Experimental Study of the effect of parapet wall location on the performance of trapezoidal labyrinth and piano-key weirs

Nonlinear weirs have relatively high discharge capacities in low hydraulic loads and their performance is reduced due to submergence of the outlet key in high hydraulic loads. Labyrinth weirs are hydraulic structures used for water level adjustment and flow control in reservoirs. The labyrinth weirs performance is not satisfactory in high hydraulic loads due to interference of flow blades, turbulence and submergence in the outlet key. Therefore, the purpose of this study was to investigate the effect of parapet wall location on weir inlet submergence. Thus, the inclined parapet walls were used on trapezoidal labyrinth and piano-key weirs, which had 60 and 30 cm length (B = 60 and 30 cm) with 1 and 0.5 in slope (B′ = 1 and 0.5). The results showed increasing the parapet wall heights and lengths leads to a 50-percent decrease in the discharge coefficient. The weirs performance also increases by 33 percent as the flow rate increases. The installation of parapet walls on the labyrinth weirs showed better performance (up to 30%) as compared to the piano-key weirs with the same plan.  Also the installation of parapet walls at the weir outlet showed a better performance (5-10%) as installed at the weir inlet. It was found that the flow rate is directly related to the discharge coefficient; hence, the efficiency drop is reduced about 40% in high discharge rates by setting the parapet walls at labyrinth and piano-key weirs.