Bridges are critical structures in river engineering, requiring robust protection against scour, 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.

One of the primary factors contributing to scour is the accumulation of floating debris around bridges, particularly near the piers. This debris reduces the flow cross-sectional area, thereby increasing the flow velocity beyond design thresholds. Consequently, especially in regions characterized by high-velocity water currents—such as mountainous and forested areas—the accumulation of floating debris causes a compounded increase in flow velocity, leading to severe scour. Thus, it is imperative to identify bridges susceptible to debris accumulation and implement mitigation measures, such as installing debris traps upstream of the bridge piers, to prevent excessive debris buildup.

This research investigates laboratory-scale scour around bridge pier groups under live-bed conditions, specifically examining the influence of pier arrangement and the presence of accumulated floating debris.

Methods The study analyzed pier groups consisting of 3, 5, and 6 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. 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 increasing the relative flow velocity leads 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 Chiew and Melville (1987). 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 theFlume was slowly drained. Following the cessation of flow, the topography around the pier group was measured using a non-contact Bed Profiler (Figure 5) to ensure accurate assessment. The 3D profile of the bed was acquired without any physical contact, achieving a measurement accuracy of 0.1 mm.

Results In the absence of floating debris, the maximum scour depths for the 3-, 5-, and 6-pier groups were recorded as 11.5 cm, 12.1 cm, and 14.8 cm, respectively. The 6-pier group exhibited a 22.31% increase in scour depth compared to the 5-pier group, and a 28.70% increase compared to the 3-pier group. Furthermore, the presence of floating debris resulted in a 35.14% increase in scour depth for the 6-pier group relative to the no-debris condition.

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.