Clear water temporal scour around oblong pier with oblong collar fixed on bed

Abstract

Any countermeasure of scouring around bridge piers is always a challenging issue. Foundations are under constant threat from scouring, which can quietly compromise their integrity over time. While completely preventing scouring is often economically unfeasible, engineers can implement strategies to manage and mitigate its impact. Bridge collapses, leading to fatalities, property damage, and halted transportation, highlight the need for bridge designers to understand the interaction between soil, water movement, and bridge structures. Existing literature indicates that the use of oblong shaped piers is advantageous in multiple contexts. Though analysis of oblong piers with oblong collars around bend channels exists, straight path analyses are unexplored substantially. Hence, the objective of the present work is to analyse scour patterns around oblong piers using oblong collars under straight channels and clear water conditions with uniform cohesionless bed material and steady flow conditions. An experimental analysis is conducted on oblong piers with collars to investigate the variations in scour depth over time and to determine the maximum scour depth. Specifically, our investigation explores non-dimensional temporal scour depth around the oblong collar fixed on the bed within a controlled flume environment. The experimental setup involved varying the collar width to pier width and pier length to pier width ratios with 12 experiments conducted in total. The temporal scour of the sand bed was measured, and equilibrium conditions were identified when the scour rate dropped below 1 mm/hour. Multiple parameters, including scour depth, width, length, area, and volume, were computed based on the experimental data. For oblong piers without oblong collars, the study found that the maximum scour depth occurred at the upstream of pier (0°). This indicates that without oblong collars, scour is more pronounced directly in front of the pier. In contrast, for piers equipped with collars, the maximum scour typically shifted to the downstream of the oblong pier (135°). The scour depths varied with the collar width to pier width ratio (W/D). As the L/D ratio increases, scour depth decreases notably without a collar. The presence of critical L/D ratios that minimize scour depth for different W/D ratios. For W/D ratios ranging from 1.5 to 3.5, these critical L/D ratios are noted for both L/D ratios of 2 and 4. These findings suggest that collars significantly alter the flow patterns around oblong piers, reducing the maximum scour depth and changing its location around the oblong pier circumference. This highlights the importance of considering both the width of the collar and the length-to-width ratio of the pier when designing bridge foundations to manage sediment transport effectively. The development of a governing equation for predicting temporal scour depth, validated by observed temporal scour depth. This non-dimensional equation may be utilized to estimate scour depth accurately in real-world scenarios, aiding in the design and maintenance of safer bridge structures. This contributes to the broader field by offering empirical data and predictive capabilities to better manage the risks associated with scouring around oblong bridge pier with oblong collars.