The effect of roughness protrusions in reducing scour around bridge piers and its impact on the vertical velocity profiles

Background and objectives: Bridge failure is a common phenomenon all around the world. The reason of the failure is not just the construction, but ignoring hydraulics’ roll in designing. Therefore, finding ways to reduce scouring is essential. There are two fundamental ways to protect bridges from scouring; increasing the resistant of bed materials and decreasing the power of erosion factors. A new method of decreasing pier scouring is to create roughness around the pier.
Materials and methods: The experiments were conducted under clear water scouring and with a constant discharge during 8 hours in hydraulic laboratory of Isfahan University of Technology using a flume of 7 m length, 0.32 m width and 0.36 depth. Sands with median diameter of 0.75, special gravity of 2.65 grams per cubic centimeters and geometric standard deviation of 1.2 are used in the experiments. In this research vertical flow structure has been investigated has been by analyzing with ADV tool and also effect of this type of flow on scouring of piers surrounding area was also evaluated.
Results: According to the results, vertical flow values were negative through central open canal line at upstream of pier which indicated that the flow was downward and also upward flow at the behind of piers was clarified by positive vertical flow values at that area. At common condition flow velocity increases by getting closer to the pier. At upstream of the flow, vertical velocity was particularly lower in existence of roughness on pier surface than control piers which didn’t have any roughness. Low negative values at upstream of the pier with roughness on its surface, the reduction of downward flow with reducing horseshoe vortex power than control pier was demonstrated.Investigation of velocity and shear profiles around the piers shows the existence of horseshoe vortex and downward flow simultaneously at upstream of piers. In downstream, existence of wake vortex causes fluctuations in flow structure. Velocity fluctuations in upstream of flow increases by entering scour hole, so inside the hole, there is a core with high intensity of turbulence in front of piers which is the result of flow separation in that region.
Conclusion: In this study, the efficiency of using roughness around the piers to decrease scouring is investigated. Results show that by increasing the size of roughness, scouring will decrease. In fact, increasing the size of roughness will decrease the power of down flow, horseshoe and wake vortexes.