Document Type : Original Article
Authors
1
Water Engineering, University of shahrekord, shahrekord,Iran
2
Shahrekord university
3
Department of Water Engineering, University of shahrekord, shahrekord,Iran
Abstract
Introduction
Bridges are critical structures in river engineering and require robust protection against scour, which is a significant hydraulic concern. Scour around bridge piers, exacerbated by flood events of varying intensity and duration, poses a substantial risk of structural failure. Furthermore, the accumulation of floating debris around piers can significantly alter local flow dynamics and scour patterns. Understanding these phenomena under diverse conditions is paramount for accurate scour depth estimation and the development of effective bridge protection strategies. Accumulation of floating debris around bridges, particularly near the piers, ranks among the primary factors contributing to scour. This debris reduces the flow cross-sectional area, thereby increasing the flow velocity to the levels beyond design thresholds. Consequently, the accumulation of floating debris causes a compounded increase in flow velocity, leading to severe scour. This ouccures especially in regions characterized by high-velocity water flows such as mountainous and forested areas. Thus, it is imperative to proactively identify bridges susceptible to debris accumulation and implement appropriate mitigation measures, such as installing debris traps upstream of the bridge piers, to prevent excessive debris buildup. This research investigated laboratory-scale scour around bridge pier groups under live-bed conditions. Also, it examined the influence of pier arrangement and the presence of accumulated floating debris.
Materials and Methods
The study analyzed pier groups consisting of three, five, and six piers, with a relative pier width (pier diameter to channel width ratio) of 1/10. A total of 18 experiments were conducted across two scenarios: with floating debris and without floating debris (control). To eliminate the influence of channel walls on scour depth, the pier diameter should not exceed 10 percent of the channel width. Therefore, piers with a diameter of 6 cm (exactly 10 percent of the channel width) were utilized in these experiments. The related tests were performed in a mobile-bed flume to evaluate how varying discharge rates (24, 39, and 54 L/s) affect scour dynamics. The experimental design aimed to demonstrate that increases in the relative flow velocity lead to greater scour depth and volume, and to quantify the significant exacerbation of scour caused by floating debris accumulation. To prevent the influence of bed particle size on scour depth, the ratio of pier diameter to the mean sediment particle diameter was maintained at 50, in accordance with previous related studies. Furthermore, to eliminate the effect of sediment non-uniformity on scour reduction, the geometric standard deviation of the sediment was kept below 1.3. Consequently, a mean particle diameter of 0.96 mm was selected for this study. Prior to initiating each experiment, the sediment bed surface was leveled using a trowel, and the area around the piers was precisely leveled using a laser level and a spirit level. The tailgate at the downstream end of the flume was fully raised to prevent initial scour and the formation of bed forms. Subsequently, the pump was started at a low discharge rate. After a few minutes, once the sediment was fully saturated, the discharge was gradually increased to the target flow rate. The previously closed tailgate was then slowly adjusted to establish the required water depth. After allowing several hours for the scour around the piers to reach an equilibrium state, the pump was shut down, and the water in the flume was slowly drained. Following the cessation of flow, the topography around the pier group was measured using a non-contact bed profiler to ensure accurate assessment.
Results and Discussions
In the absence of floating debris, the maximum scour depths for the three-, five-, and six-pier groups were recorded as 11.5 cm, 12.1 cm, and 14.8 cm, respectively. Increasing the number of piers significantly amplifies the maximum scour depth; specifically, the six-pier configuration exhibited a 28.70% increase compared to the three-pier group, which is primarily attributed to the complex interference and overlap of horseshoe and updraft vortices. Furthermore, the accumulation of floating debris acts as a critical aggravating factor by reducing the effective flow cross-section and intensifying local flow velocities. In the densest pier arrangement, this debris accumulation exacerbated the maximum scour depth by 35.14% relative to the no-debris condition. The analysis of the blockage ratio revealed that denser pier configurations, particularly at shallower flow depths, severely restrict the effective flow area, thereby forcing intense flow channelization between the piers and exacerbating localized bed erosion. Under supra-threshold live-bed conditions, a direct and substantial correlation was observed between the relative flow velocity and the expansion of the scour hole, where a 125% increase in discharge led to a remarkable 94.17% surge in scour depth when debris was present. Three-dimensional bed topography measurements further demonstrated that the presence of debris not only deepens the scour hole but also significantly increases its lateral extent and morphological complexity, particularly around the front piers where the primary downflow originates. Finally, based on dimensional analysis of the experimental data, a robust empirical predictive model was successfully formulated and statistically validated with a high coefficient of determination (R² = 0.85), providing a reliable mathematical tool for estimating maximum scour depth under these specific hydraulic conditions.
Conclusion
Based on the conducted experiments and the obtained results, an empirical relationship for predicting scour depth was derived using dimensional analysis for the three investigated groups of cylindrical piers. Statistical validation, performed with the aid of SPSS software, confirmed the high accuracy of this relationship in predicting scour depth.
Keywords
Subjects
