Reduction of Pier Scour Using an Airfoil Collar

Abstract

Scouring around the bridge pier is a natural and complex phenomenon and it is one of the major causes of a bridge failure in the alluvial channel foundation. Failure of bridges has a potential devastating effect on public safety and economic losses, which lead to political consequences as well as environmental impacts. Therefore, it is essential to countermeasure the scour around the bridge pier. Countermeasures are used to monitor, inhibit, change, delay or minimize stream instability and bridge scour problems. It is highly beneficial because it solves the existing scour problem as well as mitigating it. There are many countermeasures used today to mitigate the scour problem. One of the major and active research areas is pier modification using collars. This thesis studies the effects of four different airfoil collars (i.e., � � =1.5b, 𝑏 =2.0b, 𝑏 =2.5𝑏 and 𝑏 =3.0𝑏, where 𝑏 and 𝑏 are the diameter of the airfoil collar and pier respectively) as a scour countermeasure. A total of 69 experiments are conducted under two threshold velocities with two pier diameters of 0.05 and 0.06 m and four airfoil collars placed at four locations from the uniform sediment bed. All the experiments are conducted under clear water conditions with uniform sediment and a constant water depth (𝑦) of 0.10 m. Airfoil collar is placed at four elevations, i.e., at bed level, 𝑦/4, 𝑦/2 and 3𝑦/4 above the sediment bed level. This thesis investigates the effect of flow intensity, collar diameter on scour hole profile and computes the protection efficiencies for different locations of the collar above the sediment bed. The results show that the collar effectively reduces the scour depth. When the collar is located at bed level (i.e., 𝑦 = 0, 𝑦/4, 𝑦/2 and 3𝑦/4 above the bed level, the protection efficacies are found to be ranging between 55 to 100%, 40 to 53%, 23 to 38% and 8 to 29% for 𝑏 , 𝑏 , 𝑏 , and 𝑏 respectively. It is observed that the maximum percentages of scour reduction are 78, nearly 100 and 100% in the case of the collars 𝑏 , 𝑏 and 𝑏 respectively, when collar is located on the sediment bed. The scour hole profiles show that the length of the transverse scour hole is greater than that of the longitudinal one. This study uses nonlinear regression analysis to propose relationships for maximum scour depth and time dependent scour development around the pier in the presence of an airfoil collar. Statistical sensitivity analyses are performed and found that the most and least sensitive parameters are the collar location and dimensionless time. v A comparative study between numerical simulation and experimental results for maximum scour depth and temporal variation of scour development with and without an airfoil collar, is carried out with two collars 𝑏 and 𝑏 using FLOW-3D. A total of six simulations are carried out: pier without AirFoil Collar (AFC), pier with collar 𝑏 , 𝑏 and 𝑏 in reverse to the flow direction (denoted by 𝑏 ) on the sediment bed and pier with collars 𝑏 and 𝑏 at 𝑦/2 above the sediment bed. The turbulence model used in the study is Large Eddy Simulation (LES) and the bed-load transport model utilized is van Rijn. A nested mesh configuration is used with 12.234 million mesh cells. The percentage error between the experimental and simulated results is around 7%, indicating a good correlation between the experimental and simulation results. Temporal variation of scour depth and the percentage reduction of scour depth using the AFC are explored when placed at various locations. The percentage reduction of scour depth for various cases and different orientations of the collar ranges between 11% and 100%. The increasing order of percentages of scour reduction for collars are 𝑏 at 𝑦/2 above the sediment bed, 𝑏 at 𝑦/2 above the sediment bed, 𝑏 , 𝑏 and 𝑏 on the sediment bed respectively. The collar 𝑏 is 31.78% efficient than 𝑏 in reducing the scour around the pier. When the collar 𝑏 and 𝑏 are placed on the sediment bed, it is observed that no scouring occurred at the upstream face of the pier and around the pier. In addition, with collar � � no scouring occurred in the downstream face of the pier. Scour depth contours, morphological changes in sediment bed, flow patterns, longitudinal scour hole profiles, transverse scour hole profiles and streamlines are developed for the pier with and without the AFC and analyzed for salient features.