteria for design of TPKW are proposed.