Experimental Study on the Effect of Discharge and Tailwater Depth on Bed Topography Downstream of a Piano Key Weir

Introduction :

Non-linear Piano Key Weirs (PKW) enjoy not only a higher discharge but also a relatively simple and economic structure compared to linear weirs. Other advantages of the PKWs include the fact that they increase the discharge per unit width over the weir up to 100 𝑚3⁄𝑠, the discharge over this type of weir is at least four times that over ordinary (linear) weirs, it increases the capacity of the reservoirs, and it is cost effective with lower maintenance expenses. This type of weir is utilized mainly with the aim of increasing its capacity over the available spillways and also as controlling structures on newly constructed spillways. Despite the research studies conducted in this regard, the scour downstream of the trapezoidal Piano key weirs has not yet been investigated, and there has been no comprehensive information on the properties of the scour profile downstream of these weirs. Hence, given the several advantages of the Piano key weir and lack of a comprehensive research on the amount of scour downstream of this type of weir, the need for investigation on the scour downstream of this weir under different hydraulic conditions is obvious.

Experiments were 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 of Tarbiat Modares University, Tehran. Water flow was supplied from an underground storage tank using a pump with a maximum discharge of 85 Lit/sec. A valve installed at the end of the channel was used to adjust the flow depth in the channel. The PKW was set up and sealed at a distance of 1 m from the channel end. 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. The type-A trapezoidal piano key weir made of thermoplastic (common PLA Filament) with a thickness of 1.2 cm was utilized in this study. Weirs with 6 keys (3 inlet keys and 3 outlet keys), a width of 75 cm (the same as the channel), and the crest length (B) and a height (P) of respectively 50 and 20 cm were used. The experiments were conducted with three values of discharge 0.03, 0.04 and 0.05 𝑚3⁄𝑠 and five tailwater depths varied from 0.08 to 0.18 m.

It is observed during the experiment that with the impact of the outlet flow from the weir with a downstream bed, the scour began and the process of erosion and sedimentation continuously occurred with development of the scour hole. Observations indicated that during the erosion stage, after the impact of the water jet with the bed and upon its dispersion, the scour of the bed material begins immediately and considerably. The material washed away from the bed is mostly transported downstream along with the flow and is deposited there. Generation of the scour hole entails circulation of flow inside the scour hole. Also, a portion of circulating flow inside the hole is deviated upward. A large body of sediments washed from the bed is transported downstream with the flow leaving the hole and a part of it rotates with the circulating flow inside the hole. A part of these sediments along with the circulating flow is deposited inside the scour hole and the other part is transported downstream by the flow leaving the scour hole. After the scour hole is generated, the jet entering the hole is divided into two parts at the deepest point of the scour hole. Such division is observable from the movements of sediments inside the hole so that some of the sediments are separated and transported downstream, leaving the scour hole, and the remaining sediments move and reside in the same zone as the circulating flow due to the backflow towards the weir foot. A sedimentary ridge is created downstream of the hole as a result of the eroded sediments concentration which have left the scour hole. As the scour hole is deepened, the sedimentary ridge is also enlarged. This trend continues until the flow is able to carry the particles out of the scour hole. These variations mainly occurred at the initial 20% of the test duration. It is also observable that the scour hole downstream of the weir is not symmetrical with the longitudinal axis of the channel, which is due to the effect of inlet and outlet keys on the direction of the outlet flow over the weir. However, the overall scour pattern is nearly the same under different conditions. Variations in the maximum scour depth downstream of the weir are approximately 5.8 to 8.8 times the water depth over the weir. The location of the maximum scour depth is also a function of discharge and tailwater depth, which was measured at a distance of 15 to 27 cm from the weir foot.

Conclusions :

This study has investigated the dimensions of the scour hole as well as variations of bed topography downstream of a trapezoidal piano key wer. It was found that with a 112% increase in the tailwater depth under a constant discharge of 30 L/s, the maximum scour depth is reduced by 37%. Increasing the discharge results in an increase in the dimensions of the scour hole, in such a way that a 66.6% increase in discharge with a constant tailwater depth has increased the scour hole by 18.5%. With an increase of the tailwater depth, the development of the scour hole downstream of the piano key weir is reduced. The minimum scour hole development in downstream direction was measured in tests with a discharge of 30 L/s and tailwater depths of 15 and 17 cm. 112 and 89.5% increase in the tailwater depth for discharge values of 30 and 40 L/s reduces the area of the scour hole by approximately 46 and 25% respectively.