Experimental investigation of local scour occurring on downstream bed by free overfall weirs

The scour problem is of great importance for the safety of hydraulic structures. Local scouring in the downstream of free overfall weirs can cause severe damage to the hydraulic structure. The flow characteristics of each weir type and shape are different from each other. Flow characteristics such as jet expansion, jet thickness, jet impact velocity and angle, penetration depth, the amount of air bubbles entrained into the plunge pool, the aeration performance of the weir, and the jet trajectory have a significant effect on the scour geometry. In this study; the local scour in the downstream pool of rectangular, triangular, trapezoidal and labyrinth weirs were investigated experimentally. In this context, two experimental setups were made in the hydraulic laboratory. The first experimental setup was designed as the prototype of the second experimental setup, and the model scale was taken as 1/2.4. The experiments were carried out simultaneously in both experimental setups. Variation in time of local scour, maximum scour depth and scour geometry; different weir types (four items), different head heights (four items), different tailwater depths (two items), different median grain diameters (three items), different unit discharge values (four items) were taken into consideration. Water jet expansion, jet trajectory and jet impact velocity were also determined in the current study. The non-linear equations giving the maximum scour depth for each wier type were obtained. The relationship between the model and the prototype was determined in order to use the obtained equations safely in the prototype. Experimental results were compared with the literature and numerical results. It is concluded that weir type is a very important parameter on scouring geometry. The maximum scour depth occured by the jet from the triangular and labyrinth weirs was much less than the other weir types tested. It was shown that the free overfall jet from the labyrinth weirs impacts the downstream pool at a closer distance to the hydraulic structure than the tested weirs. Moreover, the scouring geometry in the downstream pool for the labyrinth weirs covers wider area than that of the other weirs tested.