The experimental study of downstream scouring of trapezoidal Piano key weir type A under free and submerged flow

Weir is a structure that is made in the body or support of the dam to safe drain the excess volume of water in the tank. Weirs are mainly considered for free flow mode, but in some cases there is a possibility of immersion in them. Submersion in weirs occurs in two general and local ways. General submersion will occur if the downstream water level is higher than the weir crown level. This is more likely to occur for weirs in canals and rivers and if the weir acts as a diversion dam. Local submersion is observed in the downstream part of weir due to local flow conditions. Weirs are divided into linear and non-linear weirs based on the shape in the plan. Piano key weirs are the newest type of non-linear weirs which recently due to its advantages. Non-linear Piano Key Weirs enjoy not only a higher water passage but also a relatively simple and economic structure compared to linear weirs. All experiments of this research were performed in a channel with a long 10 m, wide 0.75 m and high 0.8 m in Tarbiat Modares University hydraulic laboratory. A view of the laboratory flume is shown in Figure (3). The required water was supplied through an underground reservoir. In this research, trapezoidal piano key weir types A, made of thermoplastic with a thickness 1.2 m, slope of the input and output keys 28 degrees and a height 0.2 m was used. The weir has 6 keys (3 input keys and 3 output keys) with the same slope in the input key and the output key. Uniform bed materials with an average diameter 2.2 mm were used. Flow discharge was measured with an ultrasonic flowmeter and flow depth and bed level were measured with a laser level gauge. According to the selected discharges, the flow depth upstream of the weir was considered to be more than 3 cm so that the effect of surface tension is not significant. In this study, experiments were performed with five discharges 30, 40, 50, 60 and 70 liters per second. Under submerged flow for each discharge, two percent submersion and under free flow for each discharge, the tailwater depth was considered to be 0.13 m. Flow characteristics are affected in case of weir submersion. During the test, after the flow hit from the input switch to the tailwater surface, due to the amount of tailwater depth, surface rotations (at low tailwater depth) and surface turbulence (at high tailwater depth) are observed. Part of the flow moves downwards and after hitting the bed surface, a weak rotational zone is created in the range of the input switches. The flow enters the downstream in the form of a submerged jet after the output switch and, due to the momentum to the upper fluid, causes a severe rotational zone in the range of the output switches. In the free flow, more turbulence is observed in the range of the output switches due to the intersection of the flow passing through the weir lateral wall and the falling current from the upstream and downstream crowns. Less tailwater depth in free flow limits the deep growth and development of the sedimentary hill in downstream of the scouring hole, and sedimentation occurs with longer length and lower elevation. However under the submerged flow, due to the greater tailwater depth and lower fall height, the flow strength is more depleted and the flow strikes the bed with less energy, and result in less scouring. In this case, the flow does not have the power to transfer all the sediments to the downstream and most of the sediments accumulate on top of each other and a sedimentary hill is created as a point. As a result, a sedimentary hill with a higher height and less length is visible under submerged current than free flow. Therefore, in the free flow than in the submerged flow, the length and depth of the scour hole occur more, and sedimentation occurs with less height and longer length. Under submerged flow, most of the sediment bed remains unchanged, while under free flow most of the sediment bed is affected by scouring and sedimentation. Of course, changes vary depending on the hydraulic conditions. In the early times, in both free and submerged currents, scouring occurs with greater intensity, but over time, its severity decreases and scouring reaches a stable state. In this case, sediments are rarely transported downstream. The surface of the sedimentary hill downstream of the scour hole in the open stream is almost smooth and in the submerged stream is sharp. The results showed that most of the scour hole changes occur in 20% of the initial time of the experiment and the changes in the free flow are faster than the submerged flow. The depth and length of the scour hole in free flow is greater than in submerged flow. Relative scour depth with increasing 77% in tailwater depth for Froude numbers of particle 0.029, 0.037, 0.045, 0.052 and 0.058, respectively 42, 45, 21, 28 and 27% and with increasing the tailwater depth 123% is reduced by 95%, 92%, 90%, 75% and 68%, respectively. With increasing 18% in the submersion ratio in the Froude number of particle 0.029 and increasing 96% in the submersion ratio in the Froude number of particle 0.058, the maximum relative scour depth decreases by 92 and 56%, respectively. The location of the maximum scouring is also a function of Froude number of particle and the tailwater depth. Maximum scour depth at the tailwater depth 0.13 m at a distance 0.18 to 0.30 m from the weir toe, at the tailwater depth 0.23 m at a distance 0.18 to 0.36 m from the weir toe and at a tailwater depth 0.29 m is at a distance 0.03 to 0.30 m from the weir toe.