• dam
• piano key weirs
• TPKW
• discharge coefficient

Piano Key Weirs (PKW) are non-linear weir structures, which are relatively new weir structures. These weirs were developed in the late 1990’s, then in the following years several structures were investigated in experimental as well as numerical models and were also built in prototypes, to analyze discharge coefficients for various PKW types. Therefore, small-scaled experimental models were used.

Two main PKW applications can be separated in research projects: (1) on top of dams (flood release structures) and (2) in-channel applications (replacement of regular weirs).
The PKW design in the form of piano keys, over-hanged from both the upstream and the downstream with sloping floors founded on a base or footprint. These weirs can increase discharge capacity by increasing effective length of the weir within a fixed channel width.

Thereby, PKWs represent an improvement and further development of the traditional weirs called Labyrinth weirs, but with an additional increase of discharge capacities, hence lower free surface heads at the upstream reservoir and a reduced footprint.

Optimal design of this weirs, is an important aspect which warrants further research. Despite extensive studies on Rectangular Piano Key Weirs (RPKW), there are only a few pieces of research attributed to Trapezoidal Piano Key Weirs (TPKW).

The mainly purpose of this experimental study was analyze physical models configuration of Rectangular and Trapezoidal Piano Key Weirs. Respectively measurement data were collected from experimental models tested under different flow conditions at Technische Hochschule Lübeck Water Research Laboratory.
Each model was designed with a different inlet/outlet width ratio, consequently it has allowed to collect several data used to investigate how the discharge capacity over the weirs could be improve by increase of discharge coefficient value, which was calculated considering two cases, 1) the discharge coefficient per unit length of the weir and 2) the normalized discharge coefficient with the width weir. Additionally, the water surface elevation and the flow regime over the weir was investigated and analyzed.

Three measurement positions were utilized to collect the data along the channel, which respectively three different instruments, a Ultrasonic-sensor n.1 positioned at 5P (50 cm) from the weir, a Ultrasonic-sensor n.2 positioned at 20P (200 cm) and a Point gauge positioned at 20P (200 cm), in that way a sensitivity analysis were carried out in the tests.

The principle results in this experimental study were basically three, 1) The highest hydraulic efficiency regarding the trapezoidal piano key weir models was found for the models TPKW_1.30 and RPKW_1.57, which respectively present a similar discharge coefficient value for low and high upstream heads, 2) The highest hydraulic efficiency regarding the rectangular piano key weir models was found for the model RPKW_2.40 , which present the highest value of discharge coefficient only for low upstream heads, consequently lose of hydraulic efficiency was verified for increase of discharges and 3) An accurate comparison between trapezoidal and rectangular models has identified the trapezoidal model TPKW_1.30 present the best performance with a remarkable difference for high upstream heads, but for flow conditions with a low upstream heads the both models TPKW_1.30 and RPKW_2.40 present the highest hydraulic efficiency. This experimental study tends to recommend for further researches to implement a trapezoidal models with a inlet/outlet width ratio around 1.40, which is probably the right value to obtain a higher hydraulic efficiency.