Experimental investigation and determination of scour dimensions due to symmetric crossing jets

Introduction
One of the vital parts of a dam is its energy dissipation structure. With the construction of a dam, the flow is conveyed downstream. The outlet flow from a dam has a lot of energy and to dissipate this energy the plunge pools are used. The impact of jet from a dam outlet to the river bed might cause a large scour hole, which is one of the most important topics in the field of river engineering. The scour phenomenon may lead to damages for the dam or adjacent structures. Therefore, accurate estimation of the depth and dimensions of the scour hole is necessary. Although, the symmetric crossing jets in the dam’s outlet structures are used as a solution for the energy dissipation, but the scour at downstream of these structures may also happen. Pagliara et al. (2011) studied the scour caused by two crossing jets. They used a uniform bed material with a diameter of d50=9.5mm. They observed that in the presence of high tailwater level, a significant decrease occurs in the scour depth at all the crossing angles. The continuation of studies in this field help to collect more information and findings. Therefore, the present research uses a different bed material size with d50=1.4mm. An attempt was also made to derive specific equations which includes the crossing angle as an independent variable to interpolate the scour value in different angles.
Methodology
In this study, 54 experiments were conducted to investigate and analyze the scour caused by symmetric crossing jets, also 9 experiments with a single jet were performed as reference tests. The diameter of the equivalent single jet is equal to Deq=31.1 mm. A bed material with d50=1.4 mm and geometric standard deviation of σg=1.5 is used. The experiments were carried out in a 16 m long, 1 m wide and 0.8 m high canal at the hydraulic laboratory of the Semnan University. A hinged gate was used to adjust the tailwater level. To hold the jet pipes during the tests and change their height, a metal base was built. An electro pump with a maximum discharge of 10 lit/s was used. Accordingly, three discharge values (105, 91.5, 78 lit/min), three tailwater level (3, 6, 9 cm), two distances for the jets crossing point to the water surface (5, 10 cm) and three different crossing angles (30, 70, 110) were used. To predict the scour hole dimensions (the scour hole depth, the length and the width as well as the ending location of the downstream ridge), the linear and power regression models are also presented. The experiments performed systematically by changing the hydraulic parameters and the effect of each parameter was investigated on the scour hole dimensions. At the end of each experiment, the longitudinal and transverse profiles of the scour hole at the maximum depth section were measured using a laser measurer.
Results and Discussion
At the crossing angles of 70 and 110 with low tailwater level, the scour depth is more than that of the crossing angle of 30 and the single jet. It was also observed that at the crossing angles of 70 and 110 and low tailwater level the scour shape tends to be asymmetric. Increasing the tailwater level and the distance of the crossing point of the jets from tailwater increased the scour depth at the crossing angle of 30, but, at the angles of 70 and 110, on the contrary, the scour depth was decreased. Accordingly, it turns out that the use of crossing jets for the scour reduction is only effective in some hydraulic conditions. The linear and power equations obtained from the entire data collection were not able to estimate the scour hole dimensions accurately at all the crossing angles because of the complexity of the phenomenon. But, the power models obtained separately for each crossing angle were able to estimate the scour features satisfactorily. The longitudinal scour hole profiles are plotted and compared with each other at various crossing angles. To show the effect of each variable, the scour hole parameters are also plotted versus the independent variables.
Conclusion
The results of this research showed that the use of crossing jets necessarily does not reduce the scour, and at the same time this depends on different hydraulic factors, such as, the angle of the crossing jets, the tailwater level, and the distance of the crossing point of the jets from tailwater level. The results show that at low tailwater level, the amount of scour due to the crossing jets is more than that of the single jet at all the crossing angles.
Keywords: symmetric crossing jets, scour hole dimensions, regression equations