Previous investigations indicate that scour around bridge piers is a contributor in the failure of waterway bridges. Hence, it is essential to determine the accurate scour depth around the bridge piers. For this purpose, deep understanding of flow structures around bridge piers is very important. A number of studies on flow structures and local scour around bridge piers have been conducted in the past. Most of the studies, carried out to develop a design criterion, were based on a single column. However, in practice, bridge piers can comprise multiple columns that together support the bridge superstructure. Typically, the columns are aligned in the flow direction. The design criteria developed for a single column ignore the most important group effects for multiple columns cases such as sheltering, reinforcement and interference effects. These group effects can significantly be influenced by the variation of spacing between two columns. This is evident by the fact that insufficient investigations and development have been reported for the flow structure and maximum scour depth around bridge piers comprising multiple columns. It is therefore necessary to investigate the effects of multiple columns and spacing between them on the flow structure and local scour around bridge piers and develop a practical method to predict the maximum scour depth.
The main objectives of this research work are to analyse the effect of spacing between two in-line circular columns on the flow structure and to develop a reliable method for prediction of the maximum local scour depth around bridge piers. To meet the objectives this research, detailed experimental studies on three dimensional flow structures and local scour around two-column bridge piers were carried out. A series of laboratory experiments were conducted for no column, a single column and two in-line columns cases with different spacing. Two in-line columns were installed at the centre of the flume along the longitudinal axis. Three dimensional flow velocities in three different horizontal planes were measured at different grid points within the flow using a micro acoustic doppler velocimeter (ADV). The velocity was captured at a frequency of 50Hz. Additionally, in vertical planes, particle image velocimetry (PIV) technique was employed to measure the two dimensional instantaneous velocity components. All experiments on flow structures were conducted under no scouring and clear water flow conditions. Similarly, an array of experimental tests were conducted under different flow conditions for studying the temporal development of scour depth and the maximum local scour depth around a single column and two-column bridge piers.
The measured instantaneous three dimensional velocity components were analysed and the results for flow field and turbulence characteristics were presented in graphical forms using vector plots, streamline plots, contour plots and profile plots. The results indicated that the flow structures around two- columns bridge piers is more complex than that of a single column case. Furthermore, the spacing between two columns significantly affects the flow structures, particularly in the wake of the columns. It was observed that for the spacing-column diameter ratio (L/D) < 3, the vortex shedding occurred only behind the downstream column. Hence, the flow pattern was more or less similar to that of the single column case. However, the turbulence characteristics such as turbulence intensity, turbulent kinetic energy and Reynolds shear stresses were notably different from those of a single column case. When the spacing was in the range of 2 ≤ L/D ≤ 3, stronger turbulence structures were noticed behind the upstream column. Further increase in the spacing between two columns resulted in a decrease in the strength of turbulence characteristics.
The experimental results on temporal development of local scour depth reveal that approximately 90% of the maximum scour depth around the upstream column was achieved within the first 10 hours of the experiments. However, for the downstream column, 90% of the scour depth was achieved within 20 hours. Similarly, the results from the experiments on local scour indicated that the maximum scour depth occurred at the upstream column, when the spacing between two columns was 2.5D. The maximum value of local scour depth for the two-column case was observed about 18% higher than the value obtained for the single column case. The reasons for maximum scour depth at the spacing of 2.5D were identified as the reinforcing effect of downstream column, the strong horseshoe vortex at upstream column, strong turbulence characteristics at the wake of upstream column, and the highest probability of occurrence of sweep events at upstream side of upstream column. Furthermore, a semi empirical equation was developed to predict the maximum scour depth as a function of the spacing between two columns. The findings of this study can be used to facilitate the position of columns when scouring is a design concern.