The presence of a block downstream of a type B triangular piano key weir and its effect on increasing flow energy loss

Piano key weirs (PKWs) have rectangular, triangular, and trapezoidal shapes in terms of plan. They are available in four types: A, B, C, and D. Type A weir has an overhang upstream and downstream, type B weir has an overhang upstream, type C weir has an overhang downstream, and type D weir has no overhang. Piano key weirs are non-linear, long-crowned spillways that allow a greater flow capacity (3 to 4 times more) over a given width. The first piano key weir was built on the side channel of the Goulours dam in France. Piano key weirs are used in agricultural canals, drainage canals, and irrigation canals in addition to dams due to their lightweight foundation. Piano key weirs are the evolution of nonlinear weirs with high efficiency; therefore, it is important to investigate the flow energy loss and the solution to increase it. In the present study, for the first time, a block with a different geometry was used downstream of the triangular piano key weir type B. Also, two weirs with a height of 0.20 and 0.18 m were used. The blocks are rectangular and a combination of trapezoidal and circular blocks. Four discharges from 0.02 to 0.05 m3/s were also used. The experiments were conducted in a channel 10 m long, 0.8 m wide, and 1 m high. The slope of the channel is zero. The water temperature also varies between 8 and 13 °C. The flow was fed into the tank by a pump and an underground tank with a volume of 10,000 m3 and a monitor. Two triangular piano key weirs type B with constant geometry but different heights of 0.20 and 0.18 m were used. The width of the weir inlet keys (Wi) is 0.613 m, the width of the weir outlet keys (Wo) is 0 m, the length of the weir side walls (B*) is 0.425 m, the length of the upstream overhanging edges of the weirs (Bi) is 0.14 m, the length of the weir crest (L) is 3.04 m, and the thickness of the weirs (Ts) is 0.01 m. Blocks with rectangular and trapezoidal cross-sections with a height of 0.06 m (0.3P) were used downstream of the 0.20 m high weir. The cross-sectional area of ​​the blocks is 0.025 x 0.025 m. The height of the blocks was chosen so that they are submerged at the lowest flow rate (0.02 m3/s). The blocks are made of compressed plastic and were installed downstream of the weir with waterproof adhesive. The blocks were installed in three rows downstream of the weir. The distance between the blocks is 10.25 cm. As mentioned, the study of flow energy loss in piano key weirs is important due to their high efficiency in flow passage. Dimensional analysis was also used to extend the results to other weirs and other types of piano key weirs in channels. As the height of the weir increases, the discharge coefficient decreases, and the flow energy loss increases. The blocks downstream of the weir act as a barrier and the flow energy loss increases. The flow around the blocks has weak eddies and hydraulic jumps, which reduces the specific energy downstream. With increasing Hu/P, the flow energy loss decreases. The flow energy loss in the weir with a combination of trapezoidal and circular blocks is higher than that of rectangular blocks. Next, a relationship with a correlation coefficient of 99.92% was presented to calculate the flow energy loss.