The effect of abutment geometry and application of submerged vanes on bridge abutment scour

Scouring may occur as a result of natural changes in the flow in the waterway or as a result of human activities such as construction of structures in the flow path or removal of bed materials.The Occurrence of scouring around the abutment is considered as one of the most important 4- this phenomenon, several methods have been proposed. In the present study, the effect of two parameters of the geometric shape of the bridge abutment, as well as the presence of submerged vanes with selected arrangements, on the amount of scour around the bridge abutment was investigated. Most of the methods that are used to control and reduce scouring are divided into two categories: methods of changing the flow pattern and methods of increasing bed resistance, and researchers have used each of these methods to reduce scouring.

In this research, a physical model of four different shapes including 90 degree rectangular, rectangular abutment with fin-wall with 60 degree flow line, rectangular abutment with fin- wall with 45 degree flow line and rectangular abutment with circular fins was designed and built. Submerged vanes were also tested in four different arrangements of parallel rows, zigzag rows, oblique zigzag and pine, in two Submergence ratios of 2 and 3 and at an angle of 20° to the flow on each of the four abutments, at constant discharge. The tests were performed at a flow rate of 34 liters per second and a water depth of 12 cm. The criterion of the equilibrium time in these experiments is the same criterion that Kumar et al. considered in their research, and it is equal to the time when the changes in scouring depth do not exceed one millimeter in three consecutive hours. Therefore, in the current research, by conducting a 12-hour test for each of the abutmentmodes, this duration was estimated to be 5 hours for all four types of abutment. All the experiments of this research were done in clear water conditions. At the beginning of the tests, four control tests were performed with the support in four different states and without the submerged vanes. After completing the tests, the maximum scouring depth for each abutment was obtained as follows: the maximum relative scouring depth of the rectangular abutments 90° 1.01, the maximum scouring depth of the rectangular abutment of the fin wall of the flow line 60° 0.43, The rectangular abutment of the fin wall of the 45 degree flow line was 0.41 and the rectangular abutment with circular fins was 0.36. In the following tests, for each of the different support states, the length and width of the scour hole were measured separately for four different arrangements of sunken plates, in two absorption ratios of 2 and 3.

According to the data of this series of tests, the absorption ratio (L/H=3) in all cases of submerged vanes and also for all abutments has a better performance than (L/H=2) in reducing surrounding erosion. Installation of plates in front of the abutment causes the approaching flow to break up and as a result reduce the strength of the downward flow and the horseshoe vortex in front of the abutment. If the ratio of the height on the submerged vane bed is high compared to its length, the vanes have enough height to separate the approaching flow. Observations showed that the maximum and minimum scouring depths occurred around the 90 degree rectangular and the rectangular abutment with circular walls, respectively. In submerged vanes experiments, the pine arrangement in a Submergence ratio of 3, with a 41% reduction in scouring around the 90 ° rectangular abutment, 20% around the rectangular abutment with fin- wall with 45 degree flow line and an 18% around the rectangular abutment with circular fins showed the best performance in reducing scour compared to the control test corresponding to each of the abutments. The comparisons made in this research show the effect of the shape of the abutment, in such a way that by changing the shape of the abutment from a 90 degree rectangle to a rectangle with circular fins, scouring was reduced by 64%. Also, with the presence of the protective device of the submerged vanes under the arrangement of pine in the absorption ratio of 3, the scouring decreased around the rectangular abutment 90 degrees is 41%, the rectangular fin wall of the flow line 45 degrees is 20% and the rectangle with circular fins is 18.18% compared to the control test.