Scouring of Triangular and Trapezoidal Pianos Key Weir

Introduction: With the increasing demand for water storage in recent years, the need to build structures such as dams has increased, which increases the likelihood of flooding with larger volumes (Anderson et al., 2013). Therefore, in order to increase the safety of dams, the need to build weirs with higher efficiency is felt more (Gonzalez and Chanson, 1995). Labyrinth weirs are hydraulic structures that are used to regulate the water level and control the flow in dam reservoirs. The crown axis of this type of weir is indirect and consists of walls attached to each other. The geometric shape of this type of weir is repeated in a triangular, trapezoidal, rectangular and bended plane with alternating widths. The main hypothesis in the design of this type of weir is to increase the weir capacity of the weir by increasing their crown length in a constant width and for a certain height of the water surface upstream of the weir (Lux and Hinchcliff, 1985).
The piano key weir is a type of Labyrinth weir and is used as a suitable option to correct weirs that have difficulty in passing the maximum flow (Lempérière and Ouamane, 2003). In piano key weirs, unlike Labyrinth weirs, the openings are sloping inwards and outwards. Execution of this type of weirs requires less space than Labyrinth weirs, and therefore the foundation of this type of weirs can have smaller dimensions. This advantage has made it possible to use this type of weir on the canopy of concrete dams (Lempérière, 2017). Scour downstream of the weir can have a direct impact on the stability of the structure. For this reason, predicting the shape and dimensions of the scour hole at the downstream of these structures has been of interest to researchers. Therefore, in this paper, we compare scouring downstream of triangular and trapezoidal piano key weirs with changes in discharge and tailwater.
Methodology: Every experiment was conducted in a rectangular channel with a width of 75 cm, metal bed, glass walls and a height of 80 cm in the hydraulic laboratory of water and hydraulic structural engineering department of Tarbiat Modares University in Tehran.
The intended PKW was set up and sealed at a distance of 1 m from the beginning of the channel bend. All experiments on the weir were conducted under free flow conditions. A layer of uniform sediments with an average diameter of 1.64 mm, a geometric standard deviation of 1.24, a height of 42.5 cm, and a length of 2 m was placed downstream of the weir. A Type-A triangular and trapezoidal piano key weir made of thermoplastic (common PLA Filament) with a thickness of 1.2 cm was utilized in this study. Each weir has 6 keys (3 inlet keys and 3 outlet keys), a width of 75 cm (the same as the channel), and the crest length and a height of respectively 50 and 20 cm. The experiments were conducted with two discharge values of 35 and 45 Lit/s and five tailwater depths from 8 to 18 cm a long 5 hours.
Results and discussion: Experimental observations show that there are two dominant currents in piano switch weirs: the inlet switch draws the approaching currents to itself and the current is discharged downward from the inlet crown. The second pattern is formed on the output keys. In this section, the current flowing through the outlet crown, like a jet, is discharged downstream of the sloping section of the switch. Also, the output current from the inlet switch hits the surface of the bed downstream and, due to the existing tailwater depth, appears as surface rotation (at low tailwater depth) and surface turbulence (at high tailwater depth). Part of the flow also deflects downstream and after colliding with the sediment bed surface, creates a weak rotational zone under the inlet switch. The flow pattern on the output switch is much more complex than the input switch. In this part of the weir, due to the intersection of the flow caused by the falling jets from the side crowns with the upstream outflow, the water level rises and when the outflow enters the downstream area as a falling jet, the rotating area It forms strongly in front of the output key. For this reason, sediments from the bed in front of the outlet switch are washed more often. Downstream of the weir, the flow passing through the sediment bed causes the bed to be washed away. The scouring site begins immediately after the weir wall. Over time, the dimensions of the hole become larger, and the jet passing through the hole becomes a rotational flow that helps to transport some of the sediment down to the downstream.
Conclusions:The results showed that at low particle Froude number, the maximum depth of the lower downstream scour hole of the triangular weir occurred closer to the weir than the trapezoidal model. However, with increasing the number of Froude, this trend has changed and the maximum depth of the scour hole and its position has occurred in less trapezoidal weir and at a closer distance to the triangular weir. In general, as the particle particle Froude number increases, the maximum parameters of the scour hole depth, its location, and the length of the downstream scour hole opening of both weir models increase. Equations with appropriate accuracy were obtained to predict the maximum depth of the scour hole, its location, the length of the scour hole and the scour of the lower wall of the weir wall for the two weir forms used. It was also observed that in all discharges and for each tailwater depth, the maximum distance between the hole depth to the weir foot and the length of the scour hole opening in the triangular model is on average 37% less and 15.5% more than the trapezoidal model. The data show that the geometric parameters of the downstream scour hole of the piano key weirs depend on the characteristics of the sediments, the discharge and tailwater.