Clear water temporal scour around cylindrical pier with circular and airfoil collars fixed on bed

Abstract

Scouring around the bridge pier is a critical issue in hydraulic engineering as it affects the structural stability and safety of bridges. This natural and complex phenomenon is a leading cause of hydraulic bridge failures, resulting in public safety hazards, and economic losses. Therefore, implementing effective countermeasures to prevent scour around bridge piers is essential. An increase in scour resistance or flow alteration are two major countermeasure techniques. The goal is to counteract the erosive forces of scour-inducing mechanisms using durable engineering materials or, to shift the scour formation away from the immediate vicinity of the pier. The downflow that is responsible for clear water scour can be mitigated by using a collar plate. The present study focuses on the application of the collar as a scour countermeasure technique. This study investigates the temporal development of scour around a circular pier with circular and airfoil collars and also without any such protection. This study also aims to analyze the scour morphology, along with a new predictive model based on the nonlinear regression fonnula and Classification and Regression Tree (CART). a machine learning model to estimate temporal scour depth in the presence of a collar. The predictive ability has been compared with established empirical equations given in the works of literature. A total of 11 experiments are conducted under clear water scour conditions using cohesionless, uniform sand bed material to investigate the effects of collar shape and size on scour development around a pier. The study uses a fixed pier diameter and examines five different circular collars and five different airfoil collars, varying in diameter and length. The experiments aim to assess the effectiveness of these collars in preventing scour and to understand the time-dependent scour patterns at various points around the pier. All tests are conducted with collars fixed at bed level, with a constant flow depth, discharge, and flow intensity. Simultaneously, from the literatures as many as possible, similar experimental inputs and outputs have been collected or extracted for comparative analysis. The results reveal that the temporal development of the scour hole at the pier depends on whether the pier is protected with a collar or not. In the presence of a collar, the development of the scour hole is significantly delayed. It is observed that the larger sized circular collars (bc=36-3.56) are more effective in reducing scour where be and b are the collar and pier diameters, respectively. In the case of an unprotected pier and for smaller-sized circular collars (bc = 1.56-26), the maximum scour depth (Dscm) is observed at the upstream face of the pier (0°) but for the larger sized collars (be = 2.5b-3.56), it is found downstream of the pier (135°). On the other hand, for smaller and larger airfoil collars, Dsem is observed at the side (90°) and the downstream (135°) of the pier, respectively. A significant variation in protection efficiencies is observed when comparing circular collars to an unprotected pier, ranging from 26.23% to 84.96% at the upstream face of the pier and from 4.94% to 74.13% (collar diameter = 3.56) at the side of the pier, depending on collar dimensions. On the other hand, airfoil collars demonstrated higher protection efficiencies, ranging from 83.29% to 89.20% at the upstream face of the pier and from 32.67% to 82.80% (collar diameter = 3.5b) at the side of the pier. For both the circular and airfoil collars, maximum protection efficiency is observed for the larger collars (3.5b). It is also observed that for an unprotected pier, the scour length (1) is less than the scour width (w), and for a circular collar of 1.5b diameter, they are almost the same. However, for the remaining collars > ws. Based on the error analysis and statistical analysis, the CART model and the equation proposed in this study show a better fit with the observed data than the equations derived in previous literatures. This study may aid in protecting bridge pier designs globally under clear water scour conditions, but the predictive efficiency is limited by the use of fixed sediment types, flow conditions, and pier geometry and collar geometry.