Experimental Investigation of the Effect of Symmetric T-shaped Spur dyke on Bridge Abutment Scour in Compound Channel

Bridges are of the most important structures in rivers that are widely used for various purposes. The occurrence of scouring around the pier and abutment is one of the main factors in the destruction and failure of them. Statistics show that scouring around the abutment causes more problems compared with the pier scouring. Out of 108 bridge failures that occurred during 1960–1984 in New Zealand, 29 cases were related to bridge abutment scour. Respect to the importance of the issue, the research community has always sought to find scientific solutions to minimize the significant effects of scour around the bridge abutments. Two different methods are used for controlling and reducing erosion in river bed: increasing the bed resistance and changing the flow pattern. In the method of changing the flow pattern, different structures such as spur dike, slot and collar are used. In the method of increasing the strength of bed particles, protective tools such as concrete block and riprap are used. Spur dikes are among the structures that can reduce the scouring at the abutment by diverting the flow from the abutment and directing it towards the river axis. Although T-shaped spur dikes are of the most important forms of spur dikes, so far no comprehensive research has been done on the effect of this type of spur dikes on the reducing scouring around it. Many limited studies that have been done so far are related to rectangular channels. Therefore, their results cannot be used for supports abutment that are located in the compound channel. In the present study, by constructing a laboratory model, the effect of placing a T-shaped spur dike on reducing scour around the bridge abutment was investigated. The experiments were performed in a flume with the length of 12, width of 1 and height of 0.6 m. The length of the compound section was 6 m, which was installed at the distance of 3 to 9 m from the beginning of the channel. The main channel was made of glass and had the width of 20 and a depth of 8 cm. Both sides of the main channel were filled with a thickness of 30 cm of uniform non-cohesive natural sediments with an average diameter of d50= 1 mm. The abutments used were made of galvanized sheet with the length of 15 cm and the semicircular nose with a diameter of 10 cm. The T-shaped spur dikes with five different lengths of webs and wings were installed at three distances of 0.8 L, 1.2 L, and 1.6 L (L: support length) from the abutment. Then the effect of these variables on the reducing of scour around the abutment evaluated. The experiments repeated in three different ratios of average flow velocity to critical velocity of 0.7, 0.8 and 0.9. Results showed that the T-shaped spur dike could reduce the scour around the abutment by the average 75%. In the best case in terms of dimensions and distance, this type of spur dike would reduce the scouring around the abutment by 91%. Increasing the web length enhanced the efficiency of the spur dike. By increasing the dimensionless length of the spur dike by 100%, the spur dike efficiency enhanced by 37%. Increasing the wing length also increased the spur dike efficiency, So that with a 100% raise in the dimensionless length of the spur dike wing, the spur dike efficiency improved by 17%. At a constant length of web and wing, with increasing distance of the spur dike from the abutment, the effect of the spur dike on the abutment scour decreased and the scour depth at the abutment increased. By increasing the distance of the spur dike from the abutment by 100%, the efficiency of the spur dike was decreased by 10%. The results showed that the length of the spur dike had a greater effect on the efficiency of the spur dike compared with the length of the wing and the distance of the spur dike. The average efficiency of the spur dike in critical velocity flow ratios of 0.7, 0.8 and 0.9 was equal to 78, 75 and 70%, respectively; this showed that with increasing the ratio of flow velocity to critical velocity, the efficiency of the spur dike had not changed significantly. In other words, this type of spur dike maintained its efficiency in different flow conditions.