The Effect of Flow Conditions on Bed Topography in a 180 Degree Bend Containing 6- Inclined-Vertical Pier Groups in a Vane Perpendicular to the Flow

Document Type : Original Research

Authors
L. Eghbalnik
M. Vaghefi
M. R. Golbaharhaghighi
Persian Gulf University
Abstract
Construction of inclined piers has been observed in a great number of bridges worldwide today. With installation of the bridge piers on river path, the simple and steady flow pattern reaching the pier undergoes intense and complicated changes. Complicated vortex systems created around the pier dig around the pier a hole called a scour hole. Expansion of such a hole around the piers empties the foundations from beneath, leading to consequent destruction of the foundations and the bridge. Whereas, few studies have been conducted to address scour at inclined piers. This issue was investigated in light of the significance of severe damages caused by the scouring phenomenon, the construction of vertical-inclined piers and their untold effects specifically in meandering paths. In this study, the effect of different arrangements of inclined and vertical bridge piers (in convergent and divergent fashions), installed at the vane vertical to the flow, was analyzed along with the effect of flow conditions and the position of the pier groups at the 180 degree sharp bend on parameters such as the maximum scour depth, the maximum sedimentation level, the scour hole dimensions, etc. in the laboratory. To conduct the experiments, a channel consisting of a 180 degree sharp bend was utilized. Due to its 2-meter-long central curvature radius, it is classified as a sharp bend. The channel contains upstream and downstream straight ends respectively as long as 5 and 6.5 meters. The experiments were carried out under clear water (where u/uc is equal to 0.87), incipient motion (u/uc = 0.98), and mobile bed (u/uc = 1.03) conditions. Two vertical and four inclined piers formed the pier group. The diameter of the piers was selected 5 cm and their inclination angle was 21 degrees. The pier groups were placed at the 60, 90, and 120 degree positions of the bend. Results indicated that the maximum scour depth and level of sedimentation occurred at the 60 degree position under live bed conditions. These values were measured equal to 4.2 and 3.2 times the pier diameter. In every three position of the installation of the piers at the bend, the maximum scour depth occurred due to position of the convergent-vertical pier group. However, the maximum sedimentation level occurred in the case of positioning the divergent-vertical pier group. In both pier groups, the maximum scour depth occurred in the vicinity of the inclined pier near the outer bank in the first row. Changing the position of the piers from the 60 to 90 and 120 degree angles leads the maximum sedimentation level to occur at a distance closer to the vicinity of the pier group. Such a distance was obtained in those three positions to be respectively 42, 28, and 22 times the pier diameter on the average. In both pier groups, the minimum area of the scour hole occurred at the 120 degree position. Further, the maximum area was observed in the experiments on the 60 degree position. Shifting the flow regime from the clear flow to incipient motion resulted in an increase in the area of the scour hole. Such an increase is observed in every three position per both pier groups.

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Modares Civil Engineering journal
Volume 19, Issue 4
November and December 2019
Pages 1-16