Study on the Effect of Pipe Installation Depth on Temporal Changes of Scouring and Sedimentation Parameters Around the River Crossing Pipelines

Introduction:
By constructing a pipe containing fluids such as water and oil crosswise with the direction of the river, the pattern of river flow around the pipe changes. . These changes in the flow pattern around the pipe and an increase in shear stress on the substrate, which results in a scouring hole under the pipeline. Local scouring around pipelines across the river is one of the most important causes of their failure and destruction. Therefore, it is very important to study the mechanism of occurrence of this phenomenon around the pipelines and to evaluate the amount of scouring and the characteristics of the local scour hole around them. Wu and Chiew (2013) investigated the scour hole and the flow field around a pipeline under steady flow. The flow field in these experiments was measured by an acoustic Doppler velocimeter. The results of this study showed that the presence of vortices due to the pressure difference created upstream and downstream of the pipe causes the formation of a force for the movement and displacement of sediments. Also, the flow from under the pipe into the scour hole causes it to expand further. Zhao et al. (2015) performed laboratory and numerical study of scouring under two consecutive pipelines with different distances from each other. In moving bed conditions, it was observed that the depth of the scour hole under the upstream pipe is slightly greater than the scour hole under the single pipe, while the depth of the scour hole under the downstream pipe is much greater than the scour depth compared to the single pipe. Yan et al. (2020) numerically examined the local scour around the pipeline across the river under steady flow conditions. In their study, the CFD method and variable mesh technique were used to model the sediments transport and the results were compared with the results of the laboratory model. The results showed that the method used to model scour and sediment problems respond satisfactorily. The aim of this study was to investigate the effect of the installation depth of pipe across the river in steady flow on temporal changes in scour pattern and sedimentation around the pipeline were processed by recording video information during each experiment.

Methodology:
The present study was performed in the hydraulic laboratory of the Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz in a rectangular flume 10 m long, 0.74 m wide, and 0.6 m high. The walls of the flume were made of glass and the floor was made of steel. The flume had reservoirs at the beginning and end and a section to calm the flow. To investigate the scouring phenomenon around the pipe crossing the waterway, in the middle of the flume, in an area, 1.5 m long and 15 cm thick, uniform sediments with Medium size (d50) 0.7 mm, relative density (Sg) 2.65, and standard deviation (σg) 1.4 were poured. In this study, pipes with a diameter of 20, 40, and 60 mm at a quarter of the pipe diameter under the bed (e/D= -0.25), bed installation depth (e/D= 0), a quarter of the pipe diameter above the bed (e/D= 0.25), and half of the pipe diameter above the bed (e/D= 0.5) were used. The pipes were made of PVC and were installed perpendicular to the flow across the flume. The experiments were performed at a flow rate of 33 liters per second and a flow depth of 14 cm. The duration of all experiments was 120 minutes. The total number of experiments was 12. In these experiments, clear water conditions prevailed.



Results and Discussion:
In most of the researches in this field, a comprehensive study has not been done on the temporal changes of the scour hole parameters and their focus has been mainly on the scour depth parameter when the pipe is placed on the bed. Comparing the present laboratory study with other studies related to the study of diameter and depth of installation, one of the most relevant studies is related to the laboratory estimation of scour under the pipeline by Ataieyan (2012). In which the scour under the pipeline is investigated with emphasis on the effect of installation depth. The maximum amount of scouring was observed at a depth of one-fourth of the pipe diameter at the top of the bed. The result of the experiments performed in the present study also confirms that at the depth of installation, e/D= 0.25, due to the narrowing of a certain distance between the sub-pipe and the surface of the sedimentary bed and The formation of vertical and horizontal vortices showed the highest maximum scour depth compared to other installation depths. The results related to the effect of installation depth for different modes are as follows, e/D= 0.25, e/D= 0, e/D= -0.25, e/D= 0.5 from maximum to minimum, respectively, they had the highest amount of scour depth. Another parameter studied for scouring is the distance between the maximum scouring depth and the center of the pipe. This parameter is indicated by Xds. the location of the maximum scouring depth at the beginning of the experiment was moving upstream of the pipe. At installation depth, e/D= 0.5, the highest rate of maximum scouring depth was observed downstream compared to all cases. In all experiments, about 80 to 90% of the height of the deposition ridge occurred in the first 10 to 20 minutes of each experiment.

Conclusion:
The results of this study showed that at all installation depths, 80 to 90% of the scouring depth was performed in the first 40 minutes of each experiment. The depth of pipe installation was one of the most influential factors on the dimensions of the scour hole. In all experiments, sediment from erosion was deposited downstream of the pipe and formed a sediment ridge. The maximum and minimum deposition heights occurred at the installation depths of e/D= 0.5 and e/D= 0.25, respectively.