Effect of the peak flow of flood hydrograph on scour around the dual bridge piers using experimental model

The local scour around the bridge piers is the main cause of their destruction. Based on this, extensive studies have been done to understand this phenomenon. Most of these studies have been done under steady flow conditions. This is while the flow in the river is unsteady. The purpose of this study was to investigate the effect of peak flow of flood hydrograph on scour around the dual bridge piers at different distances from each other in uniform and unsteady flow using symmetric hydrographs with change in peak flow. The hydrographs used in the experiments are stepped hydrographs in 5 steps. The experiments were conducted under clear water condition and U/UC=0.95. In all experiments, the diameter of the bridge pier (D) was constant and equal to 2.5 cm. S/D (S: center-to-center distance between dual bridge piers and D: bridge pier diameter) was selected as 2, 3, 4 and 5. The results showed that in a given flood hydrograph, with increasing the relative distance between the dual bridge piers, the maximum dimensionless scour depth of the first and second piers increased and its maximum was measured at a relative distance of S/D=5. Whereas in uniform flow, maximum dimensionless scour depth of first pier was observed at S/D=3 and second pier at S/D=4. Also, at a constant relative distance of the pier from each other, with increasing peak flow of hydrographs, the maximum dimensionless scour depth of the first pier of the dual bridge piers increased with an increasing rate.