The stability of bridges has always been exposed to natural hazards, with one of the most important causes of bridge failure being the phenomenon of scour on bridge piers. The construction of a bridge across a river cross-section obstructs the flow, which leads to an increase in flow velocity. These changes in flow velocity lead to erosion of the river bed around the piers, creating scour downstream of the piers. Climate change has led to storms and floods occurring with long return periods along the river courses. Flood waters carry floating objects downstream, and when a bridge is in the river channel, these floating objects accumulate in front of the piers. This accumulation of floating debris narrows the flow path and alters the flow regime. This research investigates the effects of different floods (increased discharge) and the probability of floating debris accumulation on the local scour depth at bridge piers.
Keywords
Scouring at bridge piers, accumulation of obstacles, HEC-RAS software, tributary.
Methodology
The study area focuses on a bridge over the Saqqez River (in the catchment area of Lake Urmia) that serves a commercial complex. This bridge has five piers with a circular apex. In the study, the HEC-RAS hydraulic model is used to investigate the local scour depth at the bridge piers. This software uses the steady-state flow equations to calculate the required parameters and uses two empirical equations, CSU and Froehlich, to estimate the scour depth. The CSU equation calculates the scour depth considering correction factors such as pier shape, flow angle, bed conditions and reinforcement of the bed material, while the Froehlich equation determines this depth considering the pier shape coefficient. The main river on which the bridge is located is the Saqqez River with flood discharges of 106, 193, 284, 428, 552, 685 and 742 (m3/s) for return periods of 2, 5, 10, 25, 50, 100 and 200 years. The tributary (Vali Khan River), entering the main river upstream of the bridge, has discharges of 19, 35, 52, 79, 101, 127, and 153 (m3/s) for the same return periods. Boundary conditions for both upstream and downstream of the river are defined considering the normal slope. The dimensions considered for the accumulated floating objects are 20 meters by 1.2 meters, and their placement is centered on the width of the piers.
Results and Discussion
The bridge piers under study are divided into two distinct categories based on their location: side banks (two piers) and main channel (three piers). Accordingly, the Manning's flow coefficient varies for different upstream piers, with a larger coefficient for side banks, resulting in lower velocity and flow rates compared to the main channel piers. This hydraulic parameter variation leads to changes in the scour depth increment trend for three defined scenarios, including natural, branch-only, and debris-only states, for different flood events. Rahimi et al. (2020) investigated the effects of accumulated floating objects on the geometry of cylindrical-shaped scour holes around bridge piers in laboratory experiments, considering a constant value for the Manning roughness coefficient. Daneshfaraz et al. (2019) calculated the scour depth at bridge piers (case study: Seminehrood Bridge) using HEC-RAS software and compared the scour results with empirical equations CSU and Froehlich under increasing river flow conditions. In this research, initially, the scour depth at bridge piers under natural conditions for various floods was calculated. Then, the effects of the tributary inflow and accumulated floating objects on scour depth were investigated. By segregating each of the three scenarios and separately analyzing the results of CSU and Froehlich equations, the effects of each factor were compared with each other. In general, both the tributary inflow and debris accumulation deepen the scour hole depth at bridge piers. For instance, the highest percentage increase in depth is observed due to debris accumulation and side bank piers, particularly for 2- and 5-year floods. The maximum local scour depth for the intermediate piers (main channel) occurs in the presence of a tributary for larger flood events.
Conclusion
Given the importance of scour depth for the 200-year flood event, the effect of debris accumulation on depth increase for all piers is 1%. In the branch-only scenario, the percentage increase in scour hole depth for side bank piers and main channel piers is 5% and 5.7%, respectively. The average percentage increase of CSU results compared to Froehlich results is 22% for the natural scenario and 21% for both other scenarios. However, given the nature of the CSU equation, it can be considered reliable, as it takes into account the effects of flow angle of attack, pier shape, bed particle conditions, and the probability coefficient for armoring of bed material.